Radiation - Atomic Rockets

Introduction

Cherenkov Radiation If you see this in the air, the good news is you can probably live long enough write your last will and testament. If you write very quickly.
Radiation Protection poster from Oak Ridge National Laboratory (1947)
Black humor from MAD magazine (1955).
Parody of Kodacolor film ad with Nikita Khrushchev.
artwork by Kelly Freas

Everything has a price. And the price of powerful rockets with nuclear propulsion is of course the dread horror of deadly atomic radiation. But the danger can be brought under control with appropriate counter-measures, and by treating the power plant with the respect it deserves. And the same measures will come in handy if your ship is an interplanetary warship that may be facing hostile nuclear warheads.

But it is important not to over-react. There is a lot of silly media hype about plutonium being "the most toxic substance known to man", there is a general agreement among experts in the field that this is false.

The characteristic blue glow you've seen in photographs of "swimming pool" reactors is called Cherenkov Radiation. If you the blue Cherenkov glow around an object IN THE AIR (not at the bottom of a swimming pool reactor), you'd better be viewing it through several inches of lead glass or you have already taken a lethal dose, it is far too late to do anything about it, you are already dead. This comes under the heading of "not treating radiation with the respect it deserves."

As a side note, Cherenkov Radiation is caused by radioactive particles exceeding the speed of light in the medium. The term "c" is not "the speed of light", it is the speed of light in a vacuum. The maximum speed of light is much slower in air and even slower in water. The practical upshot of this is that there is no Cherenkov Radiation in the vacuum of space, and to get the same level of glow seen in a swimming pool a radiation source in air will have to be much more radioactive.

Animated GIF from Sploid

Video Clip "Nuclear Reactor 0 to 80% power steady state and down"
click to play video

Types of Radiation

The general term for dangerous unhealthy everybody-panic-now kind of radiation is "ionizing radiation." This is because the radiation is capable of ionizing atoms which compose the material being irradiated. Materials such as the poor crew's tender vulnerable pink bodies and internal organs. Non-ionizing radiation such as visible light and radio waves can be safely ignored (by which I mean a laser beam can chop you into bits but it won't give you cancer).

Ionizing the atoms composing the proteins of a living thing is much like using a machine gun to fill a running automobile engine full of bullets. Proteins are the tiny molecular machines that make cells work. If you ionize an atom of a given protein it either splits or crumples up into a tangled wad, rendering it useless. Destroy enough of the proteins and the cell dies. Destroy enough cells and you die.

Radiation also smashes DNA molecules like a jackhammer. This can kill the cell or turn it cancerous. No, it won't turn you into a mutant, but any future childen you have will be another matter.

The sun's ultraviolet light is a form of radiation that can give your skin a sunburn. Ionizing radiation is more penetrating, so it is capable of giving you a lethal "sunburn" on your internal organs.

Ionizing radiation comes in two types:

electromagnetic radiation like gamma rays and x-rays
particulate radiation like protons ("proton storm"), neutrons ("neutron radiation"), electrons ("beta particles"), alpha particles ("alpha particles"), and heavy primary nuclei ("HZE ions").

In the context of this website, you will be dealing with radiation in several areas.

Radiation from the space environment
Radiation from spacecraft nuclear propulsion and nuclear power plants
Radiation from space combat weapons

Finally it is important to understand the subtle distinction between radiation and radioactivity. Radiation are the deadly rays and particles that kill you. An element is radioactive if that element emits the deadly rays and particles that kill you.

What's the practical difference?

If you say that the nuclear rocket engine is emitting radiation, this means it is emitting deady rays, which all nuclear rockets tend to do.

But if you say that the nuclear rocket engine is emitting radioactivity, this means that the reactor core has been breached, and it is spewing powdered nuclear reactor rods in the form of a lethal cloud of atomic fallout.

Radiation from Space

Astronauts traveling from planet to planet are exposed to the natural radiation of space. This is generally always particle radiation (but little or no neutrons), with zero gamma rays or x-rays. One usually only encounters gamma rays and x-rays from artificial sources, such as nuclear reactors and nuclear weapons (unless you are near a black hole or other extreme environment). Exposure time to natural space radiation is "chronic" (see below).

There are three sources of natural space radiation:

GALACTIC COSMIC RAYS (cosmic rays or GCR)
TRAPPED RADIATION (radiation belts)
SOLAR ENERGETIC PARTICLES (solar proton storm, solar storm, solar proton event (SPE), proton storm)

Here is a good overivew of naturally occuring sources of space radiation.

From **NASA-STD-3000 MAN-SYSTEMS INTEGRATION STANDARDS**

Figure 5.7.2.1.1-1 Sources and Characteristics of Electromagnetic and Particulate Ionizing Radiations in Space.
Name	Nature of radiation	Charge	Mass	Sources
X-ray	Electromagnetic	0	0	Primary: Solar corona, stars, galaxies, terrestrial atmosphere in auroral zone. Secondary: Spacecraft structure in some parts of the radiation belts, in the auroral zone, and in interplanetary space following some solar flares
Gamma ray	Electromagnetic	0	0	Stars, galaxies, unknown sporadic sources, and spacecraft atmosphere.
Electron	Particle	-e	1m_e	Radiation belts and auroral regions.
Proton	Particle	+e	1840 m_e or 1 amu	Galaxy cosmic rays, radiation belts, and solar flares.
Neutron	Particle	0	1841 m_e	Primary: Galactic cosmic ray atmosphere albedo neutrons. Secondary: Galactic cosmic ray interaction with spacecraft structure.
Alpha particle (helium nucleus)	Particle	+2e	4 amu	Galactic and solar.
HZE particle (heavy primary nuclei)	Particle	=> +3e	=> 6 amu	Galactic and solar.

Galactic Cosmic Radiation

Galactic Cosmic Radiation is particle radiation coming from outside the solar system (some is from outside of the galaxy), from more or less the entire sky. Most of them come from supernovae, it is unclear what is the origin of the rest.

The intensity of the cosmic ray bombardment varies with the strength of the solar wind. The wind is not causing the cosmic ray flux, it is apparently providing some shielding from cosmic rays. At a distance of about 94 astronomical units from Sol the solar wind undergoes termination shock which forms the inner edge of the heliosphere. This provides the cosmic ray shielding, in an amount proportional to the current solar wind strength.

But even with maximum protection from solar wind, the cosmic ray flux never drops down to zero.

In a spacecraft or space station the crew is protected from galactic cosmic rays by hull armor. The armor has two layers: hydrogen-rich armor (e.g., paraffin) to stop particles and dense armor (e.g., tungsten) to stop electromagnetic radiation. It is vitally important that the hydrogen-rich armor is on the outside with the dense armor as an inner layer. This ensures that the cosmic rays are stopped before they hit the dense armor.

If the dense armor was on the outside so the cosmic rays hit it first, the charged particles in the GCR would create deadly "Bremsstrahlung" ("secondary") radiation and kill the crew. Basically you would have designed the spacecraft to be a huge x-ray machine with the crew at the emitter. That would be bad.

But in any event keep in mind that it is incredibly difficult to shield against GCRs.

NASA engineers fret about this because the transit time for a Mars mission with the currently available pathetically weak propulsion systems will expose the crew to more space radiation than is allowed. NASA's Curiosity space probe measured the radiation dosage inflicted by traveling through space to Mars, specifically from galactic cosmic rays and solar proton storms. The dosage was 1.84 millisieverts per day (0.00184 sieverts). However, keep in mind this was measured during the peak of Sol's 11-year activity cycle, when GCR flux is relatively low due to shielding from the heliosphere. Also keep in mind this is with zero radiation shielding.

Click to open website. — *click for larger image*

From **RADIATION ON THE WAY TO MARS, AND WHY IT ISN'T SUCH A HUGE RISK AS WE THINK IT IS** by Waz_Met_Jou (2014)

Trapped Radiation

or other planet with magnetic field

Back in the 1950s the pulp scifi stories tried to scare the readers with references to the deadly belts of radiation found in space. In reality they are not instant death, but they are unhealthy to linger inside. They damage the electronics of satellites as well.

What happens is that planets with strong magnetic fields will created zones of radiation by trapping energetic particles from the solar wind. Sort of like a cosmic roach motel: "Radiation checks in, but they don't check out!" From the standpoint of life on the planet's surface, the magnetic field is a good thing. The radiation from the solar wind is either diverted or trapped in the radiation belts, in either case it is much better than hosing the planet's surface with deadly radiation. If the planetary life becomes intelligent enough to develop space flight, then it should be intelligent enough to invent radiation shielding.

Please note that the radiation in the belts never reach the planet itself.

There are known radiation belts around Terra, Jupiter, Saturn, Uranus and Neptune.

In a spacecraft or space station the crew is protected from trapped radiation by storm cellars. The crew has to occupy the storm cellar as long as the ship is within the radiation belt. This is why ion drives are not favored for breaking out of low Terra orbit, because such low-thrust propulsion can take almost a year to climb out of Terra's Van Allen Radiation Belt.

Terran Van Allen Radiation Belts (cross-section)
note South Atlantic Anomaly

The zones around Terra are called the Van Allen radiation belts, after James Van Allen discovered them in 1958.

There are two belts.

The Inner Belt starts at an altitude from 400 km to 1,200 km, depending on latitude, and ends at an altitude of about 6,000 km, with its most lethal area 3,500 km out (1.55 Terran radii). The South Atlantic Anomaly can potentially disrupt satellites in polar orbits, but usually does not pose a problem for manned spaceflights. Except for the ISS. The radiation is high-energy protons (400 MeV).

The Outer Belt ranges from 13,000 km to 60,000 km, with its most lethal area 27,000 km out (5.22 Terran radii). The Outer Belt is affected by solar winds, and is thus flattened to 59,500 km in the area directly between the Earth and the Sun, and extends to its maximum distance in the shadow of the Earth. The radiation is high-energy electrons (7 MeV).

A safe channel exists between the belts from 9,000 km to 11,000 km.

Terran Van Allen Radiation Belt Intensity (cross-section)
Horizontal axis is ruled in Terran Radii (1.0 = 6378 km)
Blue: 1×10^-6 Gray/sec
Green: 1×10^-5 Gray/sec
Yellow: 5×10^-5 Gray/sec
Orange: 1×10^-4 Gray/sec
Red: 5×10^-4 Gray/sec

if you were in a ship with 2 g/cm² of hull shielding, the maximum radiation intensity zone of the inner Van Allen belt would give you a radiation dose of 0.2 Grays per hour. Hull shielding of 25 g/cm² would reduce that to 0.05 Grays/hr. The report cites a permissible limit of 2 Grays total for a mission. The Apollo lunar missions dealt with the belts using a trajectory that missed the inner belt and zipped through the outer belt as fast as possible.

South Atlantic Anomaly Intensity at altitude of 560 kilometers
lower boundary is at 200 km

Since Terra's rotational and magnetic axes do not intersect at Terra's Center (see diagram), there is a deadly spot in the inner belt called the South Atlantic Anomaly. The inner edge of the belt proper is usually 1,000 kilometers from Terra's surface, but the anomaly gets as close as 200 kilometers. Satellites and space stations need extra radiation shielding for when they periodically pass through the anomaly. The ISS has extra shielding for that reason. Astronauts have seen phosphene shooting lights in their eyeballs, laptops have crashed, control computers experience transient problems as they pass through the anomaly.

Also of note, the Starfish Prime nuclear test temporarily (for five years) made the radiation levels in the Van Allen belts much worse (crippled a third of all satellites in low Earth orbit). Yes, this means that somebody can easily intensify the radiation just by popping off a few fission bombs. Science fiction writers can easily imagine scenarios where this could be used to attack a planet or to defend a planet.

Since the Van Allen Belts will destroy expensive satellites as well, there have been proposals to drain the radiation out of the belt.

The planet Jupiter has radiation belts similar to Terra, except the radiation is thousands of times stronger. Io, Europa, and Ganymede are inside the radiation belt, Callisto is outside. Volcanic gas from Io makes things more complicated. In 1973 Pioneer 11 was surprised by radiation levels around Jupiter ten times greater than NASA had predicted. This is why Pioneer did not send back photos of the moon Io since the radiation belt had fried its imaging photo polarimeter. Work on the Voyager space probe came to a screeching halt as they frantically redesigned it to cope with the radiation, but still be assembled in time for the launch window.

Jupiter's Radiation Belt
L,R_j is orbit in units of Jupiter Radii (71,398 km)
Dose in units of rads per day (1 Rad = 0.01 Gray)
Lines are for dosage behind radiation shield of 0.27, 1.0, 2.2, and 5.0 gm/cm²
For instance, behind a shield of 0.27 gm/cm² the dosage at both Io and Europa is about 1000 Gray (10⁵ rad) which is enough to kill an unprotected human in a matter of hours
Callisto is at R_j = 26.4, outside of the belt
click for larger image

According to Cairan:

Satellite	Daily radiation rate (Sv)	Yearly rate (Sv)	Single-year limit	5-year cumulative limit	Mild sickness	LD 10/30	LD 35/30	LD 50/30	LD 60/30	LD 100/14	LD 100/7	LD 100/2
Io	36	13149	2 min	4 min	20 min	40 min	1.33h	2 h	2.67h	4 h	6.67h	33.33h
Europa	5	1972	13.33 min	26.66 min	2.22h	4.44h	8.89h	13.33h	17.78h	26.67h	44.44h	-
Ganymede	0.08	29.22	15h	30h	6.25d	12.5d	25d	37.5d	50d	75d	-	-
Callisto	0.0001	0.037	1.37y	2.74y	-	-	-	-	-	-	-	-

The single year limit is 50 mSv, while the maximum 5-year cumulative exposure is 100 mSv (or 20 mSv per year). LD stands for Lethal Dose, LD x/y means "x" percent of individuals die within "y" days. LD 50/30 thus means half of people exposed at this level of radiation would die within 30 days.

For shielding purposes to limit exposure to below regulatory levels for 5-year periods, the number of halving thickness of shielding material stands as follows:

Satellite	Number of shielding layers	Liquid water thickness
Io	20	3.6 m
Europa	17	3.06 m
Ganymede	11	1.98 m
Callisto	1	18 cm

Solar Energetic Particles

The sun (or any other star) occasionally belches out fast-moving clouds of deadly particle radiation (the technical term is Coronal Mass Ejection or CME). These are called a solar proton storm, solar storm, solar proton event (SPE), or proton storm. Planets lucky enough to have magnetic fields (like Terra) are protected from such storms. Planets without magnetic fields and spacecraft, are bombarded by the radiation. The International Space Station has an orbit low enough that it is protected by Terra's magnetic field, though it does periodically plough through the South Atlantic Anomaly.

The storm can be sped up or slowed down by interations with the solar wind and interplanetary magnetic field. They reach velocities from 20 to 3,200 km/s with an average speed of 489 km/s. This means the transit time from Sol to the mean radius of Terra's orbit will be from 13 hours to 86 days, with 3.5 days as the average

Solar storms typically have a duration from one to two days.

Please note that solar storms are blobs of radiation that are quite a bit larger than a planet, but smaller than the solar system. So, for instance, a storm that hits Terra might totally miss Mars or Ceres. Or force the crew of spacecraft heading to Luna to take shelter in the storm cellar, while astronauts near Mars will be safe.

In a spacecraft or space station outside of a planetary magnetic field the crew is protected from solar storms by storm cellars. The crew will have to occupy the storm cellar for several hours during the peak of the storm, maybe up to a couple of days.

Researchers have been looking into equipping spacecraft with artificial magnetic fields for protection, but so far this has proven difficult.

From NASA-STD-3000 Man-Systems Integration Standards. (ed note: Naturally any NASA document will be silent on nuclear weapons, so there is no mention of gamma-ray or x-ray shielding.)

Proton radiation levels over time from a large solar flare.
from An exploration of the effectiveness of artificial mini-magnetospheres as a potential Solar Storm shelter for long term human space missions (2014)
The energy fluence spectra of some of the largest Solar Energetic Proton (SEP) events of the last 50 years.
from An exploration of the effectiveness of artificial mini-magnetospheres as a potential Solar Storm shelter for long term human space missions (2014)

Radiation from Nuclear Power

If the spacecraft uses a nuclear propulsion system, or has a nuclear power reactor, these are also sources of both electromagnetic and particle radiation. The exposure time is "chronic." Typically the crew is protected by use of shadow shields.

In some propulsion systems, such as open-cycle gas core nuclear thermal rockets and Orion nuclear pulse rockets, the exhaust is radioactive. This means the shadow shield has to cover a broader arc.

The type of radiation emitted depends upon whether the reactor is fission or fusion, and which fuel is used.

There is a first order approximation here to calculate the radiation flux from a fission reactor or fission nuclear thermal propulsion system.

Fission fuel is radioactive. If the reactor core is breached the fuel can spread radioactive contamination. Also the neutron radiation emitted by the reactor in normal operation can cause neutron activation.

Radiation from Weapons

If the spacecraft is in a combat situation, it will be targeted by nuclear warheads and particle beam weapons. In these cases the exposure time is "acute."

Typically the crew is protected by use of storm cellars and hull armor.

Nuclear warheads emit both electromagnetic and particle radiation

Obviously particle beam weapons only emit particles. Having said that, if the particle beam hits a metal hull bremsstrahlung will create a flood of x-rays. This only happens if the particle in the beam are charged, it doesn't work with neutron beams.

Effects of Radiation

Units

The effect of radiation upon the crew is rather complicated (translation: I don't understand it) so take the following explanation under advisement.

Radiation is meaured with lots of different confusing units. To make it worse, each measurement has both a traditional obsolete deprecated unit and a new modern scientific metric unit. There are units for the amount of radiation emitted, units for the amount of radiation absorbed by an inert object, and units for the amount of radiation absorbed by a living being. And on top of that, metric units often have prefixes for various powers of 10: milli-, micro-, etc. There is a table of prefixes here.

Naturally, the United States Nuclear Regulatory Commission requires the use of the non-metric obsolete deprecated units curie, rad and rem as part of the Code of Federal Regulations 10CFR20. The United States hangs on like grim death to its stupid ramshackle non-decimal system of units, instead of adopting the metric system like the rest of the scientific and civilized world.

ACTIVITY: The amount of radiation emitted by a chunk of radioactive material is measured by the traditional obsolete unit the Curie or the new metric unit the Becquerel (Bq). One Curie is equal to the amount of radiation emitted by one gram of radium. One Becquerel is equal to one decay per second. 1 Curie equals 3.7 × 10¹⁰ Becquerels.

ABSORBED DOSE: The amount of radiation absorbed by an inert object is measured by the traditional obsolete unit the Rad or the new metric unit the Gray (Gy). One Rad is a dose of radiation causing 100 ergs of energy to be absorbed by one gram of matter. One Gray is 1 joule of radiation absorbed by one kilogram of matter. 1 Rad equals 0.01 Gray. An even more obsolete term is the Roentgen, currently it is defined as 1 Roentgen equals 0.0096 Gray.

MGy is the symbol for megagray, or 10⁶ gray. mGy is the symbol for milligray, or 10⁻³ gray. cGy is the symbol for centigray, or 10^-2 gray.

DOSE EQUIVALENT: The amount of radiation absorbed by living being is measured by the traditional obsolete unit the Rem or the new metric unit the Sievert (Sv). 1 Rem equals 0.01 Sievert.

Sieverts are determined from Grays. The effects of Acute radiation exposure are figured by the exposure in Grays. The effects of Chronic radiation exposure are figured by the exposure in Sieverts. Radiation quality factors seem to mostly matter for chronic doses.

You see, as far as an inert lump of matter is concerned, all forms of radiation are pretty much the same. But when you get to living beings, different kinds of radiation do different levels of long-term chronic internal organ damage per joule. Some types of radiation are better at killing people than other. For example, if 1 Gray of gamma radiation does 1 unit of damage to a stricken crewperson, 1 Gray of alpha particles will do 20 units of damage to the crewperson.

What this boils down to is that each type of radiation has a quality factor Q. You look up the Q factor for the radiation in question, take the radiation dose in Rads, multiply by Q, and you will have the "dose equivalent" in Rems. Or take the dose in Grays, multiply by Q and you will have the dose equivalent in Sieverts. There is a list of Q factors here.

You will sometimes see radiation exposure expressed in units of mGy/a or mGy/yr. This stands for milliGrays per annum, where 1 milliGray = 0.001 Gray and 1 annum is 8760 hours = 365 days = 1 year. This appears when talking about the radiation exposure suffered by inanimate objects on an extraterrestrial planet. For instance a rover space probe on Mars will suffer 73 mGy/a from cosmic rays and proton storms, while a space probe on Terra is typically only exposed to 0.4 mGy/a from cosmic rays.

The corresponding measure for organic creature exposure is mSv/a or mSv/yr. This is of course milliSieverts per annum.

Calculating Dosage

Logo of old United States Atomic Energy Commission. This is sometimes mentioned in old Robert Heinlein novels. Abolished in 1975 and functions assigned to the Department of Energy.

The size of the dose depends on two things: the intensity of the radiation, and the duration of exposure. Crewpersons who do not want to die a hideous radioactive death will do well to reduce the size of the dose. You reduce the intensity by getting as far away from the source of radiation that you can (allowing the inverse-square law to reduce the intensity) and trying to get some radiation shielding between you and the source of the radiation. You reduce the duration by performing the first two techniques as quickly as possible (i.e., don't just stand there with a stupid expression on your face, run for your life!). There is also a difference between "acute" and "chronic" exposure. An example of an acute exposure is being in the general neighborhood of a nuclear weapon when it goes boom: the exposure duration is measured in fractions of a second. An example of chronic exposure is the day-to-day job of a nuclear rocket engineer: exposure duration is measured in months. Obviously the only things you can do to reduce an acute dose is to always be inside plenty of shielding and only fight enemies who use tiny nuclear weapons.

To figure the dose in Grays, take the radiation from the "event." Calculate how many radiation joules managed to penetrate the shielding and intersect the cross section of a person and divide by the body mass of the unlucky crewperson.

Since I know you are impatient, I'm first going to give you the quick-and-dirty equations. I'll then give you how I derived them, to allow you to skip over it if you are not interested. The equations have build-in assumptions. Assume this is radiation from an exploding nuclear warhead. 80% of the energy is in the form of x-rays with an average energy of 10 keV. Each fissioning nuclei will produce 2 or 3 neutrons and about half will escape to become radiation. The neutrons will have an average Sievert quality factor of 10. The crewmember will have an average cross-section of 0.445 m² and a mass of 68 kilograms. Finally assume no radiation shielding.

G_x = 1.78e9 * (Y / R²)

G_n = 7.2e8 * (Y / R²)

where:

G_x = person's acute radiation dosage from x-rays (Grays)
G_n = person's acute radiation dosage from neutrons (Grays)
Y = weapon yield (kiloton TNT)
R = person's distance from weapon's detonation center (meters)

How was this derived? In a round about fashion. First you figure out the radiation flux in joules of radiation per square meter. You take the amount of joules in the detonation and divide them by the surface area of a sphere with radius R.

s_sphere = 4 * π * R²

There are about 4.19e12 joules per kiloton of nuclear detonation, and 80% of that is x-rays. Putting it all together:

F_x = joules / surfaceArea
F_x = (Y * 4.19e12 * 0.8) / (4 * π * R²)
F_x = 2.67e11 * (Y / R²)

As a general rule, figure an average person has a cross section of about 0.445 m² and a mass of 68 kilos. How was the cross section calculated? Given a mass of 68 kilos (150 pounds), and a height of 168 centimeters (5 feet, 6 inches) the Body Surface Area Calculator says the surface area is 1.78 square meters. The average cross section will be approximately one quarter of the surface area, or 0.445 m².

To obtain Grays, we take the radiation flux, multiply it by the cross section of the person (0.445), and divide by their mass (68).

G_x = ((2.67e11 * 0.445) / 68) * (Y / R²)
G_x = 1.78e9 * (Y / R²)

Figuring neutron radiation is a bit more involved. Each kiloton requires the fissioning of approximately 1.45e23 nuclei. Each fission produces 2 or 3 neutrons, with an average production of 2.5. About half (0.5) will escape the nuclear reaction to become radiation. The neutron flux is therefore:

F_n = (Y * 1.45e23 * 2.5 * 0.5) / (4 * π * R²)
F_n = 1.8e23 * (Y / R²)

Neutrons have a Sievert quality factor ranging from 2 to 20 for neutrons of energies 0.01 MeV to 2.0 MeV, with an average quality factor of 10. So according to the references I've found, in the absence of specific data, you can assume that a neutron flux of 2.5e11 neutrons per square meter is about 0.01 Sievert or 0.001 Grays. This means 1 Gray equals a neutron flux of 2.5e14 neutrons per square meter. So simply divide F_n by 2.5e14 to get Grays:

F_n = (1.8e23 * (Y / R²)) / 2.5e14
F_n = 7.2e8 * (Y / R²)

Exposure

Quality Factors
Type of radiation	Quality factor Q
X-rays	1
Gamma rays and bremsstrahlung	1
Beta particles, electrons, 1.0 MeV	1
Beta particles, 1.0 MeV	1
Neutrons, thermal energy	2.8
Neutrons, 0.0001 MeV	2.2
Neutrons, 0.005 MeV	2.4
Neutrons, 0.02 MeV	5
Neutrons, 0.5 MeV	10.2
Neutrons, 1.0 MeV	10.5
Neutrons, 10.0 MeV	6.4
Protons, greater than 100 MeV	1-2
Protons, 1.0 MeV	8.5
Protons, 0.1 MeV	10
Alpha particles (helium nuclei), 5 MeV	15
Alpha particles, 1 MeV	20
Galactic Cosmic Rays	1 to 30

There are two kinds of radiation exposure: acute and chronic.

The important point is that acute radiation damage will heal. Chronic damage does not. Acute damage goes away with time, chronic damage gradually accumulates over a lifetime. On the other hand, acute exposure can cause death by radiation burns a few weeks after the exposure. Chronic never causes death from radiation burns, instead it might kill you with cancer decades after the exposure.

Acute is a sudden dose that occurs over a few seconds to minutes. Chronic is a dose that occurs over a few days to years. Acute radiation syndrome is damage due to raw energy which burns internal organs (the technical term for such direct tissue damage is "nonstochastic effects" or "deterministic effects"). Chronic radiation syndrome is damage to the cellular DNA, leading to cancer and genetic defects in the victims future offspring (the technical is, surprise-surprise, "stochastic effects"). Chronic doses also cause skin ulceration and blindness due to cataracts scarring.

For our purposes, acute doses happen due to reactor accidents, solar storms, nearby nuclear explosions, and hits by particle beam weapons. Chronic doses are due to the unavoidable radiation filtering through the reactor shielding, the normal background radiation in space, and prolonged stays in regions like the Van Allen radiation belts.

There is no nonstochastic effects from chronic radiation dosage because the body has time to repair the damage while the dose is absorbed. There is no time to repair with an acute dose. For example, if a person suffers a (chronic) dose of 4.5 Sieverts over one year, they will have a much higher chance of cancer for the rest of their life. If they suffer an acute dose of 4.5 Grays in a second, they have an LD₅₀ dose and have a 50-50 chance of being dead within 6 weeks due to radiation burns.

Deterministic effects mean that X amount of acute radiation exposure will cause Y amount of tissue damage. Stochastic effects mean that X amount of chronic radiation exposure will raise your chance of getting cancer by Y percent.

Remember acute exposure is measured in Grays, chronic exposure is measured in Sieverts.

For acute doses, simply figure the exposure in Grays suffered by the crewperson, and refer to the Acute radiation syndrome chart below. When a person receives an acute dose, they suffer what is listed under "Immediate symptoms." Then they appear to get better, but this is only temporary. After the Latent phase time passes, the person will start to suffer what is listed under "Post-latent symptoms".

As a side note, when NASA is sterilizing containers of meat for astronaut supplies, the recommended dosage is 44,000 Grays to kill off all the bacteria. You can find dosages for other food sterilization here.

To figure chronic doses, one has to calculate the Dose Equivalent. Split the radiation into the x-ray/gamma-ray Grays and the neutron Grays. Multiply the Grays by the radiation type's Quality Factor to get the Equivalent Dose (in units called "Sieverts"). Gamma-rays have a quality factor of 1.0, neutrons have a quality factor ranging from 2 to 20 for neutrons of energies 0.01 MeV to 2.0 MeV (just use the average of 10). Add the two Sievert doses together and look it up in the Chronic radiation syndrome chart.

Doctors go further, calculating the Effective dose, which is a weighted average of the equivalent dose to each organ depending upon its radiosensitivity. But that's probably a bit too much detail for our purposes. Rocketeers will always keep close watch on their radiation dosimeters that measures their current chronic dose. There will also be a crewperson or officer who is assigned the job of logging and monitoring the chronic dose of every person on board. If a crewperson gets close to their maximum allowable dose, they may be restricted to the more shielded sections of the ship, or even grounded from shipboard duty until they recover.

Atomic rocketeers will also without fail have a package of potassium iodide tablets on their persons at all times. Why? If the reactor core is breached, the mildly radioactive fuel and the intensely radioactive fission fragments will be released into the atmosphere. While none of the fission fragment elements are particularly healthy, Iodine-131 is particularly nasty. This is because one's thyroid gland does its level best to soak up iodine, radioactive or not. Thyroid cancer or a hoarse voice from thyroid surgery might be common among atomic rocket old-timers. The tablets prevent this by filling up the thyroid first, before the Iodine-131 arrives. The instant the reactor breach alarm sounds, whip out your potassium iodide tablets and swallow one.

Radioactive contamination of an area is measured by a "swipe" or "smear" survey. A small piece of absorbent paper is rubbed over a 100 square centimeter area in a S-shaped pattern. A radiation detector then is used on the paper to measure the disintegrations per minute (dpm). Depending on the standard used, an area is considered "contaminated" if the dpm is above 100 - 500.

Note, in your research you may run across the terms "rem" and "rad." These are sort of obsolete terms. One Sievert equals 100 rems. One Gray equals 100 rads. LD₅₀ is the radiation dose that is expected to kill 50% of an exposed population.

Acute Radiation Syndrome Chart

Dose (Grays)	Immediate symptoms	Latent phase	Post-latent symptoms	Prognosis
0 - 0.5	No obvious effect	None	No obvious effect, except, possibly, minor blood changes and anorexia.	Certain survival
0.5 - 1.0	Vomiting and nausea for about 1 day in 10 to 20% of exposed personnel. Fatigue, but no serious disability.	days to weeks	In this dose range no obvious sickness occurs. Detectable changes in blood cells begin to occur at 0.25 Gy, but occur consistently only above 0.50 Gy. These changes involve fluctuations in the overall white blood cell count (with drops in lymphocytes), drops in platelet counts, and less severe drops in red blood cell counts. These changes set in over a period of days and may require months to disappear. They are detectable only by lab tests. At 0.50 Gy atrophy of lymph glands becomes noticeable. Impairment to the immune system could increase the susceptibility to disease. Depression of sperm production becomes noticeable at 0.20 Gy, an exposure of 0.80 Gy has a 50% chance of causing temporary sterility in males. At 0.75 Gy there is a 10% chance of nausea.	Almost certain survival
1.0 - 2.0	Mild acute symptoms occur in this range. Symptoms begin to appear at 1 Gy, and become common at 2 Gy. Typical effects are mild to moderate nausea (50% probability at 2 Gy) , with occasional vomiting, setting in within 3-6 hours after exposure, and lasting several hours to a day. This will be followed by other symptoms of radiation sickness in up to 50% of personnel.	10 - 14 days	Tissues primarily affected are the hematopoietic (blood forming) tissues, sperm forming tissues are also vulnerable. Blood changes set in and increase steadily during the latency period as blood cells die naturally and are not replaced. A reduction of approximately 50% in lymphocytes and neutrophils will occur. There is a 10% chance of temporary hair loss. Mild clinical symptoms return in 10-14 days. These symptoms include loss of appetite (50% probability at 1.5 Gy), malaise, and fatigue (50% probability at 2 Gy), and last up to 4 weeks. Recovery from other injuries is impaired and there is enhanced risk of infection. Temporary male sterility is universal. The higher the dosage in this range, the more likely the effects, the faster symptoms appear, the shorter the latency period, and the longer the duration of illness.	Fatality rate is about 10%
2.0 - 3.5	Nausea becomes universal, the incidence of vomiting reaches 50% at 2.8 Gy and 100% at 3 Gy. Nausea and possible vomiting starting 1 to 6 hours after irradiation and lasting up to 2 days. This will be followed by other symptoms of radiation sickness, e.g., loss of appetite, diarrhea, minor hemorrhage	7 - 14 days	Illness becomes increasingly severe, and significant mortality sets in. Hematopoietic tissues are still the major affected organ system. When symptoms recur, the may include epilation (hair loss, 50% probability at 3 Gy), malaise, fatigue, diarrhea (50% prob. at 3.5 Gy), and hemorrhage (uncontrolled bleeding) of the mouth, subcutaneous tissue and kidney (50% prob. at 4 Gy). Suppression of white blood cells is severe, susceptibility to infection becomes serious. At 3 Gy the mortality rate without medical treatment becomes substantial (about 10%). The possibility of permanent sterility in females begins to appear. Recovery takes 1 to 3 months.	Fatality rate 35% to 40%
3.5 - 5.5	Nausea and vomiting within half an hour, lasting up to 2 days. This will be followed by other symptoms of radiation sickness, e.g., fever, hemorrhage, diarrhea, emaciation.	7 - 14 days	Hair loss, internal bleeding, severe bone marrow damage with high risk of bleeding and infection. Hemopoietic Syndrome. Mortality rises steeply in this dose range, from around 50% at 4.5 Gy (LD₅₀) to 90% at 6 Gy (unless heroic medical intervention takes place). Hematopoietic tissues remain the major affected organ system. The symptoms listed for 2.0-3.5 Gy increase in prevalence and severity, reaching 100% occurrence at 6 Gy. When death occurs, it is usually 2-12 weeks after exposure and results from infection and hemorrhage. Recovery takes several months to a year, blood cell counts may take even longer to return to normal. Female sterility becomes probable. Survivors convalescent for about 6 months.	Fatality rate 50% within 6 weeks
5.5 - 7.5	Severe nausea and vomiting within 15 - 30 minutes, lasting up to 2 days, followed by severe symptoms of radiation sickness, as above.	5 - 10 days	Hair loss, internal bleeding, severe bone marrow damage leading to complete failure of blood system, high risk of infection, moderate gastrointestinal damage. Gastrointestinal Syndrome. Survival depends on stringent medical intervention. Bone marrow is nearly or completely destroyed, requiring marrow transfusions. Gastrointestinal tissues are increasingly affected. The final phase lasts 1 to 4 weeks, ending in death from infection and internal bleeding. Recovery, if it occurs, takes years and may never be complete. Survivors convalescent for about 6 months.	Death probable within 3 weeks
7.5 - 10	Excruciating nausea and vomiting within 5 - 15 minutes, lasting for several days	5 - 7 days	Hair loss, internal bleeding, severe bone marrow damage leading to complete failure of blood system, high risk of infection, severe gastrointestinal damage.	Death almost certain within 3 weeks. Complete recovery impossible.
10 - 20	Immediate nausea occurs due to direct activation of the chemoreceptive nausea center in the brain. The onset time 5 minutes.	5 - 7 days	Very high exposures can sufficient metabolic disruption to cause immediate symptoms. Above 10 Gy rapid cell death in the gastrointestinal system causes severe diarrhea, intestinal bleeding, and loss of fluids, and disturbance of electrolyte balance. These effects can cause death within hours of onset from circulatory collapse. Following an initial bout of severe nausea and weakness, a period of apparent well-being lasting a few hours to a few days may follow (called the "walking ghost"* phase)*. This is followed by the terminal phase which lasts 5 - 12 days. In rapid succession prostration, diarrhea, anorexia, and fever follow. Death is certain, often preceded by delirium and coma. Therapy is only to relieve suffering.	Certain death in one week or less.
20 - 80	Immediate disorientation and coma will result, onset is within seconds to minutes.	None	CNS Syndrome. Metabolic disruption is severe enough to interfere with the nervous system. Convulsions occur which may be controlled with sedation. Victim may linger for up to 48 hours before dying.	Certain death
> 80	Coma	None	The U.S. military assumes that 80 Gy of fast neutron radiation (from a neutron bomb) will immediately and permanently incapacitate a soldier. Lethal within 24 hours due to damage to central nervous system.	Certain death

Relationship Between Mean Survival Time and Acute Radiation Dose. Note that the chart uses Rems. This data holds only for acute total body radiation.

Chronic Radiation Syndrome Chart

Criteria	General Public	Occupational Workers	Astronauts
30-day limit	0.0004 Sieverts (0.4 milli-Sieverts)	0.004 Sieverts	1.5 Sieverts
annual limit	Adult: 0.05 Sieverts minor: 0.005 Sieverts Colonist: 0.02 Sieverts Pregnant colonist: 0.0066 Gray	0.05 Sieverts	3 Sieverts
Male career limit	N/A	2 + 0.075 x (age - 30) Sieverts	4 Sieverts
Female career limit	N/A	2 + 0.075 x (age - 38) Sieverts	4 Sieverts
Pregnancy limit (9 months)	0.005 Sieverts	N/A	N/A
accident limit	0.25 Sieverts	1 Sievert	N/A
acute limit	N/A	N/A	0.1 Sieverts

One study suggested for a pregnant woman it is 0.005 Sievert total for the duration of the pregnancy. Another later study suggested an annual limit of 0.0066 Grays, using Grays instead of Sieverts because nobody knew the Q factor for embryonic tissue and obviously nobody was about to try experimentation to find the answer.

Colorful Chronic Terms

Writer Allen Steele uses the following terms:

Singed: receiving a radiation dose that put one close to a chronic limit
Cooked: receiving a radiation dose that put one over a chronic limit, could be career-ending
Fried: receiving a radiation dose that put over the lethal LD₅₀ limit, could be life-ending

From **THE WEIGHT** by Allen Steele (1995)

Figure 1 – SFE Events During Apollo, from the NASA Document’s Figure 10

link for sharing — Figure 1 – SFE Events During Apollo, from the NASA Document’s Figure 10

Banana Equivalent Dose

Image from Watts Up With That?

An amusing unit of radiation exposure is the Banana equivalent dose. It provides some perspective, and can be used to calm down scientifically illiterate people who go hysterical when they hear the "R" word.

As it turns out, ordinary bananas are very slightly radioactive due to their potassium-40 content. Under this scale eating one banana exposes you to 0.1 μSv or 0.0000001 Sievert.

Living within 50 miles of a nuclear power plant for one year will give you a dose of half a banana. Living within 50 miles of a coal power plant for one year will give you a dose of three bananas (a pound of coal contains only small traces of radioactive elements, but such plants typically burn 4 million tons of coal every year).

Living in a stone, brick, or concrete building for a year will expose you to a dose of 700 bananas. The average dose from the Three Mile Island accident to someone living within 10 miles is 800 bananas. One mammogram is 30,000 bananas. A chest CT scan is 58,000 bananas.

If you spend one hour at the site of Chernobyl nuclear disaster in the year 2010 you will receive a dose of 60,000 bananas.

Radiation dose chart from XKCD. Click for larger image

From **SOMETIMES YOU NEED A NEW WORD** by Andrew Clough *(2019)*

Half-life

Number of half-lives elapsed	Fraction remaining	Percentage remaining
0	¹/₁	100
1	¹/₂	50
2	¹/₄	25
3	¹/₈	12.5
4	¹/₁₆	6.25
5	¹/₃₂	3.125
6	¹/₆₄	1.563
7	¹/₁₂₈	0.781
...	...	...
16	¹/_65,536	1.5×10^-5
...	...	...
n	¹/_2ⁿ	100/(2ⁿ)

Half-life applies to many things, but in our area of interest, it determines how long it takes hideously radioactive elements to decay into safe non-radioactive elements. Scientists use half-life instead of full-life because [1] we want to know the rate and [2] full-life is typically freaking huge.

Half-life is also useful for figuring out how long you'll get useful power out of an RTG.

So, say there is a slug of Strontium-90 that is emitting about 10 sieverts of radiation. We will say that "safe" means radiation at a level about equal to background radiation emitted by the ground, about 0.21 millisieverts (0.00021 sieverts). How long will it take to for the strontium-90 to decay to a safe level?

Assume that strontium-90 decays directly into a non-radioactive isotope (it doesn't, but let's not complicate things). Assume that if the amount of strontium-90 is reduced by half due to decay, the amount of radiation will also be reduced by half. The fraction of an element undecayed after n half-lives is 1/2ⁿ.

After playing around with numbers, I found that 16 half lives will have an undecayed fraction of 1/2¹⁶ = 1/65,536 = 0.000015. This means 10 sieverts will become 0.00015 sieverts (below 0.00021 sv background) after 16 half lives.

Strontium-90 has a half life of 28.8 years. Sixteen half-lives is 16 * 28.8 = 460.8 years.

Now, in reality you'd have to figure the radioactive decay products, figure their radiation level, and figure their decay time.

An atom of a radioactive element decays by emitting radiation. So as a general rule, the shorter an isotope's half-life, the more intensely radioactive it is. Specifically, the activity of a lump of isotope (in Becquerels) is:

A_bq = N * (ln[2] / t_½)

where:

A_bq = (radio)activity (Becquerels)
N = number of atoms in the lump of isotope
ln[2] = Natural logarithm of 2, about 0.69315...
t_½ = half-life of the isotope (seconds)

Isotope	Half-life
Uranium 235	7.13×10⁸ years
Uranium 233	160,000 years
Plutonium 239	24,100 years
Curium 245	8,500 years
Plutonium 238	87.7 years
Hydrogen 3 (Tritium)	12.32 years
Polonium 210	138.376 days

The question arises "how many atoms are in a gram?" The answer was told to you in chemistry class, when your eyes glazed over as the professor talked about "molar mass" and the "Avogadro constant". Avogadro constant is about 6.02214179×10²³ mol^-1. This means if you made a pile of 6.02214179×10²³ Uranium-235 atoms it would weigh exactly 235 grams. A pile of that number (one "mole") of Plutonium-239 would weigh exactly 239 grams.

The point is, you can use this to convert between atomic mass units and grams. Basically you divide Avogadro constant by the atomic mass of the element to find the number of atoms of that element in one gram. So Strontium-90 contains 6.02214179×10²³ / 90 = about 6.69126865×10²¹ atoms per gram.

The radiation flux depends upon the energy per atomic decay. These are generally listed in terms of MeV or megaelectron volts. 1 MeV = 1.6021773×10^-13 joules. For instance, strontium-90 undergoes beta-decay, at 0.546 MeV per decay. Multiply this by the Becquerels to get the total radiation flux in joules.

The total flux can then be used to calculate the dosage inflicted on anybody unfortunate enough to be exposed to the lump.

Decontamination

From **SEVENEVES** by Neal Stephenson *(2015)*

Note that Doc Smith to the contrary, chelating decontamination doesn't quite work this way.

Neutron Activation

While human beings and other living things suffer harm from nuclear radiation, inanimate objects are not fond of it either. Charged particle radiation can be deflected from your ship by magnetic fields, but vexing neutron radiation is uncharged. It hits whatever is in the way, you cannot divert it.

This is why the magnetic nozzles of fusion engines are an open lattice full of holes: to reduce the surface area of nozzle that can be hit by neutrons. Meaning if from the viewpoint at the center of the reaction the sky is 10% lattice and 90% holes, only 10% of the neutron radiation is damaging the engine.

Wonder Neutron helps kill strong bodies 3 ways: Neutron Activation, Neutron Embrittlement, and Neutron Heating.

Neutron Activation happens when an ordinary harmless atomic nucleus swallows a low-energy neutron from the radiation flux of, say, a nuclear thermal propulsion system. This changes the nucleus from a stable isotope into an unstable isotope, and all unstable isotopes are radioactive and emit gamma-rays. Or the neutron can actually split the atom, with much the same results but now including bonus fission fragments and neutrons in the induced radioactivity.

Translation: the steel girders and everything else too close to the reactor will eventually become radioactive in and of themselves, glowing with dangerous gamma-rays. Your nuclear engine structure will gradually be transformed into low-level radioactive waste. This makes it treacherous for the crew to leave the spacecraft, and drastically lowers the re-sale value. This is a good reason to make your spacecraft modular, so you can detach the nuclear engine and swap it for one that doesn't glow in the dark.

Low-activation elements are chemical elements that are resistant to neutron activation. These include tungsten, tantalum, vanadium, and chromium. Use these to construct nuclear reactors and nuclear rocket engines.

High-activation elements can easily be transmuted into glowing radioactive chunks of death by trival amounts of neutrons. These include nickel, copper, aluminium, molybdenum, cobalt and niobium. As a matter of fact, cobalt has been suggested as a component of a salted bomb. Avoid these when making your nuclear engines.

There is a nice list of common elements that can be transmuted into radioactive isotopes here, along with the half-life of said radionuclides.

Neutron activation is a good thing in a breeder reactor or a medical isotope generator, but very bad anywhere else. Heavy neutron radiation is usually never found naturally, it is found unnaturally in nuclear reactors, nuclear explosions and other artificial things made by intelligent creatures.

Neutron activation analysis can be used to determine how much neutron radiation a hapless victim was exposed to. The doctor can examine the victim to determine how much of the body's sodium was neutron activated into sodium-24 and how much phosphorus was activated into phosphorus-31. This will provide an estimate of the acute radiation dosage.

From **NUCLEAR SHUTTLE SYSTEM DEFINITION STUDY PHASE III VOLUME-8**
by Lockheed Missiles & Space Co. *(1971)*

Times after Final Irradiation	50 hr	100 hr	200 hr	500 hr
At Station 46	1600.0 mTRad/hr	151.0 mTRad/hr	25.5 mTRad/hr	17.0 mTRad/hr
At 20 feet below Station 46	60.6 mTRad/hr	5.53 mTRad/hr	0.86 mTRad/hr	0.605 mTRad/hr

Times after Irradiation	50 hr	500 hr
1st/13th	0.223	0.049
9th/13th	0.225	0.273
10th/13th	0.995	0.550

**Figure 3.7-1 Radiation Environment After Shutdown**
*click for larger image*

From **CONCEPTUAL DESIGN OF IN-SPACE VEHICLES FOR HUMAN EXPLORATION OF THE OUTER PLANETS** (2003)

Neutron activation from a Kiwi solid core nuclear thermal rocket
from Radiation Effects on a Nuclear Rocket Engine Systems (1961)

Salted Bombs

Neutron activation is used on purpose in nasty salted bombs. The bomb is intentionally cased in cobalt or other element that is particularly good at being neutron activated into a hideously radioactive isotope. This provides enhanced quantities of radioactive fallout. "Salted", as in "sowing the earth with salt so nothing ever grows there again." The salted bomb is conveniently almost non-radioactive while in storage, the radioactive isotopes are instantly manufactured at ground zero.

And let's not forget the "enhanced radiation bomb" aka "Neutron bomb". This is a nuclear warhead specially constructed so that less of the bomb energy goes into x-rays and more goes into neutrons. It was designed to do less blast damage to buildings and vehicles but do more radiation damage to people. But of course the extra neutron flux will naturally do more neutron activation to any material object near ground zero. I found mention that a main battle tank close to the detonation point would suffer enough neutron activation to render it lethally radioactive for about 48 hours.

Neutron Embrittlement

Nuclear power plant operating environments create material degradation mechanisms that may be unique or environmentally exacerbated.
In this figure, Irradiation-Assisted Stress Corrosion Cracking has resulted in cracking at the head of a baffle bolt.
source

Neutron radiation can make some materials become brittle by neutron-induced swelling and buildup of Wigner energy. High-energy neutrons striking metal gradually damage the metal's crystal lattice. This makes the metal more brittle and can eventually lead to failure. Steel has a so-called "ductile-to-brittle transition temperature", a temperature below which it becomes brittle. Neutron bombardment raises this temperature.

The brittleness can be healed by heating the material, this is called annealing. It might be possible to construct a reactor capable of annealing its structural members in place instead of removing it first. But of course you have to be very careful. You will be in for some real excitement if you accidentally catch the nuclear engine on fire.

From **NEW REACTOR-LINER ALLOY MATERIAL OFFERS STRENGTH, RESILIENCE** *(2019)*

From **CARBON NANOTUBES IMPROVE METAL’S LONGEVITY UNDER RADIATION** by David L. Chandler (2016)

Radiation Heating

Very-high energy neutrons not only cause neutron embrittlement, they also impart thermal energy to the nucleus they hit. Meaning it heats things up. Gamma rays heat things up as well. For the most part, fast neutrons tend to heat up things like liquid hydrogen, while gamma rays heat up engine components.

So here's the score:

Very-high-energy neutrons: thermal heating of engine
High-energy neutrons: embrittlement of engine
Medium-energy neutrons: SAFE ZONE
Low-energy neutrons: activation of engine (becomes radioactive)

Neutrons with energies in the medium-energy zone have no harmful effect on the engine, the metals composing the engine are transparent to the neutrons. Those neutrons have too much energy to be absorbed by nuclei and cause neutron activation, but not enough energy to interact with nuclei to cause neutron embrittlement or thermal heating.

It would be wonderful if researchers could invent a shield that would somehow cause the neutrons to enter the safe zone (the technical term is "neutron moderation"). But that would be tricky. And I am unaware of any way to speed up low-energy neutrons, moderators in nuclear reactors can only slow neutrons down.

Electronics

Semiconductor electronics are also vulnerable to radiation (including particle beam weapons) due to mechanical disruption, as you can see from Anthony Jackson and Christopher Thrash (below).

However while the vacuum tubes (thermionic valves) in the Russian MiG fighters would not provide much protection from nuclear EMP, vacuum tubes are inherently more radiation resistant compared to semiconductors.

From **AN OVERVIEW OF THE EFFECTS OF NUCLEAR WEAPONS ON COMMUNICATIONS CAPABILITIES** Signal. Vol 34, No 4, January 1980, p. 60

From **RETURN OF THE VACUUM TUBE** by Jon Cartwright *(2012)*

From **RETURN OF THE VACUUM TUBE** *(2012)*

From **VACUUM CHANNEL TRANSISTOR COMBINES BEST OF SEMICONDUCTORS AND VACUUM TUBES**
by Lisa Zyga *(2017)*

From the Wikipedia entry for **NANOSCALE VACUUM-CHANNEL TRANSISTOR**

Radiation Shielding

The three anti-radiation protection methods are Time, Distance, and Shielding. Time means minimize the duration of exposure, Distance means get as far away from the radiation source as you can, and Shielding means get some radiation armor between you and the radiation source.

Remember, outside the engine room hatch will be a decontamination booth. And I'm sure over the hatch will be mounted an alarm with a red rotating light, so you don't have to put your ear on the bulkhead to hear Astro say "Oh SH*****T!!!". Past the hatch will be a radiation-shielded corridor, with a dog-leg bend in it, so you can get in but radiation cannot get out (radiation has to travel in straight lines, but crewmen can zig-zag). Be sure you are wearing your dosimeter.

Time

Shortening the duration of exposure is difficult to do. A nuclear engine burn for a specific amount of delta V takes as long as it does, it cannot be shortened. You cannot stop a solar proton storm in progress to shorten exposure. About the only thing you can realistically do is make sure you perform any vital repairs in a radioactive zone as quickly as possible, and if you unexpected discover something radioactive nearby you should run away.

Difficult as it is, NASA scientists were looking into duration shortening strategies for a proposed Mars mission. Using a minimum delta V / maximum duration Hohmann trajectory the trip from Terra to Mars will take the better part of eight months. The Mars Science Laboratory actually traveled the route and measured the cosmic ray radiation exposure with the RAD. The scientists were aghast when they found that the round trip dose was much higher than they estimated, about 0.66 sieverts round trip (about 1.8 milliSieverts per day). Which is bad news if the career radiation limit for astronauts is 1.0 sieverts.

They tried to design radiation shielding that would reduce the cosmic ray exposure to something reasonable. Unfortunately this cut so deeply into the payload mass that there basically wasn't a mission any more.

But now the are focusing on reducing the time of radiation exposure. How? By developing more powerful propulsion systems like VASIMR. If such propulsion can increase the spacecraft's delta V capacity enough, it can afford a trajectory with trip duration reduced. Which would shorten the duration of cosmic ray radiation exposure.

Distance

There was some 1950's era spacecraft designs that attempted to substitute distance for lead shielding (since distance weighs nothing) thus utilizing the inverse-square law. They were practical designs for exploration craft but pretty silly for warships. Crew cabins on the end of hundred meter booms, or dangling at the end of kilometer long cables, that sort of thing. While you wouldn't want to use the designs, you understand the motivation. When every gram is limited, you don't want to waste it on a shield made out of one of the heaviest elements in existence.

The break-even point is where the mass of the boom or cable is equal to the mass of the shadow shield. Meaning that distance doesn't exactly weigh nothing, but you get one level of radiation reduction with far fewer kilograms than you'd need if using a heavy radiation shield. One source suggested that occurred at a cable length of one kilometer with a one megawatt reactor (reduction factor of about 10^-6).

From **TROPE-A-DAY: MILE-LONG STARSHIP** by Alistair Young (2015)

Cone at left is nuclear power reactor, on a telescoping boom
Space Station design by NASA and Marshall (1962)
click for larger image
click for larger image

Strombecker model #D-38 designed by Krafft Ehricke, 1957
Strombecker model #D-38 designed by Krafft Ehricke, 1957
Artwork by Mel Hunter
Discovery from 2001 A Space Odyssey
Pilgrim Observer
HELIOS, Revell model #H-1829 designed by Krafft Ehricke, 1959

Nuclear electric rocket desgined by Dr. Gail B. Shukom of Lockheed (1959).
Reactor is dragged on a mile-long cable as "distance type" radiation shielding. Ion engine uses cesium propellant.
click for larger image
Reactor uses thermionic cells to convert 2,100°C heat into 1,000 kW_e at 100 volts. Reactor has a mass of 34 tons since it has no weighty shadow shield.
1965 Panama postage stamp
Remolcando su unidad nuclear para protegerse contra las radiaciones
Towing its nuclear unit to protect against radiation

Reach For Tomorrow by Sir Arthur C. Clarke, 1956. This is a classic "dumb-bell" shape. Both the life-system and nuclear drive sections are spherical in order to minimize surface area. The rod in the middle is to increase the distance between the crew and the reactor. This is a strictly orbit-to-orbit ship, which cannot land on a planet
The Exploration of Space by Sir Arthur C. Clarke, 1951. This is a semi "dumb-bell" shape. The life-system is the sphere in front, while the nuclear drive is shielded by the propellant tanks. Note the outriggers containing the radar dishes, to give them a better field of view. This is also a strictly orbit-to-orbit ship.

Flyaway Engine

Lockheed Resuable Nuclear Shuttle class 3
click for larger image

This crazy/brilliant concept was described in Lockheed's study of the Class 3 Resuable Nuclear Shuttle. Sources are Nuclear Shuttle Systems Definition Study Phase III Volume 2 part A, and Volume 2 part B.

The Class 1 reusable nuclear shuttle was designed to be fully assembled on Terra, then launched into orbit on a huge booster rocket. The Class 3, however, was to be boosted piecemeal and assembled in orbit. So every component was prefereably sized to fit in the Space Shuttle cargo bay.

The Problem

Due to neutron activation, each time you run the engine reactor it transmutes more and more of the engine structure into radioactive isotopes. This will make the engine emit harmful gamma rays even when it is totally turned off. Which makes the nuclear shuttle difficult to refurbish. At least without killing the maintenance crew.

For the grim details, go here.

The Solution

Some engineer started thinking outside of the box, and came up with a solution so crazy it just might work.

The class 3 nuclear shuttle has to be assembled in orbit, right? So they designed a special little drone thingy that latches onto the nose of the nuclear engine. The drone acts like a micro-tug, using little attitude jets to drag the engine into its place on the aft end of the shuttle.

However, it is impractical for the drone to get out of the way, being in between the aft of the shuttle and the fore of the engine. It is providing all the careful course changes to ensure the engine attaches firmly. It is easier if the drone stays attached to the fore of the engine, and docks to the aft of the shuttle. All the propellant pipes pass through the body of the drone. It will become an integral part of the nuclear shuttle. A pity that at this point it is pretty much worthless penalty mass.

The engineer got a pondering look in their eyes, and figured out how to make a liability into an asset.

Nuclear shuttle performs a mission. After the mission it returns to its parking orbit, with the engine dangerously radioactive.

Here's the clever part. The drone then detaches itself and the engine from the aft of the shuttle. It then moves the engine a few hundred kilometers away from the shuttle. Now the maintenance crew can safely refurbish the shuttle, with the radioactive engine far enough away to poise no danger.

When it is time for the shuttle to perform a new mission, the drone reattaches the engine. Brilliant, eh?

From **NUCLEAR SHUTTLE SYSTEMS DEFINITION STUDY PHASE III VOLUME-2 PART-A**
by Lockheed Missiles & Space Co.(1971)

**Fig. 2-8: Engine Flyaway Structure**
*click for larger image*

Shielding

Remember the basic strategy. Use dense elements like lead, tungsten, and beryllium for x-ray and gamma-ray shielding. Use low-density elements like liquid hydrogen, dehydrated astronaut poo, lithium hydride, paraffin, hydrogenated polyethylene composite, or other hydrogen-rich compounds for particle radiation shielding.

Why? X-rays and gamma-rays are stopped by electrons, and high density elements have more electrons per cubic centimeter. Particle radiation is stopped by atomic nuclei and low density elements have more atomic nuclei per cubic centimeter than metals.

When shielding against neutron particle radiation, instead of using hydrogen compounds it is better to use neutron reflectors such as graphite, beryllium, steel, and tungsten carbide.

Beware of bremsstrahlung. If you place your shielding improperly you'll convert the inside of the spacecraft into a giant x-ray machine and fry your astronauts to death. To avoid this make sure the outermost shield layer (next to deep space or other radiation source) is particle shielding and the innermost shield layer (next to the astronauts) is the x-ray/gamma-ray shield layer. As a side note, only charged particles like protons and electrons cause bremsstrahlung, neutrons do not.

Why? Gamma-ray shielding is worse than useless against particle radiation. Charged particles hitting dense elements is the same operating mechanism used inside a dentist's x-ray machine. So if you have the gamma-ray shielding outermost, this is what happens:

A charged particle from deep space slams into the x-ray/gamma-ray shielding
This generates deadly x-rays which are emitted by the x-ray shielding
The new x-rays find there is no x-ray shielding in front of them, only pathetic particle shielding
The x-rays sail unharmed through the particle shielding, and kill the astronauts

When you place your shielding the right way, the particle shielding soaks up the particle radiation before it can hit the gamma-ray shielding.

In an attempt to avoid the penalty mass of particle shields, researchers are looking into diverting particle radiation using zero-mass bubbles of plasma or powerful magnetic or electrostatic fields.

Philip Eklund points out that in spacecraft combat an Orion drive rocket has built-in radiation armor. But it only works if you can keep the pusher plate aimed at the nuclear warhead. If you can manage that, you can laugh at most nuclear detonations.

On the other hand, there are certain propulsion systems that undergo catastrophic failure (i.e., they blow up) if minor damage happens to the fuel tanks. These include antimatter rockets, Zubrin's NSWR, and any form of metastable fuel.

Shield Rating

Radiation shielding is rated in "Tenth Value Thickness" or TVT. One TVT is the depth of shielding required to reduce the radiation to one tenth of its initial value (i.e., it stops 90% of the radiation). Twice the TVT will reduce the radiation to one one-hundredth of its initial value (stops 99%), and so on.

Sometimes one will encounter "Half Value Thickness" and "1/e". HVT is the depth required to reduce the radiation by one-half, and 1/e is the depth required to reduce the radiation to approximately 37% (specifically to 1/e where e is approximately 2.718).

Water has a TVT of 25.4 centimeters vs particle radiation (including neutrons), but only 61 centimeters vs gamma rays. Lead has a TVT of 5 cm vs gamma, and basically doesn't do diddly-squat vs particle radiation. Steel has a TVT of 11 cm vs gamma and also does poorly vs particle radiation. By my calculations carbon should have a TVT of 22.5 cm vs gamma rays, but I have no idea what its TVT is vs neutrons.

X-ray and gamma-ray shielding boils down to how much mass is between the radiation source and the crew. 45 g/cm² is a TVT (meaning that behind each square centimeter of shield surface area is 45 grams worth of shield material of a thickness determined by the material's density). As a wild guess, the interior of the spacecraft has a density of about 0.25 grams per cubic centimeter. This means a crew member would get a "free" 1 TVT from X and gamma-rays if they were 1.8 meters from the hull, from the shielding provided by the bulkheads, machinery, pipes, and structural materials (45 g/cm² / 0.25 g/cm³ = 180 cm). Keep in mind that 1 TVT isn't all that much, and the free shielding obviously goes up the further from the hull the crew is. This is an argument for putting the control room of a combat spacecraft near the center.

Cosmic rays will need a TVT of about 450 g/cm². You will need 450 g/cm² to get OSHA-legal exposure limits on a timescale of years, say for a space colony or long duration space mission.

Extremely high energy particle beam weapons act like cosmic rays, with a TVT peaking at a whopping 100 to 300 g/cm².

If you have a thickness which stops a known amount of radiation of a known and constant type, then if you have a new thickness, you can calculate how much it stops by:

Stoppage = 1 - ((1 - AmountStoppedByKnownThickness)^(NewThickness / KnownThickness))

Example: if six centimeters of polka-dotted Kryptonite will stop 90% of x-rays, then eighteen centimeters of polka-dotted Kryptonite will stop:

Stoppage = 1 - ((1 - AmountStoppedByKnownThickness)^(NewThickness / KnownThickness))
Stoppage = 1 - ((1 - 0.9)^(18 / 6))
Stoppage = 1 - (0.1^3)
Stoppage = 1 - 0.001
Stoppage = 0.999 = 99.9%

Just to make our lives more difficult, mixed radiation such as is found in space has verying penetration. So if shield material X stops 90% of the quote "radiation" unqote, this will mean something like stopping 99% of the low-penetrating radiation and 50% of the high-penetrating radiation. And doubling the thickness of the shielding might only bring the radiation stoppage up to aroung 96%.

For comparison purposes, a typical NASA space suit has 0.25 g/cm³, the hull of an Apollo command module is rated at 7 to 8 g/cm³, the Space Shuttle is rated at 10 to 11 g/cm³, the storm cellar of the International Space Station is rated at 15 g/cm³, and future lunar bases are planned to exceed 20 g/cm³.

Now to calculate the radiation that penetrates a shield:

R_d = R_h * V_f^{(A_d / V_d)}

where:

x^y = raise x to the power of y
R_d = Radiation dose that penetrates the armor (grays or whatever)
R_h = Radiation strength hitting the armor (grays or whatever)
V_f = Value Factor (0.1 for TVT, 0.5 for HVT, 0.37 for 1/e)
A_d = Armor depth (centimeters or whatever)
V_d = Value depth (e.g., 61 cm if armor is water and radiation is gamma rays)

Example (no doubt full of mistakes)

A one kiloton nuclear warhead from the Asteroid Revolutionary Navy goes off fifteen kilometer from the scoutship Peek-a-Boo. At fifteen klicks, the radiation flux will be about 1500 joules per square meter. 90% is x-rays and the rest is neutrons, or 1350 j/m² x-rays and 150 j/m² neutrons. The Peek-a-Boo has 5 centimeters of lead around the crew compartment, which is one TVT vs x-rays and zero TVT vs neutrons. Only 135 j/m² of the x-rays penetrate, but the entire 150 j/m² neutrons comes sailing on through.
Floyd is the only crewmember. He's about 68 kilos, and 168 centimeters tall, which gives him a surface area of about 1.78 square meters. Figure that Floyd's cross section is one quarter of his surface area, or 0.445 m². He will intercept 60 joules of x-rays and 67 joules of neutrons. Dividing by his mass we discover that Floyd has been exposed to 0.88 Grays of x-rays and 0.98 Grays of neutrons. This makes a grand total of 1.86 Grays. He will start upchucking his lunch in a few hours but he'll live.

Radiation flux around a NERVA nuclear rocket engine. The shadow shield does its best to protect the crew. Click for larger image. From the Unwanted Blog.
Radiation flux around a NERVA nuclear rocket engine. From NUCLEAR SPACE PROPULSION by Holmes F. Crouch

Gamma ray KERMA rates at full power
(RADS(tissue) per second)
Left: Neutron KERM rates at full power (RADS(tissue) per second)
Right: Fast Neutron flux at full power (neutrons/cm²-sec) energy > 0.9 MeV

Radiation isodose plot for 8 kilowatt thermoelectric
Left half is with empty hydrogen propellant tanks, right half is with full tanks. Basically the radiation dosage for the crew is negligible with full tanks and deadly without.
1 Rem equals 0.01 Sievert. The maximum mission dose was targeted at about 40 Rem (0.4 Sievert)
"#" means "pounds of shield material"

From **COLONIZE MARS - PART 2, SURVIVING THE TRIP** by Chris Wolfe (2016)

Shadow Shield

Shadow shields are specifically to protect the crew from radiation emitted by the spacecraft's nuclear engines and nuclear power plants.

The safe design would be to totally encase the engine and reactors in radiation shielding. But this sharply reduces the ship's payload since radiation shields literally weigh tons, and Every Gram Counts. Shadow shields are the bare minimum of shielding: it only stops the radiation heading for the habitat module and other vital parts of the spacecraft. The radiation freely sprays in all other directions, which makes it dangerous to approach an atomic rocket outside of the safe shadow cast by the shield.

Another implication is that the ship's docking port is probably best placed on the ship's nose. This will allow two ships to dock nose-to-nose, while keeping each other in the shadow of their shadow shields. Alistair Young calls this "Booping", a re-use of the word for "tapping your pet affectionately on the nose."

Note that the larger the distance between the crew and the atomic engine, the narrower the angle the shadow has to be, thus the smaller the shadow shield. Since the shadow shield is several tons of rocket mass that is not payload, the smaller it is, the better. Also note that radiation weakens with distance due to the inverse-square law, which is another argument in favor of plenty of distance. As previously mentioned, if the rocket has multiple atomic engines one wants them clustered closely or they will require a larger shadow shield, or even one shield for each engine. (for "cluster closely" read: "have the radioactive components as close to the axis of the spacecraft as possible")

I have seen a few Mars mission studies that try to use the shadow shield to protect against solar proton storms in lieu of a storm cellar. The idea is to point the engine at the Sun when a storm comes and hope it provides the same protection as the penalty-mass laden storm cellar which was omitted from the design. I do not have precise figures on how effective this strategy is, but the fact it is not a standard design feature in all Mars missions tells me the concept has problems.

Diagram adapted from Propulsion Systems for Space Flight by William R. Corliss, 1960
Shadow shield casts a shadow free from illumination by deadly radiation. Payload is also separated from the shadow shield to take advantage of distance shielding
A: Heat radiator extending into the radiation zone can backscatter radiation around the shadow shield and into the payload
B: Sodium-Potassium liquid-metal alloy (NaK) cools the reactor. But there it can become radioactive by neutron activation. It moves past the shadow shield on its way to the heat radiator, but can expose the payload to radiation
Diagram adapted from SNAP Nuclear Space Reactors by William R. Corliss (1969)

Figure II-65
**Shadow Shield Configuration**

EXPANDING THE SHADOW

Shield extension when the reactor is idling. From NUCLEAR SPACE PROPULSION by Holmes F. Crouch.
The extended shield provides a larger shadow. From NUCLEAR SPACE PROPULSION by Holmes F. Crouch.

When the reactor is idling, the shadow shield does not have to be as thick. In order to widen the area of shadow (for adding side tanks or whatever), the secondary shadow shield could extrude segments as extendable side shields.

Figure 2

Assessment of the Advantages and Feasibility of a Nuclear Rocket for a Manned Mars Mission by Dr. Steven D. Howe had a clever idea. It is a better way to widen the area of the shadow.

During a propulsive burn the neutron-stopping part of the shadow shield (Lithium Hydroxide LiH or something) and the gamma-ray-stopping part (Tungsten W or Mercury Hg) have to be on top of the engine, to shadow the rest of the spacecraft. After the burn, the reactor stops emitting neutrons and gamma rays. However, neutron activation has transmuted girders and other engine parts into radioactive isotopes. These will emit dangerous gamma-rays at a gradually decreasing rate. After a couple of days the gamma-ray flux will be reduced by about three orders of magnitude (1/1,000th). Which means a thinner shield can provide the same protection. A pity that the shield is welded in place.

In the report they say Not So Fast, there may be a way. What if you used liquid mercury instead of solid tungsten as the gamma shield? In the diagram above the mercury is in the upper tank during the burn. The thick layer can cope with burn-levels of gamma rays, at the price of only protecting a narrow shadow.

But after the burn, the neutron radiation stops and the gamma-rays are at a reduced level. The mercury is pumped into the lower tank. The mercury shield is a thinner layer which can cope with the reduced gamma-rays. The advantage is a thinner layer means there is more mercury to go around. The lower tank creates a mercury layer that covers more of the engine, expanding the size of the shadow. This could come in handy if the spacecraft is a tail-sitter.

From **Assessment of the Advantages and Feasibility of a Nuclear Rocket for a Manned Mars Mission** (1985)

RADIATION FLUX FROM ENGINE

So the theory is you calculate effective radiation flux from the atomic engine, multiply it by the appropriate attentuation factors of the shadow shield, and figure the effective dose given the total burn time for the mission. The idea is to keep the effective dose is within acceptable limts.

The effective radiation flux is the atomic engine's total radiation flux reduced by the free shielding of the remoteness of the habitat module

Divide the radiation flux by the square of the distance between the engine and the habitat module to get the effective radiation flux. That is:

Flux_eff = Flux / d²

where:

Flux = engine radiation flux (Severts/sec)
Flux_eff = effective radiation flux (Severts/sec)
d = distance between engine and habitat module (meters)
x² = square of x (x × x)

Which brings us to the problem of calculating the radiation flux from the atomic engine.

Anthony Jacks says as a quick-and-dirty first order approximation, for a fission reactor, the radiation flux is one-half the power production (in kilowatts) of the reactor core (which is greater than the power output due to inefficiency). This assumes 1 kilowatt of reactor core power produces 1.26 kilowatts of radiaion, mostly neutrons.

Flux = 0.5 * Pcore

where:

Flux = engine radiation flux (Severts/sec)
Pcore = power production of reactor core (kW)

As a rough guess, for atomic engines with a thermal power level of one megawatt to one gigawatt, the shadow shield will be from 1.0 to 0.1 kilograms per kilowatt. This assumes that the spacecraft is long and skinny, which reduces the angular size of the shield. The shield will be a composite of gamma ray shielding materials and neutron shielding materials.

In Space Propulsion Analysis and Design they give the specs on a typical shadow shield. Starting at the atomic engine, the gamma rays and neutrons first encounter 18 centimeters of beryllium (which acts as a neutron reflector), followed by 2 centimeters of tungsten (mainly a gamma-ray shield but also does a good job on neutrons), and finally 5 centimeters of lithium hydroxide (To stop the remaining neutrons. Hydrogen slows down the neutrons and lithium absorbs them.). This attenuates the gamma flux to a value of 0.00105, and neutron flux to 4.0e-9. This has a mass of 3,500 kilograms per square meter of shadow shield (ouch!).

For a rough estimate of the surface area of the shadow shield it should be a disk with a radius equal to the radius of the reactor core.

To estimate the size of the core is over my head but it is covered in SPAD. In the example from the book a 1000 megawatt reactor had an radius of 0.45 meters so the shield surface area is 0.64 m². The mass would be 3,500 * 0.64 = 2,240 kg.

A 2000 MW had a radius of 0.75 meters, surface area 1.77 m², mass 6,195 kg.

If an attenuation factor of 0.00105 for gamma and 4.0e-9 for neutrons is not enough, the factors can of course be increased by adding more thickness to the layers in the shadow shield. The SPAD has a handy table which I adapted:

Reduction Factor	Additional cm of Lithium Hydroxide for Neutron Attenuation (+kg/m²)	Additional cm of Tungsten for Gamma Attenuation (+kg/m²)
x0.5	+0.205 cm (+2.99 kg/m²)	+0.564 cm (+109 kg/m²)
x0.2	+0.477 cm (+6.96 kg/m²)	+1.308 cm (+252 kg/m²)
x0.1	+0.683 cm (+9.97 kg/m²)	+1.872 cm (+361 kg/m²)
x0.01	+1.365 cm (+19.92 kg/m²)	+3.744 cm (+722 kg/m²)
x0.001	+2.048 cm (+29.90 kg/m²)	+5.616 cm (+1084 kg/m²)

**Fig. 8.22. Radiation Attenuation with a Shadow Shield**
Nuclear rockets typically use a shadow shield to reduce the radiation levels at the payload. The shadow shield reduces the radiation flux to a level that prevents heating and boiling of the propellant. The propellant, the tank, and distance further reduce the radiation level to the payload’s allowable level.

Table 8.11. Physical Properties of Shielding Material and Effectiveness for Sample Shield Layout
Parameter/Shield Material	Be	W	Li-H₂
Density (ρ — kg/m³)	1850	19,300	500
Molecular weight (gm/mol)	9.01	183.86	Li—6.94; H—1.01
σ_a - absorption cross section (barn)	0.009	19.2	Li—71; H—0.33
Attenuation factor for 1MeV gamma rays (μ — cm^-1)	0.104	1.235	0.0444
Shield thickness (cm)	18	2	5
Gamma attenuation of incident beam	0.1538	0.0854	0.8011
Number density (atom/(barn·cm))	0.124	0.063	0.047
Σ_a - absorption cross section (cm^-1)	0.001	1.214	3.373
Attenuation of incident neutron beam	0.9802	0.0882	4.7×10^-8
Integrated gamma-flux reduction	0.1538	0.00131	0.00105
Integrated neutron-flux reduction	0.9802	0.0864	4.0×10^-9

Table 8.12. Radiation Attenuation for Shielding Example
Reduction Factor	cm Thickness Required for Neutron Attenuation (+kg/m²)	cm Thickness Required for Gamma Attenuation (+kg/m²)
0.5	622.7 (11,520)	0.571 ( )	0.205 ( )	6.665 ( )	0.564 ( )	15.611 ( )
0.2	1,446 ( )	1.326 ( )	0.477 ( )	15.475 ( )	1.308 ( )	36.249 ( )
0.1	2,069 ( )	1.897 ( )	0.683 ( )	22.141 ( )	1.872 ( )	51.861 ( )
0.01	4,138 ( )	3.794	1.365	44.281	3.744	103.721
0.001	6,206	5.691	2.048	66.421	5.616	155.581

As an example, NASA's Reusable Nuclear Shuttle concept used a NERVA NTR engine with 334 kiloNewtons of thrust with a shadow shield massing 1360 kilograms which protected a 10 degree half-angle area. The distance between the habitat module and engine (a bit less than 49 meters) provided extra protection, as did the mass of the propellant.

The crew protected by the shadow shield, distance, and propellant would still suffer a radiation dose of 0.1 sieverts every time the shuttle did a standard burn. Anybody outside of the shadow cast by the shield and closer to the engine than 16 kilometers would suffer a whopping 0.25 to 0.3 sieverts per hour. The safe distance outside of the shadow is no close than 160 kilometers.

A standard burn was a delta V between 1 and 2 kilometers per second.

NASA has a career limit of 4 sieverts for astronauts, so an astronaut exposed to 40 standard burns would be permanently grounded.

I have found minimal references to low mass shields for space nuclear reactors that were layered tungsten-lithium hydride, layered boron carbide-beryllium, and layered lithium hydride-beryllium. The lowest mass one is the tungsten-lithium hydride shield.

In Heinlein's "The Green Hills of Earth", atomic spacecraft designers are guilty of scrimping on shadow shields in order to save mass. The designers were under pressure to maximize payload mass without worrying about trivial incidentals like the health of the engine crew. This is why the jetmen working next to the atomic engines find it so hard to get insurance, and seldom have children. At least ones that are not mutants.

From **THE GREEN HILLS OF EARTH** by Robert Heinlein (1947)

From **THE ROLLING STONES** by Robert Heinlein *(1952)*

Artwork by Ed Valigursky

From Timothy's Space Book, 1952

Keep in mind that these are called "shadow" shields because it is too expensive to put radiation shielding all around the hot stuff ("expensive" in terms of reduction of payload mass). This means that if one ventures outside of the spacecraft, you run the danger of moving out of the shadow and into the deadly glow of the unshielded engine. When the spacecraft is designed, it is also important to ensure that no part of the ship scatters the lethal radiation around the shadow shield and into the crew. The heat radiators, for instance. If lifting off from a planet with an atmosphere, said atmosphere can also create pesky neutron backscatter.

This does make exiting a landed ship somewhat challenging, and makes an argument for a ground crew wearing lead suits. In the Tom Corbett books, any ship that was to be on the ground for more than three days would have its liquid fissionable fuel removed by the "hot soup" wagon. Keep in mind that the neutron flux from the engine would transmute the elements composing the rocket's structure, making the aft end of the spacecraft radioactive even if all the fissionables are removed. Spaceship designers should also construct the aft end of the spacecraft out of materials that are not only strong, but that will transmute into materials of still acceptable strength.

You will find more discussion on embarking/debarking from a radioactive rocket here.

In the interest of radiation safety, the corridor to the atomic engine room is going to have dogleg bends in it. Radiation travels in straight lines but people don't have to. This allows the crew to quickly move out of direct line of sight with the reactor. The corridor exit will have an adjacent decontamination booth.

From **BLESSED ARE THE MEEK** by G. C. Edmondson (1955)
read online at Project Gutenberg

From **FARMER IN THE SKY** by Robert Heinlein. 1950.

Stay Shadowed

"But," I hear you say "surely you only need to have the crew habitat module inside the shadow cast by the radiation shield? It's not like the ship's girders can get cancer from radiation, right?"

Nope, nice try, but you'd do best to keep every single part of the ship inside the radiation shadow. Especially the heat radiators, which are huge extended structures that want lots of room. There are three reasons why:

Neutron radiation can cause Neutron Embrittlement. Becoming brittle is not healthy for struture in general and load-bearing members in particular. Unless you have a perverse reason to want your spacecraft to snap like a twig when you goose the rockets.
Neutron radiation can cause Neutron Activation. Having components of the spacecraft transmuted into radioactive isotopes is a health hazard. Especially since the isotopes will not be behind a radiation shield. They will be free to spray the habitat module with deadly radiation.
Spacecraft strutures protruding outside of the radiation shadow can cause "backscatter", bouncing deadly radiation around the shadow shield and irradiating the habitat module. This is one reason why nuclear powered aircraft never caught on, the very atmosphere itself would cause backscatter.

In the first diagram below, note how the lower part of the external propellant tanks are cut at an angle so they do not stick outside of the shadow (the "half-cone sections"). In the other diagrams, note how the square heat radiators are trimmed to a triangular profile when they are near the shadow shield. This also gives the viewer an indication of the outline of the (otherwise invisible) radiation shadow.

The hull is tapered to fit within the shadow.
From NUCLEAR SPACE PROPULSION by Holmes F. Crouch.
Radiation may get around the shield via "backscatter." In this case the backscatter is from the rendezvous target, but can also be caused by parts of the spacecraft itself if they stick out of the protected cone.
From NUCLEAR SPACE PROPULSION by Holmes F. Crouch.
This artwork by TauCeti Deichmann shows you how it's done. Both heat radiatiors and propellant tanks are trimmed to keep inside the protected cone cast by the shadow shield
The good ship A. C. Clark (black) is safely inside the white cone shadow cast by the anti-radiation shadow shield. The purple-blue area is the zone of deady radiation created by the nuclear engines.
Increasing the spin radius will allow 1.0 g without a nausea producing spin rate. Unfortunately this will cause the hab modules to protrude into the deadly radiation zone (red).
To allow increasing the spin radius without leaving the safe zone, the star truss will have to be lengthened (gray). This unfortunately increases the structural mass at the expense of payload mass.
This is the hot end of the spaceship Hermes in Mars orbit. Radiation protection is provided in forward direction only by a disc-shaped shield casting a 'radiaton shadow'.
The heat exchangers' large wings shed the excess heat of the coolant fluid. The wings are arranged to stay in the shadow of the shield to avoid the reactors radiation. Whenever possible the spacecraft points the heat exchangers edge-on to the sun, to maximize their efficiency.
For docking Hermes shall only be approached from the front, as all other directions outside of the shadow cone are exposed to the reactors radiation.
Artwork by francisdrakex
click for larger image
Heat radiators of the Asteroid Crew Transport are cut into a triangular shape to stay inside the radiation shadow.
Note dotted line indicating Radiation Shield Shadow on the HOPE (FFRE)
In the Mini-Mag Orion the magnetic nozzle casts the radiation shadow

Electra

Discovery from 2001 A Space Odyssey, book version (the "Dragonfly" model), 1969

Pre-production sketch of the "dragonfly" version of Discovery by Roy Carnon

Physical model of "dragonfly" version of Discovery by artist Warren Zoell. Click for larger image.

Hull Armor

Hull armor is specifically to protect the ship and crew from the natural radiation from space, and from hostile weapons fire.

Different kinds of armor are required for different kinds of ionizing radiation: particle radiation or electromagnetic radiation. Neutron, cosmic rays, solar protons and the like are particle radiation (because they are subatomic particles). X-rays and gamma-rays are electromagnetic radiation. Particle shielding is generally something with lots of hydrogen in it, like water, liquid hydrogen propellant tanks, lithium hydride, paraffin or a hydrogenated polyethylene composite. X-ray/gamma-ray shielding is generally something very very dense, like lead, tungsten, or an alloy with a lot of heavy elements in it.

The hull armor will be arranged differently than than shadow shield.

First off, the armor is probably only going to be on the habitat module, and any radiation-sensitive equipment. It is not going to be over the entire spacecraft.

Secondly, unlike a reactor, cosmic rays and solar storms contain charged particles, mostly protons. Charged particles can create "Bremsstrahlung" or braking radiation. (Keep in mind that the hull of the spacecraft will probably never encounter natural gamma rays in the space environment. Gamma rays will probably only come from artificial sources, such as nuclear weapons.)

You see, gamma shielding is worse than useless against charged particle radiation. Such particles striking lead actually creates deadly x-rays, making the radiation problem much worse (the same principle is used in a doctor's x-ray machine). Please note that this only applies to charged particles, neutrons from the reactor do not generate Bremsstrahlung.

And please do not confuse "neutral particle beams" with "neutron particle beams." The former will produced Bremsstrahlung, the latter will not. Neutral particle beams are beams of protons and electrons (which are charged) in a neutral electrical balance. Neutron particle beams are beams of neutrons (which are uncharged).

So for the hull shielding it is best to arrange things so that the incoming radiation first encounters the paraffin to soak up all the particle radiation, then have a layer of tungsten to stop the gamma rays.

Anthony Jackson on the topic of Carbon as radiation shielding says:

This graph is from Proceeding of the Symposium on Manned Planetary Missions 1963/1964 page 92.

For solar proton storms occuring during missions lasting from 300 to 700 days, the graph shows the radiation dosage the crew will receive to their skin given aluminum shield weight. The dose to the crew's blood forming organs will be roughly half the skin dose.

The curves are for the probability of exceeding the listed radiation dosage, probabilities of 0.001, 0.01, and 0.1 (i.e., one in a thousand, one in a hundred, and one in ten).

For example, say you were concerned with the crew having a skin dose over 10³ rads (10 grays) and the mission was 700 days. Find 10³ on the vertical scale on the left. Look at the three curves: 700 days at P=0.001, 700 days at P=0.01, and 700 days at P=0.1

You draw a horizonal line starting at 10³, and draw a vertical line where it hits each of the three 700 day lines. Here it makes vertical red, gold, and green lines. See where the vertical lines hit the bottom scale.

The red line says that if the shielding is 3 gm/cm² of aluminum, there will be a one in ten chance that the crew will receive a skin dose of over 10³ rads on a 700 day mission. The gold says 10 gm/cm² will only have a one in a hundred chance, and the green says 17 gm/cm² will only have a one in a thousand chance.

Anti-weapon armor (lasers and kinetic energy weapons) is discussed here.

**Figure 2 A – I: Photographic data of fungal growth**
in intervals of 6 h starting with A: t₀ = 0 h continuing until I: t = 48 h, shows the development of *C. sphaerospermum* growth by means of optical density on the agar plate in the on-orbit laboratory over the first 48 h of the experiment.

Storm Cellars

Storm cellars are specifically to protect the ship and crew from the natural radiation from space, specifically when the radiation suddenly increases. Much like people take shelter in a conventional storm cellar when a tornado suddenly appears. In NASA-speak a storm cellar is called a "biowell". "Sudden increases" in radiation usually means a solar storm, though occasionally it means an unavoidable pass through a planetary radiation belt.

There are also storm cellars in Orion nuclear pulse driven spacecraft, since detonating hundreds of nuclear devices for propulsion will also cause a sudden increase in enviromental radioactivity.

A storm cellar is a radiation-shielded room near the ship's center, barely large enough for the entire crew. If it can be located in the middle of dense things, like fuel tanks or cargo, so much the better. The shielding is generally a material that contains lots of hydrogen since storm cellars typically protect against particle radiation, not x or gamma rays. NASA is currently working on a new shielding material, a hydrogenated polyethylene composite. Not only is it a better shield than aluminum, it has less mass as well.

There also might be anti-radiation suits for the crew. Not as good as a full storm cellar, but much better than nothing.

Cellars might use an as yet un-invented magnetic anti-radiation field. Such fields are currently science fiction, and in any event will only provide protection against charged particle radiation, not x or gamma rays (for that you'll need an honest-to-Doc-Smith force field or ray screen). Keep in mind that almost all natural radiation hazards are charged particle, x and gamma rays generally come from human sources (such as poorly shielded fission reactors and nuclear weapons).

To protect against a significant solar storm, the shielding on the biowell should be at least 500 grams per square centimeter (5,000 kg/m²). This will give good protection against neutrons as well.

I have seen a couple of designs for Mars missions wit solid core nuclear thermal rockets try to make a poor-man's storm cellar by aiming the rocket's shadow shield at Sol and hoping it stops enough solar storm radiation so the crew doesn't die. I am trying to find some figure on how effective this would be, but it sure looks like a dangerous mass-cutting short cut to me.

A storm cellar surrounded by water tanks can be found in John Campbell's THE ULTIMATE WEAPON, Robert Heinlein's PODKAYNE OF MARS and Lee Correy's (AKA G. Harry Stine) SPACE DOCTOR. Both Heinlein and Stine call the cellar a "caisson" or "cofferdam". A caisson is actually a pressurized working area surrounded by water that is used when building the submerged pylons of a bridge, but I suppose the description is whimsically close enough to a spacecraft storm cellar.

The crew will occupy the cellar when the sun kicks up a solar storm of radiation. As these can last for days, one had better include a few crew-days worth of food, water, and tranquilizers. And a porta-potty. If you are relying upon algae for your oxygen, it deserves space in the storm cellar as well. This probably also applies to stored food too. I have heard that particle radiation can destroy a lot of the vitamins in food, especially pyridoxine and thiamine. Alas, computers and other digital electronics are also vulnerable to radiation. Don't forget repeaters for the gauges on the major ship systems, and one monitoring radiation levels outside the cellar. The latter tells you when it is safe to come out. The former tells you if there is a critical failure outside, meaning it is time to start drawing straws to decide who gets to heroically commit suicide by saving the ship. After the storm the crew can emerge and go check the dosimeters they thoughtfully left in the modules of the spacecraft vulnerable to radiation.

As a matter of fact, in many early NASA designs for Mars missions, the storm cellar is also the control room. Don't just have repeaters in the cellar, have all the spacecraft controls. It is a really bad idea to leave the control room unmanned, and the crew will be reluctant to expose themselves to an agonizing radioactive death because the blasted control room ain't shielded. Yes the control room will be a bit crowded when the sun raises up a ruckus, but you can't have everything.

Storm cellar designed for 5 meter wide tubular space station. Cellar is a 3.5 meters cube. Holds 4 persons (reach envelope for torso-restrained 95th percentile male). 20 g/cm² shielding, plus additional shielding from equipment, consumable storage, waste storage. Integrated dose of 0.2 sievert for each of three worse known solar flare events (Feb 1956, Nov 1960, Aug 1972), and less than 0.5 sievert for all three events combined.
From Exploration Studies Technical Report FY 1988 Status, Volume II

Storm cellar on deck 3 of the Boeing IMIS

Lifesystem of Mars exploration vehicle designed by General Atomic for NASA, 1963. Note storm cellar. Spacecraft uses the "tumbling pigeon" method of centrifugal artificial gravity, which is why the decks are so weird.

From Outrim by J. Mauloni. Another scientifically accurate webcomic.

From **TURN LEFT AT THE MOON** by Robert L. Forward *(1995)*

From **SPACE DOCTOR** by Lee Correy (G. Harry Stine) 1981.

Planetary Base Shields

Lava tube skylight in Pavonis Mons region of Mars

To protect against galactic cosmic radiation and solar proton storms, lots of mass is required. A spacecraft has to carry its own shielding. But a planetary base can use regolith as shielding, i.e., bury the base by shoveling tons of the readily available local dirt over it. Alternatively the base can be located in artificial or naturally occurring caves and tunnels deep underground. This explains NASA's interest in Lunar and Martian lava tubes.

It is estimated that Lunar lava tubes can have a diameter of up to 300 meters and lying under 40 meters or more of basalt. In addition to protecting from galactic cosmic radiation, lava tubes will also protect against meteorites, micrometeorites, and ejecta from impacts. They will also provide a stable temperature of about -20 °C (instead of varying from -173 °C to +100 °C) and access to underground resources.

Martian lava tubes are estimated to have a roof thickness of around 30 meters. In 2010 a "skylight" (lava tube with hole in the roof) was observed in the Pavonis Mons region of Mars. The skylight was estimated to be about 190×160 meters wide and at least 115 meters deep.

Lunar Lava tube skylights

Radiation dose as thickness of regolith increases
From Space Environment & Planetary Civil Engineering Basics - KISS Shortcourse, R. P. Mueller (2015)
I assume that the GCR radiation dose increases from 0 to 175 g/cm³ due to Bremsstrahlung.

Using regolith is more work, but you cannot always count on a convenient lava tube near the proposed base site.

The table below assumes that regolith has a bulk density of 1.3 grams per cubic centimeter.

Radiation Shield Mass
Level	Density	Regolith Thickness
Equivalent Shielding to Terra Sea Level	1,000 g/cm²	7.7 m
Minimum Acceptable Shielding	700 g/cm²	5.4 m

In the Lunar base design below, regolith is stuffed into long bags and coiled around the dome.

Lunar outpost from NASA's 90-Day study
Dome is covered with a coiled bag of lunar regolith as radiation shielding. Private compartments are below the surface for additional radiation protection.
Artwork by John Michael Stovall.
Click for larger image

Lunar outpost from NASA's 90-Day study
Artwork by John Michael Stovall.
Lunar outpost from NASA's 90-Day study
[4] Coiled regolith bags (radiation shielding)
[5] Regolith bagging machine. Bulldozer scrapes loose regolith into coiling bags, and wraps them around the inflatable habitat
Regolith Bagger
On Mars, bags with a thickness of one meter will provide Earthlike protection from solar events. On Luna they will probably have to be thicker.
From Exploration Studies Technical Report FY 1988 Status, Volume II

From Exploration Studies Technical Report FY 1988 Status, Volume II (1988)

Force Fields

Radiation shields composed of matter are quite massive, and Every Gram Counts. Researchers have been looking into using magnetic and electrostatic fields to protect against particle radiation, since such fields have no mass. Unfortunately the generators of such fields do have mass. And the field strength will have to be strong enough that the word "superconductor" will soon be mentioned. In addition, such powerful fields might be health hazards to the astronauts. It is worthless if the field simultaneously protects the astronaut from particle radiation, but also instantly kills them by being strong enough to straighten out all their DNA molecules.

From **ASYMMETRIC ELECTROSTATIC RADIATION SHIELDING FOR SPACECRAFT** *(2004)*

1. Layers of high density polyethylene protect magnets from debris and micro-meteorites.

2. Superconducting electrical wires are coiled into looped ‘racetrack’ structures to form a magnetic shield 3000 times stronger than Earth’s, deflecting away harmful cosmic rays. Each racetrack is reinforced with tie rods in order to withstand the forces the magnet produces.

3. A protective sunshield keeps solar radiation off the superconducting wiring, helping to keep it at very low temperatures it needs to maintain superconductivity.

4. The magnets surround the cylindrical ‘habitat’ where the astronauts live on their journey to Mars and back. Superinsulating panels prevent heat trickling from the habitat and overheating the superconducting wires.

5. Only the nose cone at one end, and thrusters at the other, extend outside the protective magnetic shield.

Artwork by Anthony Calvert

From **THE SCIENCE IN THE STORY** (of the Cutting Fringe) by Paul Chafe (2004)

Electromagnetic particle shield from Soviet magazine (1963)
Radiation Protection and Architecture Utilizing High Temperature Superconducting (HTS) Magnets
The work performed showed that single layer expandable coils with diameters of 4 to 8 m and lengths of 15 to 20 m, arranged in a 6-around-1 configuration constitute the best solution among all concepts analyzed. The single compensator coil closely surrounds the habitat serving as a habitat thermal radiation shield for the outer coils and compensates for outer coil fringe fields trying to enter the habitat.

Electrostatic Sphere tree with positively charged inner spheres and negatively charge outer spheres. The yellow net is grounded.
From Analysis of a Lunar Base Electrostatic Radiation Shield Concept (2005)
Electric field potential along z-axis for negative-positive sphere tree.
From Analysis of a Lunar Base Electrostatic Radiation Shield Concept (2005)

Electrostatic sphere tree with only postively charged spheres
From Analysis of a Lunar Base Electrostatic Radiation Shield Concept (2005)
Electric field potential along z-axis for positive sphere tree.
From Analysis of a Lunar Base Electrostatic Radiation Shield Concept (2005)

Plasma bubble shield concept by John Slough of the University of Washington
Ionized hydrogen is confined by a superconducting net to provide radiation shielding. Bubble is 100 meters in diameter, is as effective at stopping radiation as ten centimeters of aluminum, but has much less mass.

Magnetic Shield
Shielding system is about nine tons, which is much less that material anti-radiation shields but still a disappointingly large chunk to remove from the payload capacity. To stop cosmic rays within a few meters the field strength will have to be about 20 teslas (600,000 times that of Terra).
Pros: much lighter than mateiral shield
Cons: offers no protection along the axis (which is why the living quarters are donut shaped). 20 teslas might be harmful to human beings.
From Shielding Space Travelers by Eugene N. Parker, Scientific American March 2006
Magnetic Shield
In this design, a second inner magnet partially cancels out the 20 tesla field inside the living quarters.
Pros: the magnetic field inside the living quarters is lower
Cons: the magnetic field inside the living quuarters is still much higher than on Terra. The system has more mass and more points of failure.
From Shielding Space Travelers by Eugene N. Parker, Scientific American March 2006

This concept uses the Mini-magnetospheric plasma sail propulsion system (M2P2) as an anti-radiation shield method.
From An exploration of the effectiveness of artificial mini-magnetospheres as a potential Solar Storm shelter for long term human space missions (2014)

The deflection of a high energy ion (green) by the electric field (Er) (red) created by the low energy plasma captured and retained by the magnetic field from the spacecraft. Augmenting the natural density, by releasing readily ionised gas from the craft, can enable protection of >> 100MeV/amu ions.
From An exploration of the effectiveness of artificial mini-magnetospheres as a potential Solar Storm shelter for long term human space missions (2014)

A conceptual design for a manned interplanetary vehicle using M2P2 shielding.
From An exploration of the effectiveness of artificial mini-magnetospheres as a potential Solar Storm shelter for long term human space missions (2014)

Medication

There are some medications that can offset the harmful effects of acute radiation exposure, but there is a limit to the protection they can offer.

If there is nuclear fallout or a release of radioisotopes/fission fragments into the air, people in the area should immediately take a potassium iodide tablet. While none of the fission fragment elements are particularly healthy, Iodine-131 is particularly nasty. This is because ones thyroid gland does its level best to soak up iodine, radioactive or not. Your thyroid will quickly become saturated with deadly iodine-131 and thyroid cancer will ensue. Potassium iodide pills load one's thyroid with safe iodine, so it be sated and thus ignore any deadly iodine-131 that passes by.

Obviously potassium iodide tablets provide zero protection against any of the many other radioisotopes.

The body's hematopoietic (blood forming) tissues are seriously damaged with exposures of 1 gray or more. This is mainly the bone marrow, causing damage to blood-cell production and the immune system.

The standard treatment is Granulocyte colony-stimulating factor (kicks the surviving bone marrow into overdrive), but G-CSF must be administered as soon as possible or it does not help. A new treatment combines thrombomodulin with activated protein C, this will help if administered within 24 hours of exposure.

The gastrointestinal tract is increasingly damage with exposures above 5.5 grays. The regenerative peptide TP508 (rousalatide acetate) will significantly increase survival and delay mortality by activating stem cells (though I am curious as to the exact definition of "significantly"). TP508 may also activate stem cells in other organs besides the GI tract, helping them recover from radiation exposure as well.

The drug dimethyloxalylglycine helps protect the GI tract from radiation damage by blocking PHD proteins. It should be administered within 24 hours of exposure.

Radiation damages a cell's DNA. If a cell discovers too much DNA damage, it commits suicide (to avoid the risk of becoming cancerous). If too many cells suicide, the person dies. Unfortunately the mechanism is set too conservatively, it will kill the cell even if the damage is slight enough to be repairable.

The drug 2-[4-(1,3-dioxo-1H,3H-benzoisoquinolin-2-yl)butylsulfamoyl]benzoic acid (mercifully abbreviated to DBIBB) delays cell suicide and speeds up DNA repair, giving the cells a fighting chance to heal themselves.

From **ANTI-RADIATION BIOLOGICAL COUNTERMEASURES: AMIFOSTINE** by Chris Stelter (2015)

From **ARE HUMANS TOO FRAGILE FOR LIFE IN SPACE?** by John G. Cramer *(2019)*

Genetic Engineering

Tardigrade

Tardigrade are microscopic animals that do not grow larger than 1.5 millimeters or so. Ordinarily they would be very forgettable creatures, were it not for the disconcerting fact that the blasted things are almost indestrutable, or at very least invincible.

They can withstand pressure of 6,000 atmospheres (about six times the water pressure at the bottom of the Mariana trench). They can withstand the zero pressure of outer space. They can survive a temperature of −20° C for about thirty years. They can survive a temperature of 151° C for a few minutes. They can officially survive being dehydrated for ten years, though there was one report of a 120-year-old dehydrated specimen waving one of its arms.

One wonders if laminated tardigrades would make good combat armor.

But more to the point, those little adamantine monsters can withstand 1,000 times more radiation than other animals. 10 grays is certain death for a human being. The median lethal dose (LD50) for a tardigrade is 5,000 freaking grays of gamma rays or 6,200 freaking grays of heavy ions. This means you could irradiate a bunch of tardigrades with sixty times the radiation that would instantly put a human into a coma and kill them in 24 hours and half of the blasted tardigrades would survive!

Naturally scientists were interested in [a] how the heck do they do this? and [b] can we teach humans to do this as well?

Scientist Takuma Hashimoto et al figured it out, and published their results in a paper Extremotolerant tardigrade genome and improved radiotolerance of human cultured cells by tardigrade-unique protein

Using tandem mass spectrometry they discovered a previously unknown protein that they gave the boring name of Damage suppressor (Dsup). The stuff stays inside the nuclei of tardigrade cells and apparently wraps itself around the nuclear DNA.

Using standard HEK 293 cells as experimental vectors, the researchers did genetic engineering to give the experimental cells the gene for Dsup. Then they subjected the cells to 10 grays of radiation. The Dsup cells had only 48% of the single-strand break radiation damage, 40% of the double strand break radiation damage, and only 25% of the reactive oxygen species radiation damage; as compared to ordinary HEK 293 cells. Which is an amazing increase in radiation resistance, expecially just from a single new stupid protein.

The bad news is that apparently the only way to obtain this radiation protection is to do genetic engineering on the astronaut's cells. Which has quite a few ethical problems. But study of the Dsup protein will eventually reveal the mechanism of its protection, and may lead to radiation protection that is a bit less invasive than mutating your cells.

Dsup is probably not the only anti-radiation measure in the tardigrade's genome. For example, its genome contains more copies of an anti-oxidant enzyme and a DNA-repair gene than any other animal.

From **SUPER SPACE SUNBLOCK MADE FROM SKIN PIGMENT COULD SHIELD ASTRONAUTS FROM RADIATION**
by Adam Mann *(2020)*

Example: Atomic Engine Design

click for larger image

This section is sort of a case study on the many ways atomic radiation impacts on atomic engine design. This is taken from Radiation Effects on a Nuclear Rocket Engine Systems (1961)

The report is about the Kiwi-B2 experient of the Los Alamos Scientific Laboratory. It goes into great detail on the design challenges created by the radiation enivronment. The report details what it would take to alter the Kiwi-B2 to create a practical engine.

Obviously the radiation affects everything. Neutron heating and Gamma heating puts thermal stress on the engine components, structure, and propellant tanks. Neutron embrittlement damages sensitive materials and electronic equipment. Ionizing radiation disrupts electrical systems. The grim spectre of neutron activation will transmute once-safe engine components into radioactive isotopes spitting death in all directions. This will present a severe radiation hazard to the crew tasked with maintenance, disassembly, and inspection.

If that wasn't enough, allowances must be made to ensure the blasted engine will actually operate in the space environment. Passing tests on the ground with flying colors is no guarantee it will work in space. On the ground there is an atmosphere and the ground. Both can backscatter neutrons back into the reactor core, acting like additional neutron moderators that will not be present in space. Real embarrassing if the reactor won't react once you get it into orbit. Plus the atmosphere will give additional convective cooling to the reactor, which will also be absent in space. Melting in orbit is even more embarrassing.

The priority focus of the report is trying to optimize the balance between "reactor — tank" separation and radiation shield weight. Because the rest of the design hinges on it. Remember that radiation shields have outrageous high weights but are compact. Distance is anything but compact. In theory distance has no mass, but in reality you at least need a structural spine whose mass increases with distance. By using graphs one can determine the sweet spot: that perfect balance of shield mass and distance mass which adds the lowest additional mass for a given radiation protection level.

Secondary focus was on specifying components such that they were light-weight, resistant to radiation damage, and resistant to neutron activation.

Kiwi-B2 Description

As per the design parameters the engine assumes the use of a LASL Kiwi-B3 reactor, a Rocketdyne Mark 9 pump + turbine, and a modified Kiwi B nozzle. The diagrams above show the nozzle with lines for expansion ratios of 50:1, 75:1, and 100:1.

The turbines can be energized in any of three different ways

Generator: turbine driven by hot gases created in a separate gas generator
Bleed Cycle: turbine driven by hot gasses tapped from the reactor exhaust
Topping Cycle: turbine driven by hot gasses created by pumping stolen cold propellant through special heat exhanger pipes passing through nozzle and reactor

Kiwi-B2 design chose to use Bleed Cycle because it has a performance advantage over the gas generator method, and because the topping cycle lacks over-all engine flexibility.

The reactor power is 1500 megawatts, propellant mass flow through reactor is 83 lb/sec, chamber pressure is 816 psia, and propellant temperature at reactor outlest is 4500° R.

In the design process it became glaringly obvious that the propellant tank was going to need protection from reactor radiation. Or the propellant would start violently boiling and the tank would explode, this is a bad thing. This is the reason for the focus on balancing the distance separation with the radiation shield weight: if the boiling is a show-stopper you must prevent it, but every gram counts.

Radiation Shadow Shield

True, the primary job of the radiation shield is to prevent lethal radiation from destroying the ship and killing the crew. But there are other concerns besides how much radiation they stop.

The shield should be as light as possible because every gram counts. It should be strong enough to withstand rocket acceleration, requiring additional supporting structure is adding more grams. Since it is stopping radiation it is going to grow hot, so it will need adequate cooling. This also means it has to be constructed of materials that have good heat transfer properties and high-temperature strength.

Obviously it will have to stop both neutrons and gamma rays. Just stopping one type will be a deadly mistake.

An important point is it should not reflect neutrons. Otherwise it will increasing the amount of free neutrons in the core, and could cause a runaway reaction.

In the report, they figured that the best shield material that fulfilled all the requirements was boronated graphite.

Balancing "Reactor — Tank" Separation with Radiation Shield Weight

In the three graphs below, Reactor — Tank Separation is displayed on the bottom scale ( abscissa ) and Radiation Shield Weight is indirectly displayed at the "Attenuation Factor" curves (the attenuation implies the thickness of the radiation shield). The side scale (ordinate) is the system weight change. The idea is to find the lowest value for system weight change.

There are three graphs because there are three mathematical propellant flow models: Potential Flow Model, Completely Mixed Flow Model, and Recirulation Flow Model.

The minimum system weights for each of the three models is:

Fig.	Flow Model	Power Level (Mw)	Run Duration (sec)	Separation Distance (ft)	Attenuation Factor	System Weight (lb)
3	Potential	1500	1200	14	2	4100
4	Completely mixed	1500	1200	16	6	5200
5	Recirculating	1500	1200	17	6	5500

which you can see is the lowest curve point of all the curves, in the three graphs below.

The report is vague, but I gather they went with figure 5: separation distance of 17 and attenuation factor of 6. These were used in the radiation flux plot below. Figure 3 was thrown out because an attenuation factor of 2 is not enough to prevent severe radiation damage to the engine and crew.

Radiation at the "top" has been attenuated by a factor of 6 by using a 14-inch 20% void boronated graphite shadow shield. This protects the crew, but also the feed system and propellant tank (and the propellant, if it starts boiling that will be very bad).

Neutron (left) and Gamma (right) flux around the engine at 1500 Mw operation

The Kiwi test engine included some explosive devices for emergency break-up and scattering of the reactor core. Alas the reliability of these drastically degrades as they are exposed to gamma radiation (neutron radiation has little effect because explosives have relatively small numbers of hydrogen atoms). There are two radiation effects. First the explosive force is diminished as the radiation breaks down the molecular structure. This can be measure by the amount of gas evolved by a unit dose of radiation, to allow selection of the most resistant explosive type. Second, the radiation can make the explosive unstable and sensitive, so it might "predetonate" (explode whenever it damn well pleases instead of waiting for you to push the button).

A glance at the graph shows that the obvious winner in the explosives contest is good ol' TNT. Its gas evolved line is practically flat, meaning that the radiation has little effect. The maximum dose at the horizontal centerline of the engine is about 10¹¹ erg gm^-1 (C) (i.e., way off the right edge of the graph) but the TNT line is flat enough that it seems safe to extrapolate.

The higher the gas evolved, the weaker the explosive type becomes under radiation bombardment

Atomic Maintenance

Waldoes

The various controls, tongs, and remote control "waldoes" will reach around or penetrate the anti-radiation shadow shield, and there may be auxiliary lead baffles. Peeking around the baffles is how Rhysling lost his sight in Heinlein's "The Green Hills of Earth".

Waldo by Robert A. Heinlein, 1940
Remote manipulators, AKA "Waldoes"
Anti-radiation waldoes

Robots

Galaxy magazine, July 1952

Remember that the shadow shield will be in the floor, with the engine below that. Closed-circuit TV monitor will help Astro see what he is doing, but if they are damaged, he'll have to make do with mirrors and/or doing it by touch. What he really needs is one of Tom Swift Jr.'s Giant Robots, which were designed to do maintenance inside nuclear power plants. There is more about robots here.

For external repairs, the chief engineer might use something similar to the amazing Canadarm 2, which is currently on active duty on the International Space Station. Unlike the first Canadarm, this one is not attached at either end. Instead, either end can plug into special sockets ("power data grapple fixtures") built at strategic spots on the surface of the station. Canadarm 2 can literally walk on the surface of the station to where it is needed, moving end-over-end like a giant metal inch worm. The main limitation is that each "step" must end at a socket, but this is due to power and control signal issues. A more advanced version might be self contained enough to not require sockets, just hand-holds or other protrusions that it could grab.

Canadarm 2 is quite large, 17.6 meters (57.7 feet) long when fully extended. It can move payloads with a mass up to 116 metric tons.

On your atomic rocket, one would use arm(s) long enough to reach any spot on the radioactive engine.

Canadarm 2
Canadarm 2 socket. The socket plugs into Power Data Grapple Fixtures.
A Power Data Grapple Fixture. These provide the arm with power and a computer/video link to astronaut controllers inside.
"Canada Hand" attachment for the Canadarm 2.
The Canada Hand is used for precision work.
Artwork by Roy Scarfo
Artwork by Roy Scarfo

Hot Soup Wagon

Refueling and maintenance on radioactive spacecraft out on a landing pad will need something with lots of waldoes and probably treads. In olden days (the 1950's) they figured these things would be controlled by men inside lead-lined control cabs, using TV cameras. Nowadays it would make more sense for the vehicles to be remotely controlled drones.

In the old Tom Corbett Space Caded novels, such vehicles were called "hot soup wagons", because the spacecraft in the novels used liquid core nuclear thermal rocket propulsion. Though in reality I doubt that a landed rocket would keep the plutonium fuel molten just for ease of pumping it into the wagon.

Atom-powered jet transport of the future will be refueled by lead-shielded tank-like vehicles. Operators inside vehicles see through TV iconoscope cameras and do work with robot arms. Trailer under plane contains shielded coffin to receive spent but still radioactive slugs of plutonium which will be replaced with fresh fuel.
Artwork by Frank Tinsley for Popular Mechanics September 1956.
Click for larger image. Courtesy of James Vaughan
Nuclear powered aircraft has its reactor replaced. Artwork by Eberhard Binder-Staßfurt. From Gigant Atom (Giant Atom), 1956

The Beetle

Beetle
Length	19' 0"
Width	12' 7.5"
Height - cab down	11' 7"
Height - cab up	26' 7"
Weight	170,000 lbs
Ground Pressure	35 lbs/in²
Cab
Elevation max	15'
Rotates	360° at 0.8 rpm
Elevates	4'/minute
Lead wall thickness	12"
Hatch opens	1 minute
Speed
0% grade forward	8 mph
0% grade reverse	5 mph
10% grade both	5 mph
Manipulators
Max reach from operator	17' 9"
Max weight lift straight down shoulder	2,000 lbs
Max weight lift with arms extended	100 lbs
Other
Window thickness	23.24"
Flood light	250 foot-candles at 15'
Operating ambient temperature	-30°F to 130°F
Drawbar pull	85,000 lbs
Continuous operation	8 hours

One of the more interesting examples of a hot soup wagon is the Beetle. It was built in 1961 by Jered Industries on contract for General Electric's Nuclear Materials and Propulsion Operation division. It was going to be used in the US Air Force Special Weapons Center to service and maintain a planned fleet of nuclear powered Air Force bombers. The bombers were never constructed and Beetle was scrapped.

You can find all the details in the report AD0402748 USAF Shielded Cab Vehicles Test And Evaluation. This includes the layout of all 120-odd buttons on the control panels, which I didn't bother to include.

On the Beetle, the engine and transmission are located in the front of the chassis, while the operator cab and manipulators are mounted on the rear. The cab walls are solid lead 12 inches thick, clad with a one inch steel shell on the outside and a 0.5 inch steel shell on the inside. The five operator windows are 23.25 inches thick made out of seven panes of leaded glass (same radiation shielding level as 12 inches of solid lead). The cab can lift up to a height of 26 feet off the ground since nuclear bombers are quite tall. Each arm can lift 2,000 pounds yet are delicate enough to pick up an egg without breaking it.

With top hatch in the open position
View into cab from top hatch.
Looking down into crew compartment, with front facing window at top
Operator looking through front window (23.25" of leaded glass)
Top view, front of Beetle is towards bottom. Area visible through windows is indicated by arcs
Front of Beetle is to the left. Visibility from front window
Front of Beetle is to the left. Cone of visibility from rear TV cameras
Front of Beetle is to the left
Cab front view (numbers are radiation test exposures)
Cab rear view
Cab top view
Cab left view
Cab right view
Motions of the manipulator arms
Manipulator motions
Manipulator motions
Manipulator motions

General Atomic Remotes

This is from General Atomic Company - ANL THE-NEXT-STEP Scoping Studies - Vol VII Remote Maintenance Systems

The Aerojet Manufacturing Company got a contract from the General Atomic Company to develop a remote maintenance system for the Ignition Test Fusion Reactor. The neutron flux from the fusion reaction is going to neutron-activate the heck out of the reactor and everything near it. The equipment proposed will be similar to a hot soup wagon used to service future nuclear powered rockets.

The equipment is to have a service life of 10 years, and the replaceable sub-components are to have a service life of 5 years.

7.3.1. BRIDGE MOUNTED SHIELDED CAB MANIPULATOR (BMSCM)

click for larger image

The BMSCM is a manned shielded cab equipped with two manipulators. The cab is suspended from a bridge crane via a telescoping mast and provides maintenance coverage for the upper portion of the reactor. The cab has sufficient space and radiation shielding for two operators. The BMSCM weight is 50 tonne.

The cab is an octagonal shape, 183 cm across the flats and 267 cm in height. The wall thickness on three forward sides and the bottom is 25.4 cm (2.5 cm of steel and 22.9 cm of lead). The top and five remaining sides are 12.7 cm thick (2.5 cm of steel and 10.2 of lead). Three shielded windows (45.7 cm x 45.7 cm x 45.7 cm thick) are located in the 25.4 cm thick walls at a 15° downward viewing angle. A hinged personnel access hatch with a 20.3 cm diameter shielded viewing port is provided at the rear of the cab.

Two electromechanical manipulators are mounted on the lower portion of the cab and telescope in a horizontal direction. Manipulator arm length and telescope reach are 138 cm and 165 cm respectively, providing a total reach of 306 cm. The telescoping tube also provides rotation of 360°. The manipulator arm consists of three main elements (upper arm, forearm, and wrist) with an extendable hand which is capable of continuous rotation at the wrist in either direction. Load capacities at the hand are rated at 181 kg with the manipulator elements in any position.

Closed circuit television (CCTV) is provided to supplement viewing through windows. One CCTV camera with zoom lens, pan/tilt head and illumination is mounted on a telescopic mast. The TV mast is attached to the lower portion of the cab and is capable of projecting 3.8 meters.

A three bay control console is located within the cab. The center bay contains controls for the bridge, cab mast extension, BMSCM trolley, 45 tonne auxiliary crane trolley, cab rotation, CCTV system, cab lighting, air conditioning system, intercom system, alarm system, status monitors and radiation monitor. The left-hand and right-hand bays provide controls for the manipulators. Each motion of the manipulator is controlled by a knob, spring centered to the "off" position. Velocities in both directions are proportional to the displacement of the knobs. Rotary motions are controlled by rotary knobs and the others by sliding knobs. All are flush to minimize accidental movements. Color coding on the console and on the manipulator provides direct correlation between motions of the two. Thus, movement of a pivot knob toward the "yellow" bar on the controller will move the corresponding manipulator element in the direction of the "yellow" side.

The control panel provides the necessary controls for grip force, power on-off, and tool power.

The CCTV monitor is mounted above the center window for convenient viewing by operator(s). The air conditioning unit and the lighting fixtures are mounted on the cab ceiling. A fire extinguisher, first aid kit, and life support equipment are stowed within easy access of the operator(s). Tools and personal effects are stowed in cabinets. Two adjustable chairs are provided and may be folded and stowed when not in use.

A three bay control console is located in the master control room with controls and monitors identical to those within the BMSCM cab. Thus all BMSCM functions may be controlled remotely and viewed with the CCTV system from within the master control room.

The BMSCM bridge span is 50 meters with the bridge rail elevation at 16.8 meters above the operating floor. The BMSCM trolley has a load capacity of 50 tonne and supports an extendable cab mast assembly with a 8.2 meter vertical travel distance. The BMSCM floor clearance is 12.5 meters with the mast retracted and 4.3 meters with cab mast extended.

An auxiliary 50 tonne capacity crane runs on the same bridge as does the BMSCM trolley. This crane controlled from either the BMSCM cab or the remote control console in the master control room.

In the event of a failure in the BMSCM bridge crane drive system, the 150 tonne facility bridge crane may be used to move the BMSCM to a safe position.

7.3.10.2. Overhead Bridge Manipulator

The bridge, trolley, mast and auxiliary manipulation are the principal components of the Overhead Bridge Mounted Manipulator (OBMM). This system capable of servicing the hot cell during assembly and disassembly operations. The traveling bridge spans the width of the hot cell (18.28 meters). The trolley travels the length of the bridge and supports the 3 section telescoping mast (7.62 meters retracted, 16.76 meters extended), which has an azimuth drive for the lower section. The auxiliary manipulator is mounted within the mast. Bridge and trolley movement allows the vertical axis of the mast to be positioned over any set of horizontal coordinates within its range of travel. A control located in the operating gallery provide for the remote operations of the bridge, trolley, mast, manipulator, and positioning heads.

7.3.2. VEHICLE SHIELDED CAB MANIPULATOR (VSCM)

The VSCM is a manned shielded vehicle equipped with two manipulators. The cab is mounted on an industrial truck chassis and provides maintenance coverage for the lower portion of the reactor. The cab has sufficient space for two operators. The VS CM weight is 50 tonne.

The cab is a rectangular shape 213 cm x 213 cm x 229 high. The thickness of the three sided front wall is 25.4 cm (2.5 cm of steel and 22.9 cm of lead). The top, bottom and three remaining sides are 12.7 cm thick (2.5 cm of steel and 10.2 cm of lead). Three shielded windows (45.7 cm x 45.7 cm x 45.7 cm thick) are located in the 25.4 cm thick front wall. A hinged personnel access hatch with a 20.3 cm diameter viewing port is provided at the rear of the cab.

Two knuckle-boom cranes are mounted in front of the cab. These cranes have 180° rotation capability at the base and provide a telescoping boom. One electromechanical manipulator is mounted to the end of each boom crane. The manipulator arm consists of three main elements (upper arm, forearm, and wrist) with an extendable hand capable of rotating 300° at the wrist. Load capacities are rated at 181 kg at the hand with the manipulator elements in any position. Manipulator arm reach is 138 cm. Total reach with boom and manipulator extended is 719 cm.

Closed circuit television (CCTV) is provided to supplement viewing through the windows. One CCTV camera with zoom lens, pan/tilt head and illumination is mounted on a telescopic mast. The TV mast is attached to the shielded cab between the two knuckle-boom cranes and is capable of projecting 3.8 meters.

A three bay control console is located within the cab. The center bay provides controls for the CCTV system, cab lighting, air conditioning system, intercom system, alarm system, status monitors, radiation monitor and vehicle drive functions.

The left-hand and right-hand bays provide controls for the knuckleboom cranes and manipulators. Each motion of the manipulator is controlled by a knob spring centered to the "off" position. Velocities in both directions are proportional to the displacement of the knobs. Rotary motions are controlled by rotary knobs and the others by sliding knobs. All are flush to minimize accidental movements. Color coding on the console and on the manipulator provides direct correlation between motions of the two. Thus movement of a pivot knob toward the "yellow" bar on the controller will move the corresponding manipulator element in the direction of the "yellow" side.

The control panel provides the necessary controls for grip force, power on-off, and tool power.

The CCTV monitor is mounted above the window for convenient viewing by the operator. An air conditioning unit and lighting fixtures are mounted on the cab ceiling. Fire extinguishers, first aid kit, and life support equipment are stowed within easy access of the operator(s). Tools and personal effects are stowed in cabinets. Two adjustable chairs are provided and may be folded and stowed when not in use.

The vehicle is a modified industrial lift truck, 549 cm long, 386 cm wide and 350 cm high (top of shielded cab). The truck is powered by a LP-gas engine. The truck has a flat area behind the cab suitable for transporting items. The power control center, containing individual panels for motor control units, is mounted on the truck chassis adjacent to the cab. A cable reel is mounted on the rear end of the truck to provide power, communication and monitoring.

In the event of a failure of the VSCM, an electric cable winch mounted on the rear of the truck, may be used to move the vehicle to a safe position.

Design: Power Plant▶

◀Design: Sizing

Design: Power Plant▶

◀Design: Sizing

Transistors	10¹¹—10¹⁵ neutron/cm²
MOS Transisitors	10⁴ rads (silicon)
Capacitors	10¹⁵ neutrons/cm²
Precision Resistors	10⁷ rads (carbon)/s
	10¹⁴ neutron/cm²
NiCd Batteries	10⁷ rads (air)/s
	10¹³ neutron/cm²
Hg Batteries	10^16 neutron/cm²
Wiring Insulation:
Silicon Rubber	2x10¹⁵ neutron/cm²
Polyethylene	10⁷ rads (carbon)
Teflon TFE	10⁴ rads (carbon)
Teflon FEB	2×10⁶ rads (carbon)
Polyolefins	5×10⁹ rads (carbon)

Characteristics	COTS	Rad Hard
Total Dose	10³—10⁴ rads	10⁵—10⁶
Dose-Rate Upset	10⁶—10⁸ rads(Si)/s	>10⁹ rads(Si)/s
Dose-Rate Induced Latchup	10⁷—10⁹ rads(Si)/s	>10¹² rads(Si)/s
Neutrons	10¹¹—10¹³ n/cm²	10¹⁴—10¹⁵ n/cm²
Single-Event Upset	10^-3—10^-7 error/bit-day	10^-8—10^-10 error/bit-day
Single-Event Latchup
Single-Event Burnout	<20 MeV-cm²/mg (LET)	37-80 MeV-cm²/mg (LET)

Reduction Factor	cm Thickness Required for Neutron Attenuation (+kg/m²)			cm Thickness Required for Gamma Attenuation (+kg/m²)
Φ(x)/Φ(0)	Be	W	LiH₂	Be	W	LiH₂
0.5	622.7 (11,520)	0.571 ( )	0.205 ( )	6.665 ( )	0.564 ( )	15.611 ( )
0.2	1,446 ( )	1.326 ( )	0.477 ( )	15.475 ( )	1.308 ( )	36.249 ( )
0.1	2,069 ( )	1.897 ( )	0.683 ( )	22.141 ( )	1.872 ( )	51.861 ( )
0.01	4,138 ( )	3.794	1.365	44.281	3.744	103.721
0.001	6,206	5.691	2.048	66.421	5.616	155.581

Observation

Two Risks:

NASA’s Astronaut Exposure Rules:

Effectiveness of Shielding Materials:

Lessons for Spacecraft Design:

Exposures Calculated for a Mars Mission at 60 REM/Year (0.6 Sv/year) GCR with Three 1972-Class SFE Events:

Final Comments:

3.7 RADIATION ENVIRONMENT

4.5 ORBITAL ENVIRONMENT

4.5.1 Residual Activation Analysis

Salted Bombs

EMP

History

Simplified operation

Advantages

High speed

Operation at high temperature

Immunity to radiation

Disadvantage

The Problem

The Solution

4.6.2.1 Summary

4.6.2.2 Operations

2.6 ENGINE FLYAWAY MODULE STRUCTURE

3.2 PRIMARY ENGINE SYSTEM

3.3 ENGINE FLYAWAY MODULE

3.3.1 Module Requirements

3.3.2 Module Description

3.3.3 Module Characteristics

A Self-Replicating Radiation-Shield for Human Deep-Space Exploration: Radiotrophic Fungi can Attenuate Ionizing Radiation aboard the International Space Station

Abstract

Introduction

Conclusion

1. INTRODUCTION

Electronic Effects

Biological Effecfs

Secondary Radiation

2. THE RADIATION

Impulsive SPEs

Gradual SPEs

Galactic Cosmic Radiation

3. SHIELDING ALTERNATIVES

Passive (Materials) Shielding

Magnetic Shielding

Radially Symmetric Electrostatic Shielding

Plasma Shielding

Asymmetric Electrostatic Shielding

4. ELECTROSTATICS FEASIBILITY STUDY

Methodology

Simulated Shield Geometry

5. RESULTS

Protons with Nominal Shield Configuration

Electrons with Nominal Shield Configuration

Iron Ions with Nominal Shield Configuration

Shielding Eficiency as a Function of Voltage

Shielding Eficiency as a Function of Geometry

6. DISCUSSION OF FEASIBILITY

7. CONCLUSIONS

Kiwi-B2 Description

Radiation Shadow Shield

Balancing "Reactor — Tank" Separation with Radiation Shield Weight

7.3.1. BRIDGE MOUNTED SHIELDED CAB MANIPULATOR (BMSCM)

7.3.10.2. Overhead Bridge Manipulator

7.3.2. VEHICLE SHIELDED CAB MANIPULATOR (VSCM)