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| Nuclear physics |
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High-energy processes |
Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoesnuclear fission. Typically, a largenucleus like that ofuranium fissions by splitting into two smaller nuclei, along with a fewneutrons, the release of heat energy (kinetic energy of the nuclei), andgamma rays. The two smaller nuclei are thefission products. (See alsoFission products (by element)).
About 0.2% to 0.4% of fissions areternary fissions, producing a third light nucleus such ashelium-4 (90%) ortritium (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 undergobeta decay. This releases additional energy in the form ofbeta particles,antineutrinos, andgamma rays. Thus, fission events normally result in beta and additional gamma radiation that begins immediately after, even though this radiation is not produced directly by the fission event itself.
The producedradionuclides have varyinghalf-lives, and therefore vary inradioactivity. For instance,strontium-89 andstrontium-90 are produced in similar quantities in fission, and each nucleus decays bybeta emission. But90Sr has a 30-year half-life, and89Sr a 50.5-day half-life. Thus in the 50.5 days it takes half the89Sr atoms to decay, emitting the same number of beta particles as there were decays, less than 0.4% of the90Sr 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.[1] In fact the short lived products are so predominant that 87 percent decay to stable isotopes within the first month after removal from the reactor core.[2]
The sum of theatomic mass of the two atoms produced by the fission of onefissileatom is always less than the atomic mass of the original atom. This is because some of the mass is lost as freeneutrons, 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 thenucleons inuranium-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. stablezirconium-90 is 56% neutrons compared to unstablestrontium-90 at 58%). The initial fission products therefore may be unstable and typically undergobeta decay to move towards a stable configuration, converting a neutron to aproton with each beta emission. (Most fission products do not decay viaalpha 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-calleddelayed neutrons, which play an important role in control of anuclear reactor.
The first beta decays are rapid and may release high energybeta particles orgamma radiation. However, as the fission products approach stable nuclear conditions, the last one or two decays may have a longhalf-life and release less energy.
Fission products have half-lives of 90 years (samarium-151) or less, except for sevenlong-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 containsactinides. This fuel is produced in the so-called "open" (i.e., nonuclear 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 theIntegral Fast Reactor andmolten salt reactor, use this fact to claim that within 200 years, their fuel wastes are no more radioactive than the originaluranium ore.[3]
Fission products primarily emitbeta radiation, while actinides primarily emitalpha radiation. Many of each also emitgamma 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 ofternary fission.)
While fission products include every element fromzinc through thelanthanides, the majority of the fission products occur in two peaks. One peak occurs at about (expressed by atomic masses 85 through 105)strontium toruthenium while the other peak is at abouttellurium toneodymium (expressed by atomic masses 130 through 145). 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 thefissile atom increases, the valley between the two peaks becomes more shallow.[4]For instance, the curve of yield against mass for239Pu has a more shallow valley than that observed for235U when the neutrons arethermal neutrons. The curves for the fission of the lateractinides tend to make even more shallow valleys. In extreme cases such as259Fm, only one peak is seen; this is a consequence of symmetric fission becoming dominant due toshell effects.[5]
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 witheven 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.[6]
Small amounts of fission products are naturally formed as the result of eitherspontaneous fission of natural uranium, which occurs at a low rate, or as a result of neutrons fromradioactive decay or reactions withcosmic ray particles. The microscopic tracks left by these fission products in some natural minerals (mainlyapatite andzircon) are used infission track dating to provide the cooling (crystallization) ages of natural rocks. The technique has an effectivedating 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, anatural 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 innuclear weapon explosions, with the amount depending on the type of weapon.The largest source of fission products is fromnuclear reactors. In currentnuclear 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 isfuel element failure or anuclear accident, or the fuel isreprocessed.
Commercialnuclear fission reactors are operated in the otherwise self-extinguishingprompt subcritical state. Certain fission products decay over seconds to minutes, producing additionaldelayed neutrons crucial to sustaining criticality.[7][8] An example isbromine-87 with a half-life of about a minute.[9] Operating in thisdelayed critical state, power changes slowly enough to permit human and automatic control. Analogous tofire dampers varying the movement of wood embers towards new fuel,control rods are moved as the nuclearfuel burns up over time.[10][11][12][13]
In a nuclear power reactor, the main sources of radioactivity are fission products along withactinides andactivation products. Fission products are most of the radioactivity for the first several hundred years, while actinides dominate roughly 103 to 105 years after fuel use.
Most fission products are retained near their points of production. They are important to reactor operation not only because some contribute delayed neutrons useful for reactor control, but some are neutron poisons that inhibit the nuclear reaction. Buildup of neutron poisons is a key tohow long a given fuel element can be kept in the reactor. Fission product decay also generates heat that continues even after the reactor has been shut down and fission stopped. Thisdecay heat requires removal after shutdown; loss of this cooling damaged the reactors atThree Mile Island andFukushima.
If the fuelcladding around the fuel develops holes, fission products can leak into the primarycoolant. Depending on the chemistry, they may settle within thereactor core or travel through the coolant system and chemistry control systems are provided to remove them. In a well-designed power reactor running under normal conditions, coolant radioactivity is very low.
The isotope responsible for most of the gamma exposure infuel reprocessing plants (and the Chernobyl site in 2005) iscaesium-137.Iodine-129 is a major radioactive isotope released from reprocessing plants. In nuclear reactors both caesium-137 andstrontium-90 are found in locations away from the fuel because they're formed by thebeta decay ofnoble gases (xenon-137, with a 3.8-minute half-life, andkrypton-90, with a 32-second half-life) which enable them to be deposited away from the fuel, e.g. oncontrol rods.
Some fission products decay with the release ofdelayed neutrons, important to nuclear reactor control.
Other fission products, such asxenon-135 andsamarium-149, have a high neutron absorptioncross section. Since a nuclear reactor must balance neutron production and absorption rates, fission products that absorb neutrons tend to "poison" or shut the reactor down; this is controlled with burnable poisons and control rods. Build-up of xenon-135 during shutdown or low-power operation may poison the reactor enough toimpede restart or interfere with normal control of the reaction during restart or restoration of full power. This played a major role in theChernobyl disaster.
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, the134Cs/137Cs ratio provides an easy method of distinguishing between fallout from a bomb and the fission products from a power reactor. Almost nocaesium-134 is formed by nuclear fission (becausexenon-134 is stable). The134Cs is formed by theneutron activation of the stable133Cs which is formed by the decay of isotopes in theisobar (A = 133). So in a momentary criticality, by the time that theneutron flux becomes zero too little time will have passed for any133Cs to be present. While in a power reactor plenty of time exists for the decay of the isotopes in theisobar to form133Cs, the133Cs thus formed can then be activated to form134Cs only if the time between the start and the end of the criticality is long.
According to Jiri Hala's textbook,[14] the radioactivity in the fission product mixture in anatom bomb is mostly caused by short-lived isotopes such asiodine-131 andbarium-140. After about four months,cerium-141,zirconium-95/niobium-95, andstrontium-89 represent the largest share of radioactive material. After two to three years,cerium-144/praseodymium-144,ruthenium-106/rhodium-106, andpromethium-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 istechnetium-99 that dominates.
Some fission products (such as137Cs) are used in medical and industrialradioactive sources.99TcO4− (pertechnetate) ion can react with steel surfaces to form acorrosion resistant layer. In this way these metaloxo anions act asanodiccorrosion inhibitors - it renders the steel surface passive. The formation of99TcO2 onsteel surfaces is one effect which will retard the release of99Tc fromnuclear waste drums and nuclear equipment which has become lost prior todecontamination (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 byadsorption on metal surfaces within the nuclear plant.[15] Much of the other work on the iodine chemistry which would occur during a bad accident has been done.[16]
For fission ofuranium-235, the predominant radioactive fission products include isotopes ofiodine,caesium,strontium,xenon andbarium. The threat becomes smaller with the passage of time. Locations where radiation fields once posed immediate mortal threats, such as much of theChernobyl Nuclear Power Plant on day one of theaccident and theground zero sites ofU.S. atomic bombings in Japan (6 hours after detonation) are now relatively safe because the radioactivity has decreased to a low level.Many of the fission products decay through very short-lived isotopes to formstable isotopes, but a considerable number of theradioisotopes havehalf-lives longer than a day.
The radioactivity in the fission product mixture is initially mostly caused by short lived isotopes such as131I and140Ba; after about four months141Ce,95Zr/95Nb and89Sr take the largest share, while after about two or three years the largest share is taken by144Ce/144Pr,106Ru/106Rh and147Pm. Later90Sr and137Cs are the main radioisotopes, being succeeded by99Tc. 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 airnuclear 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 localfallout shelters and other measures provide the best protection.[18]
At least threeisotopes of iodine are important.129I,131I (radioiodine) and132I. Open airnuclear testing and theChernobyl disaster both released iodine-131.
The short-livedisotopes of iodine are particularly harmful because thethyroid collects and concentratesiodide – radioactive as well as stable. Absorption of radioiodine can lead to acute, chronic, and delayed effects. Acute effects from high doses includethyroiditis, while chronic and delayed effects includehypothyroidism,thyroid nodules, andthyroid cancer. It has been shown that the active iodine released fromChernobyl andMayak[19] has resulted in an increase in the incidence of thyroid cancer in the formerSoviet Union.
One measure which protects against the risk from radio-iodine is taking a dose ofpotassium 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 tofallout shelters. A low-cost alternative to commercially available iodine pills is asaturated solution of potassium iodide. Long-term storage of KI is normally in the form ofreagent-grade crystals.[18]
The administration of knowngoitrogen substances can also be used as aprophylaxis in reducing the bio-uptake of iodine, (whether it be the nutritional non-radioactiveiodine-127 or radioactive iodine, radioiodine - most commonlyiodine-131, as the body cannot discern between different iodineisotopes).Perchlorate ions, a common water contaminant in the USA due to theaerospace industry, has been shown to reduce iodine uptake and thus is classified as agoitrogen. 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).[20]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.[21]
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,[20][22] and perchlorate use at lower doses to treat thyroid problems continues to this day.[23] 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.[20][21]
Current regimens for treatment ofthyrotoxicosis (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.[20][24]
Prophylaxis with perchlorate-containing water at concentrations of 17ppm, 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%[20] 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.[25]
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.[26] this may well be attributable to sufficient daily exposure or intake of healthy iodine-127 among the workers and the short 8 hrbiological half life of perchlorate in the body.[20]
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 250ppm, assuming people drink 2 liters of water per day, to be truly beneficial to the population at preventingbioaccumulation when exposed to a radioiodine environment,[20][24] independent of the availability ofiodate oriodide 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.[27]
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 againstcarcinogenic radioiodine bioaccumulation.
The ingestion of goitrogen drugs is, much like potassium iodide also not without its dangers, such ashypothyroidism. 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 ofcaesium 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 of137Cs, which can be transferred to humans through thefood chain.
One of the best countermeasures indairy farming against137Cs is to mix up the soil by deeply ploughing the soil. This has the effect of putting the137Cs 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 thegamma rays from137Cs will be attenuated by their passage through the soil. The deeper and more remote the trench is, the better the degree of protection.Fertilizers containingpotassium can be used to dilute cesium and limit its uptake by plants.
Inlivestock farming, another countermeasure against137Cs is to feed to animalsprussian blue. This compound acts as anion-exchanger. Thecyanide 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 thebiological half-life (different from thenuclear half-life) of the caesium. The physical or nuclear half-life of137Cs 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 thepigment grade used inpaints have not been successful.[28]
The addition oflime to soils which are poor incalcium can reduce the uptake ofstrontium by plants. Likewise in areas where the soil is low inpotassium, the addition of a potassium fertilizer can discourage the uptake of cesium into plants. However such treatments with either lime orpotash should not be undertaken lightly as they can alter thesoil chemistry greatly, so resulting in a change in the plantecology of the land.[29]
For introduction of radionuclides into an 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.[30]
| Isotope | Radiation | Half-life | GI absorption |
|---|---|---|---|
| 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%[a] |
Paul Reuss,Neutron Physics, chp 2.10.2, p 75
The Live Chart of Nuclides - IAEA Color-map of product yields, and detailed data by click on a nuclide.
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