Fission product

Fission products are the residues of fission processes.

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Physical process of nuclear fission

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 heavy nuclei that can 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 56% ruthenium-100 is 56% neutrons, stable xenon-134 is 60%), and therefore undergo beta decay towards stable nuclei, converting a neutron to a proton with each beta emission. The first beta decays are rapid, but in some fission products, the last one or two decays have a long halflife, so that the fission product which has not yet undergone these last decays is a nuclear waste that must be stored safely for a long time.

Some beta decays and isomeric transitions are accompanied by gamma radiation. However, fission products do not emit alpha particles; only the heavy actinide nuclei produced by neutron absorption without fission do that.

Many of the early decays of short-lived nuclei are high-energy, but the beta and gamma emissions of most of the long-halflife fission product nuclides are of relatively low energy and therefore of relatively less biological risk. 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)

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.

What They Are

The fission products include every element from zinc through to the mid to late lanthanides. The majority of the mass yield of the fission products occurs in two peaks. One peak gives a peak (expressed by atomic number) at about strontium to ruthenium while the other peak is at about tellurium to neodymium

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 Pm-147 are the bulk of the radioactivity. After a few years, the radiation is dominated by Sr-90 and Cs-137, whereas in the period between 10,000 and a million years it is Technetium-99 that dominates. A more detailed description of individual products, organized by element, may be found here

Mass vs. yield curve

If a graph of the mass or mole yield of fission products against the atomic mass of the fragments is drawn then it has two peaks, one in the area zirconium through to palladium and one at Xenon through to neodymium. This is due to the fact that the fission event causes the nucleus to split in an asymmetric manner.[2]

Yield vs. Z - This is a typical distribution for 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. It tends to alternate.

In general the higher the energy of the state that undergoes nuclear fission the more likely a symmetric fission is, 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.

What uses do they have

Fission in nature

A comparison of the isotope signatures of ruthenium and neodymium at the site of the natural nuclear fission reactors in Africa was able to show that in the past nuclear fission had occurred at that site.

Supply of radioactive isotopes

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

The chemical nature of the Fission products

For fission of Uranium-235 the most common radioactive fission products include isotopes of iodine, cesium, 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. Please see Fission products (by element) for a discussion of the main fission products.

Some fission products are useful as beta and gamma sources in medicine and industry, see common beta emitters and commonly used gamma emitting isotopes for more details. Few fission products are alpha particle emitters, but a few lanthanide isotopes which are fission products can decay through alpha emission.

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 to an open air nuclear detonation where all the fission products are dispersed.

Fission products in power reactors

This is a section devoted to the fission products and other radioactivity in the coolant and primary circuit of power reactors. For details of the nature of used nuclear fuel (uranium dioxide or MOX) please see the article which is about these fuels and the article about the spent fuel.

Radioactivity of coolant

In a nuclear reactor, the buildup of fission products as reaction poisons in the fuel eventually leads to loss of efficiency, and in some cases to instability. They contribute most of the short and medium term radioactivity of high-level nuclear waste produced from spent reactor fuel. Depending on the quality of the fuel cladding, fission products can appear in the primary coolant. In a well-designed power reactor running under normal conditions, the radioactivity of the coolant is very low.

Light water reactors

In the BWR reactors the bulk of the activity in the coolant is due to the activation of oxygen to form O-19 and N-16. Because these are very shortlived radioisotopes, the simple erection of a shielding wall around the turbine is enough to protect the staff who anyway are excluded from the turbine hall while the reactor is in use.

Gas cooled reactors

If a fuel element leaks, then noble gas fission products will mix with the coolant. Some of these have half lives long enough to travel to the steam generators or the gas driven turbines (depending on the reactor setup) and there decay into solid elements like isotopes of cesium (such as ¹³⁷Cs) and strontium (such as ⁹⁰Sr).

Isotope signature of an atomic bomb vs. that of used reactor fuel

The examination of the ¹³⁴Cs/¹³⁷Cs ratio is 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.

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 [3]. 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^[1] 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).

1. Cresson Kearny, Nuclear War Survival Skills, available on line at 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.

Cesium

The Chernobyl accident released a large amount of cesium 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 Behaviour and Decontamination of Artificial Radionuclides in the Urban Environment. Also see chapter four of the NEA reports Chernobyl ten years on and 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 cesium. 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. Cesium in humans normally has a biological half life of between one and four months. An added advantage of the prussian blue is that the cesium which is stripped from the animal in the droppings is in a form which is not available to plants. Hence it prevents the cesium 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.[4]

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 cesium release from the Chernobyl accident can be found at [5] . A definitive report on Chernobyl is at [6] - 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 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.

Fission products as nuclear poisons

Main article: Nuclear poison

Some of the fission products generated during a nuclear reaction have a high neutron absorption capacity, such as xenon-135 and samarium-149. Because these two fission product poisons remove neutrons from the reactor, they will have an impact on the thermal utilization factor and thus the reactivity. The poisoning of a reactor core by these fission products may become so serious that the chain reaction comes to a standstill. This can be countered in at least two different ways.

Burnable poison

In some fuels a small amount of a poison such as boron is added, as the reactor operates this is consumed through neutron capture and it helps to counteract the decrease in reactivity caused by the consumption of the fissile isotope and the appearance of fission products in the fuel. Such fuel was involved in the SL-1 reactor.

Control rods

Some plants have one set of control rods which are used to increase the core reactivity as the reactivity of the fuel decreases.

Fission products within the back end of the nuclear fuel cycle

Cesium

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[7] was written on this subject at The Nuclear chemistry department[8] 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.[9][10][11]

References

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

DOE: Key Radionuclides and Generation Processes