Fission product


Fission product

Fission products are the atomic fragments left after a large nucleus fissions. Typically, a large nucleus like Uranium fissions by splitting into two smaller nuclei, along with a few neutrons and a large release of energy in the form of heat (kinetic energy of the nuclei), gamma rays and neutrinos. The two smaller nuclei are the "fission products".

Formation and decay

The sum of the atomic weight of the two atoms produced by the fission of one atom is always less than the atomic weight of the original atom. This is because some of the mass is lost as free neutrons and large amounts of energy.

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 almost always more neutron-rich than stable nuclei of the same mass as the fission product (e.g. stable ruthenium-100 is 56% neutrons, stable xenon-134 is 60%). The initial fission products therefore may be unstable and typically undergo beta decay towards stable nuclei, converting a neutron to a proton with each beta emission. (Fission products do not emit alpha particles.)

A few neutron-rich and short-lived initial fission products first decay by emitting a neutron. This is the source of 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 halflife and release less energy. Exceptions are:
*Sr-90 (high energy beta, halflife 30 years)
*Cs-137 (high energy gamma, halflife 30 years)
*Sn-126 (even higher energy gamma, but long halflife of 230,000 years means a slow rate of radiation release, and the yield of this nuclide per fission is very low)

Yield

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 % per parent fission; therefore, yields total to 200% not 100%.

While fission products include every element from zinc through the lanthanides, the majority of the fission products occurs 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 exact yield is somewhat dependent on the parent atom, and also on the energy of the initiating neutron. [http://www.science.uwaterloo.ca/~cchieh/cact/nuctek/fissionyield.html]

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 Pu-239 has a more shallow valley than that observed for U-235 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.

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. (But note that the curve against mass number is smooth [http://www.science.uwaterloo.ca/~cchieh/cact/nuctek/fissionyield.html] .)

Characteristics

The adjacent tables provides information on the half-life, yield and decay energies for some more important fission products. A more detailed description of individual products is provided in Fission products (by element) and in Long-lived fission products, and in articles on specific radionuclides.

The following chart provides information on the electronegativity of the fission products.

Fission product production

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 can be used to provide a method of dating old materials.

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. More details are provided in the linked article.

Fission products are produced in nuclear weapons, with the amount depending on the type of weapon.

The largest source of fission products is from nuclear reactors. In current nuclear power reactors, a small percentage of the uranium in the fuel is converted into fission products as an unavoidable by-product of energy generation. Most of these fission products remain in the fuel unless there is fuel failure, or an accident, or the fuel is reprocessed.

Applications

Supply of radioactive isotopes

Some fission products (such as Cs-137) are used in medical and industrial radioactive sources.

Nuclear reactor control

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 release 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.

Nuclear reactor poisons

Some of the fission products have a high neutron absorption capacity, such as xenon-135 and samarium-149. Since a nuclear reactor depends on a balance in the neutron production and absorption rates, these fission product remove neutrons from the reactor and will tend to shut the reactor down or "poison" the reactor. Nuclear fuels and reactors are designed to address this phenomena through such features as burnable poisons and control rods. More details are provided in the article on nuclear reactor poisons.

Fission Product Decay with time

For fission of Uranium-235 the most common radioactive fission products include isotopes of iodine, caesium, strontium, xenon and barium. It is important to understand that the size of the threat becomes smaller with the passage of time, locations where radiation fields which posed immediate mortal threats (such as much of the Chernobyl power plant on day one of the accident and the ground zero sites of Japanese atomic bombings [6 hours after detonation] ) are now safe as the radioactivity has decayed away. Please for instance see the graph below of the gamma dose rate due to Chernobyl fallout as a function of time after the accident. Many of the fission products decay through very shortlived isotopes to form stable isotopes, but also a considerable number of the radioisotopes have half lives longer than a day.

The radioactivity in the fission product mixture is mostly short lived isotopes such as I-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. Note that in the case a release of radioactivity from a power reactor or used fuel that 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.

Fission products in power reactors

In a nuclear power reactor, the main types of radioactivity are fission products, actinides and activation products. Fission products are the largest amount 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 (as noted above) 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 shutdown and the fission reactions stopped. It is this decay heat that sets the requirements for cooling of a reactor after shutdown. More details on these topics are provided in the articles on nuclear power plants and used nuclear fuel.

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 among other purposes, will tend to remove such fission products. In a well-designed power reactor running under normal conditions, the radioactivity of the coolant is very low.

Fission products in 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 Cs-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 (due to an atom bomb) is mostly caused by short-lived isotopes such as I-131 and Ba-140. After about four months Ce-141, Zr-95/Nb-95, and Sr-89 represent the largest share of radioactive material. After two to three years, Ce-144/Pr-144, Ru-106/Rh-106, and Promethium-147 are 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.

Countermeasures against the worst fission products found in accident fallout

The purpose of radiological emergency preparedness is to protect people from the effects of radiation exposure after an accident at a nuclear power plant. Evacuation is the most effective protective measure in the event of a radiological emergency because it protects the whole body (including the thyroid gland and other organs) from all radionuclides and all exposure pathways. However, in situations where evacuation is impossible, calling for in-place sheltering, there are measures which lend some degree of protection against harmful radioisotopes

The mixture of radioactive fission products found in the fallout from a nuclear bomb are very different in nature to those found in spent power reactor fuel. This is because the reactor fuel will have had more time for the short lived isotopes to decay, and because for many accident types that the volatile elements are liberated while the involitiles are retained at the accident site. As a result the contribution of many shortlived (eg 97Zr) and/or involtiles to the off site gamma dose is less for accident fallout than it is for local fallout from a bomb detonation.

Iodine

At least three isotopes of iodine are important. 129I, 131I (Radioiodine) and 132I. An overview of iodine exposure in the USA (resulting from bomb tests) can be seen at [http://rex.nci.nih.gov/massmedia/Fallout/contents.html] . Open air nuclear testing and the Chernobyl disaster both released iodine-131.

The shortlived isotopes of iodine are particularly harmful because the thyroid collects and concentrates iodide -- radioactive as well as non-radioactive -- for use in the production of metabolic hormones. 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 [G. Mushkacheva, E. Rabinovich, V. Privalov, S. Povolotskaya, V. Shorokhova, S. Sokolova, V. Turdakova, E. Ryzhova, P. Hall, A. B. Schneider, D. L. Preston, and E. Ron, "Thyroid Abnormalities Associated with Protracted Childhood Exposure to 131I from Atmospheric Emissions from the Mayak Weapons Facility in Russia", "Radiation Research", 2006, 166(5), 715-722] has resulted in an increase in the incidence of thyroid cancer in the former Soviet Union.

One measure which may protect against this risk is taking large doses of potassium iodide before exposure to radioiodine -- the non-radioactive iodide 'saturates' the thyroid, causing less of the radioiodine to be stored in the body. Because this countermeasure simply takes advantage of the pharmacokinetics regarding iodide uptake, it affords no protection against other causes of radiation poisoning.

Administering potassium iodide reduces the effects of radio iodine by 99%, and is a prudent, inexpensive supplement to sheltering. The Food and Drug Administration (FDA) has approved potassium iodide as an over-the-counter medication. As with any medication, individuals should check with their doctor or pharmacist before using it.

A low-cost alternative to commercially available iodine pills is a saturated solution of potassium iodide. It usually possible to obtain several thousand doses for prices near US$ 0.01/dose. Long term storage of KI is normally in the form of reagent grade crystals, which are convenient and available commercially. The purity is superior to "pharmacologic grade". Its concentration depends only on temperature, which is easy to determine, and the required dose is easily administered by measuring the required volume of the liquid. At room temperature, the U.S. standard adult radiological protective dose of 130mg is four drops of a saturated solution. A baby's dose is 65mg, or two drops. It should be noted that these doses are sufficient to cause nausea and sometimes emesis in most individuals. It's normally administered in a ball of bread, because it tastes incredibly bad. Use is contraindicated in individual known to be allergic to iodine; for such persons sodium perchlorate is one alternative (see chap 13, Kearney).

#Cresson Kearny, Nuclear War Survival Skills, available on line at [http://www.oism.org/ Oregon Institute of Science and Medicine] , created with the permission of the author. The information on KI is near the end of chapter 13. This manual has proven technical info on expedient fallout shelters, and assorted shelter system needs that can be created from common household items. OISM also offers free downloads of other civil defense and shelter information as well.

Caesium

The Chernobyl accident released a large amount of caesium isotopes, these were dispersed over a wide area. For instance they can be found in the soil of France at low levels while in some areas of the former Soviet Union the concentration in soil is sometimes much higher. For a review of the methods used to decontaminate an urban environment please see the scope report [http://www.icsu-scope.org/downloadpubs/scope50/chapter06.html Behaviour and Decontamination of Artificial Radionuclides in the Urban Environment] . Also see chapter four of the NEA reports [http://citeseer.ist.psu.edu/cache/papers/cs/28102/http:zSzzSzwww.nea.frzSzhtmlzSzrpzSzchernobylzSzchernobyl-1995.pdf/chernobyl-ten-years-on.pdf Chernobyl ten years on] and [http://www.nea.fr/html/rp/reports/2003/nea3508-chernobyl.pdf Chernobyl twenty years on] for details of how farming methods can be changed to reduce the impact of accident fallout.

Prussian blue

In livestock farming an important countermeasure against 137Cs is to feed to animals a little prussian blue. This iron potassium cyanide compound acts as a ion-exchanger. The cyanide is so tightly bonded to the iron that it is safe for a human to eat 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. This is a constant which can not be changed but the biological half life is not a constant. It will change according to the nature and habits of the organism it is expressed for. 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 humans or animals is a special grade. Attempts to use the pigment grade used in paints have not been successful. For further details of the use of prussian blue please see the IAEA report on the Goiânia accident. [http://www-pub.iaea.org/MTCD/publications/PDF/Pub815_web.pdf]

Ploughing or the removal of the top layer

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 after a nuclear war or serious accident the removal of top few cm of soil and its burial in a shallow trench will reduce the long term gamma dose to humans due to 137Cs as the gamma photons will be attenuated by their passage through the soil. The deeper and more remote the trench is, the better the degree of protection which will be afforded to the human population.

Release from the Chernobyl fire

More details about the caesium release from the Chernobyl accident can be found at [http://www.icsu-scope.org/downloadpubs/scope50/chapter02.html#t2.7] . A definitive report on Chernobyl is at [http://www.nea.fr/html/rp/chernobyl/allchernobyl.html] - table 1 in chapter two lists the radioisotopes released in the fire. The percentage of the inventory which was released was controlled largely by how volatile the fission product is. Hence a greater proportion of the xenon and iodine than the cerium and plutonium were released.

Strontium

Also by the addition of lime to soils which are poor in calcium the uptake of strontium by plants can be reduced, likewise in areas where the soil is low in potassium, the addition of a potassium fertiliser can discourage the uptake of caesium 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.

Fission products within the back end of the nuclear fuel cycle

Caesium

It is known that the isotope responsible for the majority of the external gamma exposure in fuel reprocessing plants (and the Chernobyl site in 2005) is Cs-137. 137Cs does appear to be an indicator of nuclear fission, as it is only formed by nuclear fission of an actinide.

137Cs is often removed from waste waters in the nuclear industry by means of solid ion exchangers. A range of zeolites can be used for this task. In nuclear reactors both 137Cs and 90Sr are found in locations remote from the fuel, this is because these isotopes are formed by the beta decay of noble gases (xenon-137 {halflife of 3.8 minutes}and krypton-90 {halflife 32 seconds}) which enable these isotopes to be deposited in locations remote from the fuel (eg on control rods and in the space inside a fuel pin between the fuel and the cladding)

Iodine

133I decays by beta particle decay (with a half life of 20.8 hours) to 133Xe which in turn decays by beta decay (with a half life of 5.2 days) to 133Cs. The isotopes which decay to 133I are very short lived. 129I is very long lived and this is one of the major radioactive elements which enter the sea from reprocessing plants.

Fission products which form anions

Some fission products are very long lived, examples of these include iodine-129 and technetium-99. Both of these are very mobile in solid/water as they form anionic species (Iodide and 99TcO4-).

Absorption of fission products on metal surfaces

Tc

It is interesting to note that in common with chromate and molybdate that 99TcO4- ion can react with steel surfaces to form a corrosion resistant layer. In this way these metaloxo anions act as anodic corrosion inhibitors. 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 (eg submarine reactors which have been lost at sea). This 99TcO2 layer renders the steel surface passive, it inhibits the anodic corrosion reaction.

I

In a similar way the release of iodine-131 in a serious power reactor accident could be retarded by absorption on metal surfaces within the nuclear plant. A PhD thesis [http://www.nc.chalmers.se/AVHANDL/DRAVH/HGLAN.HTM] was written on this subject at The Nuclear chemistry department [http://www.nc.chalmers.se/PUBLIC.HTM] at Chalmers University of Technology in Sweden.

* H. Glänneskog. Interactions of I2 and CH3I with reactive metals under BWR severe-accident conditions, "Nucl. Engineering and Design", 2004, 227, pages 323-329.

* H. Glänneskog. Iodine chemistry under severe accident conditions in a nuclear power reactor, Ph.D. Thesis, Chalmers University of Technology, October, 2005.

A lot of other work on the iodine chemistry which would occur during a bad accident has been done. [http://www.sbf.admin.ch/htm/services/publikationen/international/frp/eu-abstracts/html/fp/fp5/5eu99.0423.html] [http://www.nea.fr/html/nsd/docs/2000/csni-r2000-12.pdf] [http://www.ing.unipi.it/~dimnp/CD/supporto/pdf/paci03.pdf]

References

Radioactivity, Ionizing Radiation and Nuclear Energy, by J. Hala and J.D. Navratil

[http://www.em.doe.gov/stakepages/wmdi_llwapxc.aspx DOE: Key Radionuclides and Generation Processes]


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