Fuel Types

RocketCat sez

Your atomic rocket's fuel is gonna be Uranium-233, Uranium-235, and/or Plutonium-239.

These are rare, so they will also use breeder reactors to turn worthless Uranium-238 into valuable Plutonium-239, and worthless Thorium-232 into valuable Uranium-233. Since the perversity of the universe tends towards a maximum, the ore has about a hundred times more of the worthless stuff than the good stuff. Good ol' O'Toole, you can count on him.

The propellant is gonna be liquid hydrogen, but you knew that already, right?

Plutonium-239 can also be used to make nuclear bombs. This is why atomic fuel gives the authorities sleepless nights. This is also the insane reason that "spent" reactor fuel rods are thrown away even though they contain 85% of their valuable uranium unburnt! Reprocessing the rod to recover the un-burnt uranium would also recover unwelcome freshly-made plutonium. The authorities figure it makes more sense to throw away the valuable uranium rather than being stuck with the job of guarding the plutonium. You never know if Somebody Evil might steal it in order to make atom bombs. Or Somebody Opportunistic might steal it in order to sell it on the black market to Somebody Evil.

For nuclear fission, the main fuel types are Uranium and Plutonium, specifically 235U, 233U, and 239Pu. Plutonium-239 is also used in nuclear weapons. In science fiction stories, these are often called "power metals."

Also very valuable are Thorium-232 and Uranium-238. They are worthless as fuel, but they are about a hundred times more plentiful and an application of neutrons transmutes them into useful fuels (the technical term is "fertile"). 238U transmutes into 239Pu, and 232Th transmutes into 233U. One generally sees these reactions used in a Breeder Reactor or a Thorium Fuel Cycle reactor.

In a breeder reactor, the worthless but fertile Thorium-232 captures a neutron, becoming Thorium-233. It does a beta-decay becoming Protactinium-233. It then does a second beta-decay, becoming valuable Uranium-233. In a breeder, worthless but fertile Uranium-238 does much the same thing, becoming Uranium-239, then Neptunium-239, and finally valuable Plutonium-239.

Currently most of the governments of the world are rather hostile to the idea of breeder reactors, due to fears of nuclear proliferation. It would be different if the breeders produced 235U, but the blasted things make plutonium (aka the sine qua non of nuclear weapons). The governments are also opposed to fuel reprocessing for the same reason. This puts the nuclear industry in the ridiculous position of trying to find ways of safely throwing away used reactor rods that still contain 85% of their valuable 235U un-burnt.

From a commercial power standpoint, it would have made more sense back in the 1940's to have developed thorium power reactors. Unfortunately for commercial power, back then the priority was creating large stockpiles of plutonium for the US military's nuclear weapon needs. Commercial power was only a secondary concern. So plutonium producing uranium reactors were developed instead.

Now that the cold war is over, commercial power is stuck with mature but inconvenient nuclear technology that creates unwanted plutonium. By comparison, thorium reactor technology is very immature. Lots of research money will have to be spent to bring it to maturity. Recently India announced that they were pursuing thorium reactor technology, due to that country's large thorium ore deposits.

Having said that, Luke Campbell points out thorium power reactors are not quite as weapon-proliferation free as the proponents like to think. For one thing it is quite possible to make a nuclear weapon out of 233U. There are some notes about proliferation risks of thorium reactors in this report.


In my recent appearance on The Space Show I made the observation that we have (or, rather, the nation-states of the world have) decided to forego some of the existential opportunities of nuclear technology in order to mitigate the existential risk of nuclear proliferation. We have not entirely eliminated nuclear technology, but what nuclear technology we do use operates under strict regulation intended to prevent the proliferation of fissionable materials.

I said that nuclear technologies represent an existential opportunity because nuclear rockets could have opened up the solar system to human exploration and settlement. Both rocket and nuclear technologies emerged during the Second World War and were rapidly developed during the post-war period. There were many nuclear rocket technologies on the drawing board, and nuclear power might also have provided power for plasma engines or for human settlements in space or on other worlds. But the nuclear rocket industry was strangled in the cradle due to nuclear proliferation concerns (which I discussed in Secrecy and the STEM Cycle).

So, with this in mind, here is my thought experiment, which takes the form of an impossible choice. Suppose that a genie appeared to you (perhaps an evil genie) and offered you this choice: either your society can accept nuclear technology, with all its dangers and opportunities, or it can reject nuclear technology and go without both its dangers and opportunities. Your society can have a proliferation of uses of nuclear technology, with nuclear reactors in cars, trains, ships, and planes, but also the pervasive risks due to easily available fissionables, or nuclear technology can be eliminated or so limited that it poses little threat because it provides few opportunities. Suppose that the genie ups the ante by being a little more specific: accept nuclear technology, and with it you will lose one city every 25 years to a nuclear attack; reject nuclear technology and there will be no more cities lost to nuclear attacks. This is the “trolley problem” for civilization entire, and not merely for a handful of persons.

As impossible as this choice is, human society did, on the whole, face something like this choice. For all practical purposes, human society was offered the genie’s deal, and it chose safety, existential risk aversion, and the foregoing of existential opportunity over a more dangerous world, which latter would also have been a world with greater opportunity.

With the atomic bombs that brought an end to the Second World War, the nuclear genie was out of the bottle. But then something almost unprecedented in human history occurred: though the nuclear genie wasn’t quite stuffed back into the bottle, it was put on a very short leash. The uses of nuclear technology came under intense scrutiny, and fissionables in particular were tightly regulated. Moreover, this regulatory regime was not limited to a few nation-states, but was global in its efficacy, which it had to be, because uranium was mined all over the world, and came to be used all over the world.

Suppose human society had made the other choice, allowing nuclear technology to proliferate, which would have led to dramatic developments, perhaps a nuclear industrial revolution, but at the same time this choice would have meant more nation-states possessing nuclear weapons and terrorist groups being able to acquire nuclear weapons. And, according to the genie’s dictates, we lose a city every 25 years – four cities a century.

Within a couple of hundred years of the beginning of the nuclear era, many of the planet’s largest and busiest cities would have been destroyed – London, Paris, Moscow, Beijing, New York, LA, Cairo, Delhi, and so on – whether in local nuclear conflicts or due to nuclear terrorism. This would mean the deaths of tens of millions of people, and these sites contaminated by radioactivity. But this level of destruction would not be sufficient to cause a generalized collapse of civilization on a planetary scale. Many would die, many more would suffer horribly, but life would go on. There would probably have been some dud devices as well, and a few fizzles, in which the yield of the explosion was too small to destroy the target city. Some targets would dodge the bullet, but not all of them.

This picture of Earth experiencing nuclear conflicts and nuclear terrorism incidents every two or three decades would occur in the context of human civilization experiencing an early spacefaring inflection point and a breakout to spacefaring civilization beginning in the last quarter of the twentieth century. (On early, mediocre, and late breakouts to spacefaring civilization cf. The Spacefaring Inflection Point.) The horror, then, would be counterbalanced by great human accomplishments. Perhaps it might even be the case that accepting the existential risk of nuclear war and nuclear terrorism would have coincided with the overall existential risk to civilization being reduced, because human civilization would have outposts on other worlds, and therefore redundancy.

I’ve formulated horrific thought experiments previously, such as An Horrific Thought Experiment, though in this previous example the mortal risk was to the individual, whereas this present thought experiment involves mortal risk to millions, and perhaps also existential risk to civilization. In retrospect we can see what choice was made, and we can argue if this was the right choice or the wrong choice, but I think that most of us can understand why this choice was made. And I will go farther and speculate that most individuals would have made the same choice.  

John Maynard Keynes once wrote, “There is no subtler, no surer means of overturning the existing basis of society than to debauch the currency. The process engages all the hidden forces of economic law on the side of destruction, and does it in a manner which not one man in a million can diagnose.” I might say the same thing about the future: the lowering of horizons debases our hopes for the future, but not one man in a million understands the true source of despair. Surrendering existential opportunities in order to avoid existential risks is like this: it is a surrender of a part of the future, and the more reduced the future is in stature, the less hope there is in the present. 

Energy Content

RocketCat sez

So, how far can your ship go before it runs out of gasoline? Look at the table, find the fuel your ship uses, and look up the J/kg number (joules per kilogram, if you care). Divide the watts your engine produces by the J/kg and you'll see how many kilograms of uranium or plutonium you'll use up per second of thrust. It will probably be a tiny fraction of a gram, that stuff is potent.

FuelMeV/fissionJ/kg1000 MW burn
6Li? MeV? J/kg? gram/sec
235U202.5 MeV83.14×1012 J/kg0.01208 gram/sec
233U197.9 MeV81.95×1012 J/kg0.01220 gram/sec
239Pu207.1 MeV83.61×1012 J/kg0.01196 gram/sec
245Cm5.623 MeV2.22×1012 J/kg0.450450 gram/sec

In the table, the column you probably will be most interested in is the "1000 MW burn" or "burn rate requred to generate 1000 megawatts." This is how much nuclear fuel must be totally burnt (fissioned) each second to produces 1000 megawatts of thermal energy. As you can see, nuclear energy has a power concentration that makes petroleum look pathetic. The table tells us that if you wanted to generate 1000 megawatts for an entire year (3.15×107 seconds), it would only take a measly 380 kilograms of uranium-235. That's concentrated, a coal-fired power plant typically burns closer to 4 million tons in a year. The equation is:

burnRate = powerReq / Jkg


  • burnRate = nuclear fuel burn rate (kg/sec)
  • powerReq = power to be generated (watts)
  • Jkg = joules per kilogram for the fuel, from table (J/kg)

Say your nuclear lightbulb engine runs on uranium-235 and produces 4,600 megawatts of thermal energy. What is its nuclear fuel burn rate? Remember that 4,600 megawatts = 4,600,000,000 watts

  • burnRate = powerReq / (Jkg)
  • burnRate = 4,600,000,000 / 83,140,000,000,000
  • burnRate = 0.000055 kilograms per second = 0.055 grams per second

Keep in mind that the reactor or engine is probably going to require 2 to 50 kilograms of nuclear fuel to create a critical mass. So even if your reactor only needs to burn a couple of grams a week, the reactor still needs several tens of kilograms of fuel to be present in order to allow the few grams to burn.

Also keep in mind that your fuel rods will become so choked with nuclear poisons that they will stop producing energy even though most of the fuel is un-burnt. See reprocessing below.

The Details

Now I'm going to go into the boring scientific details, so if you are not interested you'd best skip to the next section.

Each fuel type has a certain amount of energy given off when each of its atoms split (or "fission"). This is measured in units called "electron volts" or "eV". For nuclear physics, it is useful to use units of "millions of electron volts" or "MeV". Uranium-235 fissions produces 202.5 MeV per atom, Uranium-233 produces 197.9 MeV and Plutonium-239 produces 207.1 MeV. You can find these values in Wikipedia or any nuclear physics textbook. If you want to calculate the values yourself, the equations are here.

There are 1.602602214179×10-13 joules in 1 MeV, so Uranium-235 fissions produces 3.244×10-11 joules per atom, Uranium-233 produces 3.171×10-11 joules and Plutonium-239 produces 3.318×10-11 joules.

The question then becomes "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 Uranium-235 contains 6.02214179×1023 / 235 = about 2.5626135×1021 atoms per gram.

Now simply multiply each element's joules per fissioned atom by the number of atoms per gram and you'll have the amount of joules produced by totally burning the entire gram of nuclear fuel. For example: Uranium-235 produces 3.244×10-11 joules per fission, times 2.5626135×1021 atoms per gram gives us 8.3131182×1010 joules per gram. Divide by 109 to obtain 83.14 terajoules per kilogram (109 means multiplying by 103to get joules per kilogram then dividing by 1012 to get terajoules per kilogram).

One watt is one joule per second. So if you want to produce 83.14 terawatts, you'll have to burn 1 kilogram per second.


How much deadly radiation does the engine or reactor spew out? That is complicated, but Anthony Jackson has a quick-and-dirty first order approximation:

r = (0.5*kW) / (d2)


  • r = radiation dose (Sieverts per second)
  • kW = thermal power of the engine/reactor. For a reactor this will be greater than the power output of the reactor due to reactor inefficiency (kilowatts)
  • d = distance from the engine/reactor (meters)

This equation assumes that a 1 kW reactor puts out an additional 1.26 kW in penetrating radiation (mostly neutrons) with an average penetration (1/e) of 20 g/cm2.


Let's say that the previously mentioned 4,600 megawatt thermal energy nuclear lightbulb was mounted in a NASA resuable nuclear shuttle. That is 49 meters long, with the crew module as far as it possibly can be from the engine. Remember that 4,600 megawatts = 4,600,000 kilowatts

  • r = (0.5 * kW) / (d2)
  • r = (0.5 * 4,600,000) / (492)
  • r = 2,300,000 / 2,401
  • r = 958 Sieverts per second

80 Sieverts is enough to instantly put a person into a coma, with certain death in 24 hours. Since this is over ten times that amount, per second, this engine is going to need lots of radiation shielding.

Nuclear Fission Product

As any Generation X person growing up in the Atomic Age knows, scientists discovered atomic energy when they "split the atom."

A neutron slams into a uranium-235 nucleus, which promptly explodes into several neutrons, some x-rays or gamma-rays, and of course the two split halves of the nucleus.

The neutrons go on to split other U235 nuclei (the fact there are more than one neutron produce is what allows the reactor to have a self-propagating chain reaction). The x-rays and gamma-rays are harvested by the reactor, they are the nuclear energy or atomic power.

But what about all the split nuclei produced? These are called "nuclear fission products", and are frankly a pain in the butt.

They get in the way of the neutrons, wastefully gobbling up the neutrons and preventing them from making more nuclear energy. After about 15% of the U235 has been burned into energy, the nuclear fuel rod is so clogged with split nuclei that it will no longer sustain a chain reaction. All you can do is remove the fuel rods and send them to a reprocessing plant to filter out the crap so that the rods will sustain a reaction again.

To make it worse, the fission products are incredibly radioactive. They are part of the reason that a nuclear reactor that has been running a while is basically a can full of glowing radioactive death.

This is why atomic rocketeers always carry a bottle of potassium iodide tablets. 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, since one's thyroid gland does its best to suck up iodine, radioactive or not. The tablets fill up the rocketeer's thyroid with safe non-radioactive iodide, so the thyroid won't stuff itself with Iodine-131 and become cancerous.


Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons, the release of heat energy (kinetic energy of the nuclei), and gamma rays. The two smaller nuclei are the fission products. (See also Fission products (by element)).

About 0.2% to 0.4% of fissions are ternary fissions, producing a third light nucleus such as helium-4 (90%) or tritium (7%).

The fission products themselves are usually unstable and therefore radioactive. Due to being relatively neutron-rich for their atomic number, many of them quickly undergo beta decay. This releases additional energy in the form of beta particles, antineutrinos, and gamma rays. Thus, fission events normally result in beta and gamma radiation, even though this radiation is not produced directly by the fission event itself.

The produced radionuclides have varying half-lives, and therefore vary in radioactivity. For instance, strontium-89 and strontium-90 are produced in similar quantities in fission, and each nucleus decays by beta emission. But 90Sr has a 30-year half-life, and 89Sr a 50.5-day half-life. Thus in the 50.5 days it takes half the 89Sr atoms to decay, emitting the same number of beta particles as there were decays, less than 0.4% of the 90Sr atoms have decayed, emitting only 0.4% of the betas. The radioactive emission rate is highest for the shortest lived radionuclides, although they also decay the fastest. Additionally, less stable fission products are less likely to decay to stable nuclides, instead decaying to other radionuclides, which undergo further decay and radiation emission, adding to the radiation output. It is these short lived fission products that are the immediate hazard of spent fuel, and the energy output of the radiation also generates significant heat which must be considered when storing spent fuel. As there are hundreds of different radionuclides created, the initial radioactivity level fades quickly as short lived radionuclides decay, but never ceases completely as longer lived radionuclides make up more and more of the remaining unstable atoms.

Formation and decay

The sum of the atomic mass of the two atoms produced by the fission of one fissile atom is always less than the atomic mass of the original atom. This is because some of the mass is lost as free neutrons, and once kinetic energy of the fission products has been removed (i.e., the products have been cooled to extract the heat provided by the reaction), then the mass associated with this energy is lost to the system also, and thus appears to be "missing" from the cooled fission products.

Since the nuclei that can readily undergo fission are particularly neutron-rich (e.g. 61% of the nucleons in uranium-235 are neutrons), the initial fission products are often more neutron-rich than stable nuclei of the same mass as the fission product (e.g. stable zirconium-90 is 56% neutrons compared to unstable strontium-90 at 58%). The initial fission products therefore may be unstable and typically undergo beta decay to move towards a stable configuration, converting a neutron to a proton with each beta emission. (Fission products do not decay via alpha decay.)

A few neutron-rich and short-lived initial fission products decay by ordinary beta decay (this is the source of perceptible half life, typically a few tenths of a second to a few seconds), followed by immediate emission of a neutron by the excited daughter-product. This process is the source of so-called delayed neutrons, which play an important role in control of a nuclear reactor.

The first beta decays are rapid and may release high energy beta particles or gamma radiation. However, as the fission products approach stable nuclear conditions, the last one or two decays may have a long half-life and release less energy.

Radioactivity over time

Fission products have half-lives of 90 years (samarium-151) or less, except for seven long-lived fission products that have half lives of 211,100 years (technetium-99) or more. Therefore, the total radioactivity of a mixture of pure fission products decreases rapidly for the first several hundred years (controlled by the short-lived products) before stabilizing at a low level that changes little for hundreds of thousands of years (controlled by the seven long-lived products).

This behavior of pure fission products with actinides removed, contrasts with the decay of fuel that still contains actinides. This fuel is produced in the so-called "open" (i.e., no nuclear reprocessing) nuclear fuel cycle. A number of these actinides have half lives in the missing range of about 100 to 200,000 years, causing some difficulty with storage plans in this time-range for open cycle non-reprocessed fuels.

Proponents of nuclear fuel cycles which aim to consume all their actinides by fission, such as the Integral Fast Reactor and molten salt reactor, use this fact to claim that within 200 years, their fuel wastes are no more radioactive than the original uranium ore.

Fission products emit beta radiation, while actinides primarily emit alpha radiation. Many of each also emit gamma radiation.


Each fission of a parent atom produces a different set of fission product atoms. However, while an individual fission is not predictable, the fission products are statistically predictable. The amount of any particular isotope produced per fission is called its yield, typically expressed as percent per parent fission; therefore, yields total to 200%, not 100%. (The true total is in fact slightly greater than 200%, owing to rare cases of ternary fission.)

While fission products include every element from zinc through the lanthanides, the majority of the fission products occur in two peaks. One peak occurs at about (expressed by atomic number) strontium to ruthenium while the other peak is at about tellurium to neodymium. The yield is somewhat dependent on the parent atom and also on the energy of the initiating neutron.

In general the higher the energy of the state that undergoes nuclear fission, the more likely that the two fission products have similar mass. Hence, as the neutron energy increases and/or the energy of the fissile atom increases, the valley between the two peaks becomes more shallow. For instance, the curve of yield against mass for 239Pu has a more shallow valley than that observed for 235U when the neutrons are thermal neutrons. The curves for the fission of the later actinides tend to make even more shallow valleys. In extreme cases such as 259Fm, only one peak is seen; this is a consequence of symmetric fission becoming dominant due to shell effects.

The adjacent figure shows a typical fission product distribution from the fission of uranium. Note that in the calculations used to make this graph, the activation of fission products was ignored and the fission was assumed to occur in a single moment rather than a length of time. In this bar chart results are shown for different cooling times (time after fission). Because of the stability of nuclei with even numbers of protons and/or neutrons, the curve of yield against element is not a smooth curve but tends to alternate. Note that the curve against mass number is smooth.


Small amounts of fission products are naturally formed as the result of either spontaneous fission of natural uranium, which occurs at a low rate, or as a result of neutrons from radioactive decay or reactions with cosmic ray particles. The microscopic tracks left by these fission products in some natural minerals (mainly apatite and zircon) are used in fission track dating to provide the cooling (crystallization) ages of natural rocks. The technique has an effective dating range of 0.1 Ma to >1.0 Ga depending on the mineral used and the concentration of uranium in that mineral.

About 1.5 billion years ago in a uranium ore body in Africa, a natural nuclear fission reactor operated for a few hundred thousand years and produced approximately 5 tonnes of fission products. These fission products were important in providing proof that the natural reactor had occurred. Fission products are produced in nuclear weapon explosions, with the amount depending on the type of weapon. The largest source of fission products is from nuclear reactors. In current nuclear power reactors, about 3% of the uranium in the fuel is converted into fission products as a by-product of energy generation. Most of these fission products remain in the fuel unless there is fuel element failure or a nuclear accident, or the fuel is reprocessed.

Power reactors

In commercial nuclear fission reactors, the system is operated in the otherwise self-extinguishing prompt subcritical state. The reactor specific physical phenomena that nonetheless maintains the temperature above the decay heat level, are the predictably delayed, and therefore easily controlled, transformations or movements of a vital class of fission product as they decay. Delayed neutrons are emitted by neutron rich fission fragments that are called the "delayed neutron precursors." Bromine-87 is one such long-lived "ember", with a half-life of about a minute and thus it emits a delayed neutron upon decay. Operating in this delayed critical state, which depends on the inherently delayed transformation or movement of fission products to maintain the temperature, temperatures change slowly enough to permit human feedback. In an analogous manner to fire dampers varying the opening to control the movement of wood embers towards new fuel, control rods are comparatively varied up or down, as the nuclear fuel burns up over time.

In a nuclear power reactor, the main sources of radioactivity are fission products, alongside actinides and activation products. Fission products are the largest source of radioactivity for the first several hundred years, while actinides are dominant roughly 103 to 105 years after fuel use.

Fission occurs in the nuclear fuel, and the fission products are primarily retained within the fuel close to where they are produced. These fission products are important to the operation of the reactor because some fission products contribute delayed neutrons that are useful for reactor control while others are neutron poisons that tend to inhibit the nuclear reaction. The buildup of the fission product poisons is a key factor in determining the maximum duration a given fuel element can be kept within the reactor. The decay of short-lived fission products also provide a source of heat within the fuel that continues even after the reactor has been shut down and the fission reactions stopped. It is this decay heat that sets the requirements for cooling of a reactor after shutdown.

If the fuel cladding around the fuel develops holes, then fission products can leak into the primary coolant. Depending on the fission product chemistry, it may settle within the reactor core or travel through the coolant system. Coolant systems include chemistry control systems that tend to remove such fission products. In a well-designed power reactor running under normal conditions, the radioactivity of the coolant is very low.

It is known that the isotope responsible for the majority of the gamma exposure in fuel reprocessing plants (and the Chernobyl site in 2005) is caesium-137. Iodine-129 is one of the major radioactive elements released from reprocessing plants. In nuclear reactors both caesium-137 and strontium-90 are found in locations remote from the fuel. This is because these isotopes are formed by the beta decay of noble gases (xenon-137, with a 3.8-minute half-life, and krypton-90, with a 32-second half-life) which enable these isotopes to be deposited in locations remote from the fuel (e.g. on control rods).

Nuclear reactor poisons

Some fission products decay with the release of a neutron. Since there may be a short delay in time between the original fission event (which releases its own prompt neutrons immediately) and the release of these neutrons, the latter are termed "delayed neutrons". These delayed neutrons are important to nuclear reactor control.

Some of the fission products, such as xenon-135 and samarium-149, have a high neutron absorption cross section. Since a nuclear reactor depends on a balance in the neutron production and absorption rates, those fission products that remove neutrons from the reaction will tend to shut the reactor down or "poison" the reactor. Nuclear fuels and reactors are designed to address this phenomenon through such features as burnable poisons and control rods. Build-up of xenon-135 during shutdown or low-power operation may poison the reactor enough to impede restart or to interfere with normal control of the reaction during restart or restoration of full power, possibly causing or contributing to an accident scenario.

Nuclear weapons

Nuclear weapons use fission as either the partial or the main energy source. Depending on the weapon design and where it is exploded, the relative importance of the fission product radioactivity will vary compared to the activation product radioactivity in the total fallout radioactivity.

The immediate fission products from nuclear weapon fission are essentially the same as those from any other fission source, depending slightly on the particular nuclide that is fissioning. However, the very short time scale for the reaction makes a difference in the particular mix of isotopes produced from an atomic bomb.

For example, the 134Cs/137Cs ratio provides an easy method of distinguishing between fallout from a bomb and the fission products from a power reactor. Almost no caesium-134 is formed by nuclear fission (because xenon-134 is stable). The 134Cs is formed by the neutron activation of the stable 133Cs which is formed by the decay of isotopes in the isobar (A = 133). So in a momentary criticality, by the time that the neutron flux becomes zero too little time will have passed for any 133Cs to be present. While in a power reactor plenty of time exists for the decay of the isotopes in the isobar to form 133Cs, the 133Cs thus formed can then be activated to form 134Cs only if the time between the start and the end of the criticality is long.

According to Jiri Hala's textbook, the radioactivity in the fission product mixture in an atom bomb is mostly caused by short-lived isotopes such as iodine-131 and barium-140. After about four months, cerium-141, zirconium-95/niobium-95, and strontium-89 represent the largest share of radioactive material. After two to three years, cerium-144/praseodymium-144, ruthenium-106/rhodium-106, and promethium-147 are responsible for the bulk of the radioactivity. After a few years, the radiation is dominated by strontium-90 and caesium-137, whereas in the period between 10,000 and a million years it is technetium-99 that dominates.


Some fission products (such as 137Cs) are used in medical and industrial radioactive sources. 99TcO4 ion can react with steel surfaces to form a corrosion resistant layer. In this way these metaloxo anions act as anodic corrosion inhibitors - it renders the steel surface passive. The formation of 99TcO2 on steel surfaces is one effect which will retard the release of 99Tc from nuclear waste drums and nuclear equipment which has become lost prior to decontamination (e.g. nuclear submarine reactors which have been lost at sea).

In a similar way the release of radio-iodine in a serious power reactor accident could be retarded by adsorption on metal surfaces within the nuclear plant. Much of the other work on the iodine chemistry which would occur during a bad accident has been done.


For fission of uranium-235, the predominant radioactive fission products include isotopes of iodine, caesium, strontium, xenon and barium. The threat becomes smaller with the passage of time. Locations where radiation fields once posed immediate mortal threats, such as much of the Chernobyl Nuclear Power Plant on day one of the accident and the ground zero sites of U.S. atomic bombings in Japan (6 hours after detonation) are now relatively safe because the radioactivity has decayed to a low level. Many of the fission products decay through very short-lived isotopes to form stable isotopes, but a considerable number of the radioisotopes have half-lives longer than a day.

The radioactivity in the fission product mixture is initially mostly caused by short lived isotopes such as Iodine-131 and 140Ba; after about four months 141Ce, 95Zr/95Nb and 89Sr take the largest share, while after about two or three years the largest share is taken by 144Ce/144Pr, 106Ru/106Rh and 147Pm. Later 90Sr and 137Cs are the main radioisotopes, being succeeded by 99Tc. In the case of a release of radioactivity from a power reactor or used fuel, only some elements are released; as a result, the isotopic signature of the radioactivity is very different from an open air nuclear detonation, where all the fission products are dispersed.

Fallout countermeasures

The purpose of radiological emergency preparedness is to protect people from the effects of radiation exposure after a nuclear accident or bomb. Evacuation is the most effective protective measure. However, if evacuation is impossible or even uncertain, then local fallout shelters and other measures provide the best protection.


At least three isotopes of iodine are important. 129I, 131I (radioiodine) and 132I. Open air nuclear testing and the Chernobyl disaster both released iodine-131.

The short-lived isotopes of iodine are particularly harmful because the thyroid collects and concentrates iodide – radioactive as well as stable. Absorption of radioiodine can lead to acute, chronic, and delayed effects. Acute effects from high doses include thyroiditis, while chronic and delayed effects include hypothyroidism, thyroid nodules, and thyroid cancer. It has been shown that the active iodine released from Chernobyl and Mayak has resulted in an increase in the incidence of thyroid cancer in the former Soviet Union.

One measure which protects against the risk from radio-iodine is taking a dose of potassium iodide (KI) before exposure to radioiodine. The non-radioactive iodide 'saturates' the thyroid, causing less of the radioiodine to be stored in the body. Administering potassium iodide reduces the effects of radio-iodine by 99% and is a prudent, inexpensive supplement to fallout shelters. A low-cost alternative to commercially available iodine pills is a saturated solution of potassium iodide. Long-term storage of KI is normally in the form of reagent-grade crystals.

The administration of known goitrogen substances can also be used as a prophylaxis in reducing the bio-uptake of iodine, (whether it be the nutritional non-radioactive iodine-127 or radioactive iodine, radioiodine - most commonly iodine-131, as the body cannot discern between different iodine isotopes). Perchlorate ions, a common water contaminant in the USA due to the aerospace industry, has been shown to reduce iodine uptake and thus is classified as a goitrogen. Perchlorate ions are a competitive inhibitor of the process by which iodide is actively deposited into thyroid follicular cells. Studies involving healthy adult volunteers determined that at levels above 0.007 milligrams per kilogram per day (mg/(kg·d)), perchlorate begins to temporarily inhibit the thyroid gland’s ability to absorb iodine from the bloodstream ("iodide uptake inhibition", thus perchlorate is a known goitrogen). The reduction of the iodide pool by perchlorate has dual effects – reduction of excess hormone synthesis and hyperthyroidism, on the one hand, and reduction of thyroid inhibitor synthesis and hypothyroidism on the other. Perchlorate remains very useful as a single dose application in tests measuring the discharge of radioiodide accumulated in the thyroid as a result of many different disruptions in the further metabolism of iodide in the thyroid gland.

Treatment of thyrotoxicosis (including Graves' disease) with 600-2,000 mg potassium perchlorate (430-1,400 mg perchlorate) daily for periods of several months or longer was once common practice, particularly in Europe, and perchlorate use at lower doses to treat thyroid problems continues to this day. Although 400 mg of potassium perchlorate divided into four or five daily doses was used initially and found effective, higher doses were introduced when 400 mg/day was discovered not to control thyrotoxicosis in all subjects.

Current regimens for treatment of thyrotoxicosis (including Graves' disease), when a patient is exposed to additional sources of iodine, commonly include 500 mg potassium perchlorate twice per day for 18–40 days.

Prophylaxis with perchlorate-containing water at concentrations of 17 ppm, which corresponds to 0.5 mg/kg-day personal intake, if one is 70 kg and consumes 2 litres of water per day, was found to reduce baseline radioiodine uptake by 67% This is equivalent to ingesting a total of just 35 mg of perchlorate ions per day. In another related study where subjects drank just 1 litre of perchlorate-containing water per day at a concentration of 10 ppm, i.e. daily 10 mg of perchlorate ions were ingested, an average 38% reduction in the uptake of iodine was observed.

However, when the average perchlorate absorption in perchlorate plant workers subjected to the highest exposure has been estimated as approximately 0.5 mg/kg-day, as in the above paragraph, a 67% reduction of iodine uptake would be expected. Studies of chronically exposed workers though have thus far failed to detect any abnormalities of thyroid function, including the uptake of iodine. this may well be attributable to sufficient daily exposure or intake of healthy iodine-127 among the workers and the short 8 hr biological half life of perchlorate in the body.

To completely block the uptake of iodine-131 by the purposeful addition of perchlorate ions to a populace's water supply, aiming at dosages of 0.5 mg/kg-day, or a water concentration of 17 ppm, would therefore be grossly inadequate at truly reducing radioiodine uptake. Perchlorate ion concentrations in a region's water supply would need to be much higher, at least 7.15 mg/kg of body weight per day, or a water concentration of 250 ppm, assuming people drink 2 liters of water per day, to be truly beneficial to the population at preventing bioaccumulation when exposed to a radioiodine environment, independent of the availability of iodate or iodide drugs.

The continual distribution of perchlorate tablets or the addition of perchlorate to the water supply would need to continue for no less than 80–90 days, beginning immediately after the initial release of radioiodine was detected. After 80–90 days passed, released radioactive iodine-131 would have decayed to less than 0.1% of its initial quantity, at which time the danger from biouptake of iodine-131 is essentially over.

In the event of a radioiodine release, the ingestion of prophylaxis potassium iodide, if available, or even iodate, would rightly take precedence over perchlorate administration, and would be the first line of defense in protecting the population from a radioiodine release. However, in the event of a radioiodine release too massive and widespread to be controlled by the limited stock of iodide and iodate prophylaxis drugs, then the addition of perchlorate ions to the water supply, or distribution of perchlorate tablets would serve as a cheap, efficacious, second line of defense against carcinogenic radioiodine bioaccumulation.

The ingestion of goitrogen drugs is, much like potassium iodide also not without its dangers, such as hypothyroidism. In all these cases however, despite the risks, the prophylaxis benefits of intervention with iodide, iodate, or perchlorate outweigh the serious cancer risk from radioiodine bioaccumulation in regions where radioiodine has sufficiently contaminated the environment.


The Chernobyl accident released a large amount of caesium isotopes which were dispersed over a wide area. 137Cs is an isotope which is of long-term concern as it remains in the top layers of soil. Plants with shallow root systems tend to absorb it for many years. Hence grass and mushrooms can carry a considerable amount of 137Cs, which can be transferred to humans through the food chain.

One of the best countermeasures in dairy farming against 137Cs is to mix up the soil by deeply ploughing the soil. This has the effect of putting the 137Cs out of reach of the shallow roots of the grass, hence the level of radioactivity in the grass will be lowered. Also the removal of top few centimeters of soil and its burial in a shallow trench will reduce the dose to humans and animals as the gamma photons from 137Cs will be attenuated by their passage through the soil. The deeper and more remote the trench is, the better the degree of protection. Fertilizers containing potassium can be used to dilute cesium and limit its uptake by plants.

In livestock farming, another countermeasure against 137Cs is to feed to animals prussian blue. This compound acts as an ion-exchanger. The cyanide is so tightly bonded to the iron that it is safe for a human to consume several grams of prussian blue per day. The prussian blue reduces the biological half-life (different from the nuclear half-life) of the caesium. The physical or nuclear half-life of 137Cs is about 30 years. Caesium in humans normally has a biological half-life of between one and four months. An added advantage of the prussian blue is that the caesium which is stripped from the animal in the droppings is in a form which is not available to plants. Hence it prevents the caesium from being recycled. The form of prussian blue required for the treatment of animals, including humans is a special grade. Attempts to use the pigment grade used in paints have not been successful.


The addition of lime to soils which are poor in calcium can reduce the uptake of strontium by plants. Likewise in areas where the soil is low in potassium, the addition of a potassium fertilizer can discourage the uptake of cesium into plants. However such treatments with either lime or potash should not be undertaken lightly as they can alter the soil chemistry greatly, so resulting in a change in the plant ecology of the land.

Health concerns

For introduction of radionuclides into organism, ingestion is the most important route. Insoluble compounds are not absorbed from the gut and cause only local irradiation before they are excreted. Soluble forms however show wide range of absorption percentages.

IsotopeRadiationHalf-lifeGI absorptionNotes
Strontium-90/yttrium-90β28 years30%
Caesium-137β,γ30 years100%
Promethium-147β2.6 years0.01%
Cerium-144β,γ285 days0.01%
Ruthenium-106/rhodium-106β,γ1.0 years0.03%
Zirconium-95β,γ65 days0.01%
Strontium-89β51 days30%
Ruthenium-103β,γ39.7 days0.03%
Niobium-95β,γ35 days0.01%
Cerium-141β,γ33 days0.01%
Barium-140/lanthanum-140β,γ12.8 days5%
Iodine-131β,γ8.05 days100%
Tritiumβ12.3 years100%
From the Wikipedia entry for NUCLEAR FISSION PRODUCT

Nuclear Fuel Cycle


Percent 235UTerm
0.7%Natural Uranium
0.9%-2%Slightly Enriched Uranium
2%-20%Low Enriched Uranium
Low-Enriched Uranium (HALEU)
20%-85%Highly Enriched Uranium (HEU)
85%-100%Weapons-Grade Uranium

The life cycle of nuclear fuel is a complicated subject.

In nature, uranium is found as uranium-238 (99.2742%), uranium-235 (0.7204%), and a very small amount of uranium-234 (0.0054%). This means that only seven-tenths of one percent of a given lump of uranium is useful as fuel. Luckily the 238U can be turned into plutonium fuel by a breeder reactor.

Plutonium does not occur naturally at all.

Pretty much all naturally occurring Thorium is Thorium-232. Thorium is more plentiful than Uranium.

For lack of any better information, I'd assume that the above figures would hold true for uranium deposits on other planets, moons, and asteroids.

Separating the 235U from the 238U (the technical term is "enrichment") is a royal pain. This is because the two are isotopes of the same element, which means quick and easy chemical techniques will not work at all (or only with great difficulty). As far as chemistry is concerned, 235U and 238U are the same thing. Chemistry works on an atom's electron structure, and both isotopes have an identical 92 electrons, of which 6 are valence electrons. The only difference is inside the atomic nucleus, out of the reach of chemistry but vital to nuclear reactions.

There are several uranium enrichment methods, all of which require a very high technology base and are annoyingly expensive. When a rogue nation starts investing in such technology it is cause for alarm.

The dissasembler of a Santa Claus machine can easily create enriched fissionables out of raw ore with its mass spectrometer. It can reprocess fuel rods as well. Which is why they will be strictly controlled by the Santa Guard.

Some heavy-water nuclear power reactors can actually manage to run with the thin gruel of natural uranium, with only 0.7% 235U. Other require Slightly enriched uranium (SEU) with 235U concentration of 0.9% to 2%. Low-enriched uranium (LEU) has a concentration of 235U from 2% to 20%, and is used in light water reactors. Anything above 20% is Highly enriched uranium (HEU) (used in fast-neutron reactors) and above 85% is Weapons-grade uranium (used in nuclear weapons).

Actually, the limit on weapons-grade uranium is a bit more vague than "above 85%." The actual definition is more like "whatever we can make explode." The original Little Boy atomic bomb used 80% enriched uranium.

And the 20% enrichment line separating low-enriched uranium from highly enriched uranium is also a bit arbitrary. The official reason is that below 20% a runaway criticality (atom bomb go boom!) was impossible. This paper explains how that really ain't true. You can have a runaway criticality with as little as 5.5% enrichment. 20% was chosen because:

  • Yes, 20% can be used to make a weapon, but an impractical one
  • 20% will make a worthwhile reactor fuel for a small reactor
  • Politically it would be nice to set a limit on the enrichment allowed to be exported to friendly nations such that the uranium could make a worthwhile reactor instead of a worthless reactor

So 20% was at the political sweet spot, for the year 1954. It is still at 20% due to inertia more than anything else.

I'm still trying to find some solid figures on the levels of enrichment on the reactor elements in a nuclear thermal rocket. The only source I've found suggests it will be from 60% to 93% 235U!! (apparently the old NERVA ran on 90%) In 2014 NASA contracted BWXT to design a NTR engine that would run on 20% enriched uranium, which will make the military less paranoid.

The opposite of enriching is downblending; surplus HEU can be downblended to LEU to make it suitable for use in a power reactor.


      The decision between HEU (in this context, uranium with an enrichment greater than 90 percent) and HALEU (less than 20 percent enrichment) fuel involves more than feasibility and system performance. No such comprehensive assessment that compares the fuel types head-to-head (as distinct from a standalone assessment of the feasibility of an HALEU system) for either an NTP or NEP system was available to the committee. Key factors to be included in a comparative assessment of HEU and HALEU for both systems are as follows:

  • Technical feasibility and difficulty. A HALEU reactor has never been built, tested, or flown for either NTP or NEP applications, and there are no experimental data on the behavior of HALEU NTP reactors to benchmark modeling and simulation codes. In contrast, HEU NTP reactors have been built, tested, and benchmarked using prior M&S software. Technical feasibility and difficulty considerations favor HEU for NTP systems, but they do not clearly favor one fuel enrichment level over the other for NEP systems.

  • Performance. Fuel enrichment affects the performance of the system. For example, the relative mass and size of NTP and NEP systems (including shielding) is a function of fuel enrichment and other parameters such as each reactor’s power level and neutron spectrum (fast vs. moderated). Data from the Rover/NERVA programs provide insight into the operational performance of HEU NTP reactors; equivalent data does not exist for HALEU reactors for NTP or NEP systems. Performance considerations do not clearly favor one fuel enrichment level over the other.

  • Proliferation and security. HEU fuel, by virtue of the ease with which it could be diverted to the production of nuclear weapons, is a higher value target than HALEU, especially during launch and reentry accidents away from the launch site. As a result, HEU is viewed by nonproliferation experts as requiring more security considerations. In addition, if the United States uses HEU for space reactors, it could become more difficult to convince other countries to reduce their use of HEU in civilian applications. Proliferation related concerns also affect other factors such as cost, schedule, the ability of the commercial space sector to participate in reactor development, and the extent to which domestic politics becomes a factor in obtaining launch approval. Proliferation and security considerations favor HALEU.

  • Safety. The selection of fuel enrichment, in conjunction with the reactor’s neutron spectrum, can affect the design approach and difficulty in preventing inadvertent criticality events during launch and reentry accidents. This may require different emergency planning, accident response, and recovery protocols, even if there are no radiological consequences to the public. Safety considerations are design dependent, and do not clearly favor one enrichment level over the other.

  • Fuel availability. It may be possible to obtain HEU from DOE’s National Nuclear Security Administration stockpile. Producing HALEU would either require downblending HEU from the stockpile or enriching lower enriched uranium. The latter would require new infrastructure. DOE is investigating production of HALEU to support nearterm terrestrial power reactor needs, but there are concerns about the long-term availability of HEU. Overall, fuel availability considerations do not clearly favor one enrichment level over the other.

  • Cost. The costs of HEU and HALEU systems differ because of factors such as safeguards and physical security, facilities, fuel procurement and fabrication, and system development. From a launch approval perspective, HEU systems require Presidential approval. While this may have schedule implications, it may not have cost implications as the cost of launch approval will likely be dominated by the safety analysis, which will be similar for HEU and HALEU systems. Cost considerations do not clearly favor one enrichment level over the other.

  • Schedule. Use of different enrichment levels will affect the design, development, testing, and launch preparations schedule. Possible locations for test facilities may be more limited for HEU due to the different security requirements, which could protract schedule, but there is a more historical data on HEU reactors. Schedule considerations do not clearly favor one fuel enrichment level over the other.

  • Supply chain. The use of HEU would restrict the number of private-sector organizations which are able to participate in system development and manufacture. HEU would limit participation to DOE laboratories and the small number of private companies with licenses to work with HEU. Use of HALEU, on the other hand, would permit the involvement of a larger number of private companies and enable a variety of publicprivate partnerships. Supply chain considerations favor HALEU.

     While there is some clarity on each of the criteria above, they are not equally important. Performance, security, and safety concerns are significantly more important than those related to the supply chain. This weighting must be considered prior to making a fuel enrichment decision.

FINDING. Enrichment of Nuclear Fuels. A comprehensive assessment of HALEU versus HEU for NTP and NEP systems that weighs the key considerations is not available. These considerations include technical feasibility and difficulty, performance, proliferation and security, safety, fuel availability, cost, schedule, and supply chain as applied to the baseline mission.

RECOMMENDATION. Enrichment of Nuclear Fuels. In the near term, NASA and DOE, with inputs from other key stakeholders, including commercial industry and academia, should conduct a comprehensive assessment of the relative merits and challenges of highly enriched uranium and high-assay, low-enriched uranium (HALEU) fuels for nuclear thermal propulsion and nuclear electric propulsion systems as applied to the baseline mission.


RocketCat sez

As you burn your uranium fuel rods, the blasted things clog up with nuclear poison crap. Eventually they won't burn any more, even though they still got plenty of fuel in them. Too much clog. All you can do is replace the rods, and send the clogged rods to a fuel reprocessing plant. You better have some spare rods aboard, Captain Peter Peachfuzz, or your ship might be out of gas a long way from nowhere.

The plant will melt 'em down, filter out the crap, and make new rods. Which is a royal pain in the posterior, see Processing above.

The experts ain't gonna tell us how much is "too clogged with poisons" is. Best guess I've read is at 15% fuel burnt (85% unburnt) for standard rods, 50% fuel burnt for rods made out of weapons-grade uranium salted with burnable poisons.

As a fuel rod undergoes a chain reaction, it gradually fills up with nuclear poisons. Eventually it is so full of poisons that it will no longer react. There is still plenty of fuel left in the rod (only about 15% of the fuel has been burnt), but it is too clogged with poison. The rod has to be removed and sent to a fuel reprocessing plant. The plant filters out the poisons and can recover 55 to 95% of the un-burnt fuel, to be made into a new fuel rod.

With reprocessing, in the long term each totally consumed kilogram of plutonium or highly enriched uranium (HEU) will yield ~1 × 1010 newton-seconds of impulse at a specific impulse of ~1000 seconds. Dr. John Schilling also warns that there is a minimum amount of fissionable material for a viable reactor. Figure a minimum of 50 kilograms of HEU.

The higher the level of enrichment, the longer the fuel rod can burn until it becomes clogged with nuclear poisons. That's why the nuclear thermal rocket uses HEU (or even weapons-grade) instead of LEU.

Dr. Schilling figures that as an order of magnitude guess, about one day of full power operation would result in enough fuel burnup to require reprocessing.

Another source (a certain Mr. Wilde) suggested that if your rods are weapons grade but salted with "burnable poisons", you could get 10,000 to 20,000 effective full power hours out of your rods. Your rods will become clogged after 50% of the fuel has been burnt, instead of only 15%. At this point, the principal concern starts becoming neutron embrittlement of the reactor vessel rather than fuel burnout. He goes on to say that the purpose of burnable poisons is to allow you to use very highly enriched uranium as fuel during early core life. The highly enriched 235U would generate far too much positive reactivity at BOL (beginning of life) to work with, and this is dampened with elements that absorb neutrons and then decay to or become an isotope of something that which does not (the burnable part), over core life so that the average positive reactivity add of the uranium is relatively even throughout core life.

As another data point, there are some indications that US Navy nuclear submarines use fuel rods that are above 90% 235U. Their reactors are designed to run for 30 years, but the reactors are NOT designed to be re-fueled. The exact details are classified.

One must always keep in mind that all this life-cycle and reprocessing stuff only applies to solid-core rockets (and nuclear-lightbulb close-cycle gas core). Liquid-core and (open-cycle) gas-core nuclear thermal rockets eventually blow all their nuclear fuel out their exhaust nozzles into the vast depths of space, so there is no way to take the expended fuel back to a reprocessing plant. This is why such propulsion systems put a premium on keeping the nuclear fuel inside the reaction chamber as long as humanly possible, if the unburnt nuclear fuel loss is too high such propulsion systems are too uneconomical to be used.

There is another option. closed-cycle gas-core nuclear thermal rockets use uranium gas, not solid rods. This means they can take partially fissioned uranium and send it through an on-board fuel reprocessing plant to filter out the nuclear poisons. A gas-core NTR so equipped could in theory burn every atom of uranium during a flight. This could also be used with a bimodal gas-core NTR, which lets the power-generation mode not waste a single atom.

Extraterrestrial Sources

One theory of solar system formation is that there are more metals in the inner solar system. That would mean most of the uranium can be found at Mars, Mercury, Earth, Luna, Venus and asteroid belt.

Dr. Ethan Siegel is of the opinion that the planet Mercury has more deposits of fissionables than Terra.

Dr. Luke Campbell said "Fissionables dissolve well in rocky stuff, and poorly in iron/nickel. High density in solid stuff floating around in the solar system is usually because of high concentrations of iron/nickel, while lower concentrations mean more rock (and even lower density means ice). So high density is good to look for stuff that dissolves in iron/nickel, like gold and platinum. For uranium and thorium, look in rock." 

Remember that in naturally occurring uranium only 0.72% (that is, 0.0072) of it is actually valuable fissionable uranium 235, the remaining 99.28% are the other worthless uranium isotopes. Also remember there are other fissionable isotopes of uranium besides 235U but they do not occur naturally. Naturally occuring thorium-232 is not fissionable, but neutron bombardment will turn it into fissionable 233U

In 2009, the Japanese Kaguya spacecraft detected uranium with a gamma-ray spectrometer as it orbited the Moon. Unfortunately it detected that uranium was in short supply on the Moon, less than the concentration in terrestrial granite. Uranium and thorium seem to be concentrated in KREEP. Lunar maria regolith is from 0 to 2 ppm uranium and up to 7 ppm thorium. urKreep is about 5 ppm uranium, and some Apollo samples had up to 20 ppm. On Terra, uranium ore is considered to be low grade if it is only 100 ppm.

The uranium/thorium ratio in lunar materials is about 0.27. The Kaguya map has a resolution of only 130 kilometers, so 7 ppm in a pixel could mean a smaller area with far more.

The Mars-5 space probe did detect uranium and thorium on Mars.

The asteroid Vesta is what astronomers call an "evolved object" or "protoplanet." This means it has a distinct core, mantle, and crust; unlike common asteroids that are more homogeneous. Objects become evolved if they are formed with enough radioactive material inside to melt the rock. I am unsure if this implies that Vesta has deposits of uranium large enough to be worth mining, but it's a start (some of my reading suggests that asteroid melting is caused by the decay of Aluminum-26, which is worthless as atomic fuel).

Other known protoplanets are Ceres, Pallas, and Kuiper Belt dwarf planets.

For what it is worth, meteors tend to only have 0.008 ppm uranium.

Unsurprisingly the Virtual Moon Colony Project figures a good place for a colony is at Lalande Crater, for ready access to thorium and KREEP.


So let's say, you've got some compelling reason to mine uranium on Mars. Perhaps a certain deranged Urth president has imposed crippling tariffs on your precious fission fuels, and your atomic rockets'fuel rods are steadily becoming depleted. You can scoop up propellant with a shovel and put it into the propellant tanks, but without the atomic pile to work its hot magic you're going nowhere. What do you do? Well, after rolling into a ball and crying a lot, you can dig out the geology maps for Mars because fortunately it's one of the few places in the solar system besides Earth that you can find anything more than the poorest-grade ores. People tend to assume that asteroids are full of everything from platinum to antimatter and rainbows — they're not.

In theory you can process almost any amount of a given rock and get something out of it — but processing a million tonnes of rock to get even one tonne of uranium is ridiculously uneconomical. Therefore, as with most minerals, you go looking for where it's concentrated.

There are two sorts of questions one has to ask about the possibility of uranium sources besides the Earth. The first question is, is the planet or asteroid core differentiated? That is, was it molten long enough for the elements to separate out into a heavy core, mantle and crust floating on top? The second is, was there some way for the uranium to be concentrated?

Before we talk about that, let's talk about what uranium is like — it's a "lithophile" element, like the rare earths and also thorium. Rare earths go by certain rules, which also sort of apply to U and Th. It makes them a useful geochemical marker, and you can often find uranium where you find rare earths, but that's a loose relationship. So when we talk about finding rare earths we're also talking about the fissile fuels, thorium and uranium. Rare earth tailings are often slightly radioactive because of the leftover U and Th.

But why is it so difficult to get a hold of the rare earths? Well, for a start you need some kind of separation process, and rare earths are notoriously difficult to concentrate in "veins" and so on. A magma sea allows molten minerals to precipitate out of solution and become concentrated. Generally, as magma cools other minerals and elements leave the magma party first by dissolving out as crystals. The rare earth elements are the hard partiers who you can't really get to leave. Eventually, by repeated heating and cooling, all that's left are the rare earths and you get some high purity stuff, along with tough crystals like garnet. But usually that's not the case and you get a mishmash of different minerals with some enrichment of rare earth elements. Generally, you know when a mineral is not going to hang out with rare earths because they are just incompatible due to electron valences etc. For example, olivine is always going to have very little in the way of rare earths.

I'm sure there's a Steven Universe joke in here somewhere.

Most asteroids are never heated to this degree or if they are, are not molten long enough for this to happen. So, they are a random collection of accreted space dirt. You won't find any ore concentration processes happening on them. No veins of uranium unless there happens to have been hit by a freak uranium-rich rock, which will probably turn out to be a derelict atomic rocket. Typically, the concentrations of U in asteroids are in the ranges of less than 1 part per million, which is pathetic as far as ores go. Marginally economic uranium ores on Earth are 100 times richer. This is also why you can't economically mine lunar rare earths — at best they are equivalent to low-grade ores on Earth. So forget it. Nothing short of desperation or some clever way to recover it as byproduct is going to make it worthwhile over simply ordering it from alibaba dot com.

Now, onto the next stage of concentration, which typically involves the use of liquid water — not something you'll ever find on an asteroid.

In general, uranium is leached out of rocks when water is oxidative, that is, has an excess of electrons to give away. Compounds like H2O2 and the notorious perchlorates widely present on Mars can do this. The dissolved uranium then precipitates out under acidic conditions, as would be the case in the drying Martian seas. Then, as it is eroded to sand, it can be sorted by wind and wave action, for example. Processes like this are what makes uranium and rare earths start to part ways, especially the lighter rare earths like lanthanum. There are a large number of ways uranium can become concentrated. It can even be concentrated in plant matter, becoming coal.

Mars had both a differentiated core and the possibility of hydrothermal action. Heck, we know Mars was wet thanks to the various Mars rovers. And, as the planet became dryer, the water became more and more acidic: the right conditions for uranium to precipitate out into deposits.

Wind action, presumably when the atmosphere was more dense, would have worked with wave action to sift sands and gradually shuffle the heaviest stuff to the bottom. This is how mineral sands work. As Mars' seas gradually shrank, the stuff would wind up left behind on the dry seabed.

Martian thorium concentrations. The red zone is about 1ppm.

Thorium is often found in association with uranium, and a useful nuclear fuel in its own right. It has quite a strong gamma decay signal and so is fairly easy to pick up a gamma spectrometer in orbit. The Mars Odyssey orbiter picked up quite a strong concentration of the stuff in Chryse Planitia just where a shrinking ocean would have receded to.

These are probably monazite sands, which are eroded out of granitic rocks which have been subject to fractional crystallisation (that process mentioned above). This was probably fed by volcanic eruptions and lavas from Pavonis Mons etc. to the south-west.

Along with monazite grains, fluorine-enriched minerals from Mars have been found, and even fluorapatite. This is nice because we can make uranium hexafluoride with the fluorine, an important part of the enrichment process (you can't just shovel uranium-rich sand into a nuclear reactor after all). Fluorine also makes a terrifyingly effective rocket fuel. And by terrifying, I mean the rocket engineers have not been able to make a fluorine-based rocket engine that didn't blow up. Fluorine is also necessary for a number of plastics, as well as teflon, used in liquid hydrogen tankage and plumbing and of course non-stick frying pans.

Fluorapatite — Ca5(PO4)3F. Like most minerals, usually occurs just as tiny crystals mixed in with other stuff.

Other potential sources include iron-oxide rich laterite clays, as well as certain carbonate rocks. Laterite clays are normally only found in tropical conditions on Earth, but are expected to be common on Mars. The nice thing about have sands as opposed to veins is that the sands are easy to mine. Just scoop them up — no rock crushing required, which requires extremely heavy duty machinery. Clays are also quite soft and require minimal processing. The rare earths, thorium and uranium can be chemically leached out of the rock with a series of hot acidic and alkaline solutions. It won't be the most efficient operation, but then again you are not trying to sell the stuff on the Earth market — possibly because the Urth-people have already nuked themselves into oblivion.

Perhaps the Asteroid Belters will come and trade rare heavy metals like platinum and rhenium for uranium and dysprosium (an important rare earth for high-strength electric motors).

This handy overview of Martian habitability for both human explorers and potential Martian life shows that the best locations with ice are mostly above or below 30 degrees latitude, which is also where solar power becomes less useful and temperatures drop. Fortunately the Th concentrated regions shown above in Chryse Planitia will have some subsurface ice to support the miners without being too far north as to expose them or their machinery to freezing long Martian winter nights.

So, there you have it. As far as uranium is concerned, there's no place like home (Earth) — but you can at least ask the Martians if they have some.


Today's Ogg-Nat War Trivia

     "Lincoln's Plan" is the Martian codeword for the discovery and exploitation of radioactive elements, during the Revolutionary Charter years (They simply updated the original Martian Charter to remove mention of Earth or the UN, and to focus on their own sovereignty, if you were wondering.). The UN is aware of existing Martian thorium mining, but has been kept out of Martian orbit and is limited in their surveillance. Also, the UN hasn't been particularly successful in penetrating the ASG's secrets, of which Lincoln's Plan is but one of many.

     Why Lincoln? Is he a colonist? Are they talking about the President during the US Civil War? No, they're referring to the Earl of Lincoln, which has for 12 generations lived in Australia. Which happens to have Earth's largest deposits of uranium.

     And why would a relatively egalitarian society like Mars' own, care about baronets?
     Because if you're looking for radioactive ores, you're bound to run across some noble gases.

     (Yes, I insist on keeping this horrible joke in the story. I might switch names but as there's a number of knights and baronets in Australia, I got plenty of material to work with.)
     (Also, there is at least one hydrous mineral, amphibole, that can trap noble gases. But that's not as reliable a source of noble gases, since Mars' mantle solidified over three billion years ago. Best to find some uranium ore and extract the noble gases during refinement — and then capture and recycle them as their reactors produce them.)

From THE OGG-NAT WAR: LIFT UP YOUR EYES by Lilith Dawn (2016)

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This week's featured addition is NTR ALTERNATIVES TO LIQUID HYDROGEN

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