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.
|Fuel||MeV/fission||J/kg||1000 MW burn|
|6Li||? MeV||? J/kg||? gram/sec|
|235U||202.5 MeV||83.14×1012 J/kg||0.01208 gram/sec|
|233U||197.9 MeV||81.95×1012 J/kg||0.01220 gram/sec|
|239Pu||207.1 MeV||83.61×1012 J/kg||0.01196 gram/sec|
|245Cm||5.623 MeV||2.22×1012 J/kg||0.450450 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)
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.
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.
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.
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)).
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.
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.
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 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.
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.
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.
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.
Isotope Radiation Half-life GI absorption Notes Strontium-90/yttrium-90 β 28 years 30% Caesium-137 β,γ 30 years 100% Promethium-147 β 2.6 years 0.01% Cerium-144 β,γ 285 days 0.01% Ruthenium-106/rhodium-106 β,γ 1.0 years 0.03% Zirconium-95 β,γ 65 days 0.01% Strontium-89 β 51 days 30% Ruthenium-103 β,γ 39.7 days 0.03% Niobium-95 β,γ 35 days 0.01% Cerium-141 β,γ 33 days 0.01% Barium-140/lanthanum-140 β,γ 12.8 days 5% Iodine-131 β,γ 8.05 days 100% Tritium β 12.3 years 100%
|0.9%-2%||Slightly Enriched Uranium|
|2%-20%||Low Enriched Uranium|
Low-Enriched Uranium (HALEU)
|20%-85%||Highly Enriched Uranium (HEU)|
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.
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.
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).
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.