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On this page, a discussion of each of the main elements in the fission product mixture from the nuclear fission of an actinide such as uranium or plutonium is set out by element.

Krypton 83-86

Krypton-85 is formed by the fission process with a fission yield of about 0.3%. Only 20% of the fission products of mass 85 become ⁸⁵Kr itself; the rest passes through a short-lived nuclear isomer and then to stable ⁸⁵Rb. If irradiated reactor fuel is reprocessed, this radioactive krypton is released into the air. This krypton release can be detected and used as a means of detecting clandestine nuclear reprocessing. Strictly speaking, the stage which is detected is the dissolution of used nuclear fuel in nitric acid, as it is at this stage that the krypton and other fission gases like the more abundant xenon are released.

Increase of fission gases above a certain limit can lead to fuel pin swelling and even puncture, so that fission gas measurement after discharging the fuel from the reactor is most important to make burn-up calculations, to study the nature of fuel inside the reactor, behaviour with pin materials, for effective utilisation of fuel and also reactor safety.

Rubidium 85,87

Strontium 88-90

The strontium radioisotopes are very important as strontium is a calcium mimic which is incorporated in bone growth and therefore has a great ability to harm humans. On the other hand, this also allows ⁸⁹Sr to be used in the open source radiotherapy of bone tumors. This tends to be used in palliative care to reduce the pain due to secondary tumors in the bones.

Strontium-90 is a strong beta emitter with a half-life of 28.8 years. Its fission product yield decreases as the mass of the fissile nuclide increases. A map of ⁹⁰Sr contamination around Chernobyl has been published by the IAEA. [1]

Yttrium 89

The only stable yttrium isotope, ⁸⁹Y, will be found with yield somewhat less than 1% in a fission product mixture which has been allowed to age for months or years, as the other isotopes have half-lives of 106.6 days or less.

⁹⁰Sr decays into ⁹⁰Y which is a beta emitter with a half life of 2.67 days. ⁹⁰Y is sometimes used for medical purposes and can be obtained either by the neutron activation of stable ⁸⁹Y or by using a device similar to a technetium cow.

Zirconium 90-96

A significant amount of zirconium is formed by the fission process; some of this are short-lived radioactives (⁹⁵Zr and ⁹⁷Zr which decay to molybdenum), while almost 10% of the fission products mixture after years of decay consists of five stable or nearly stable isotopes of zirconium plus ⁹³Zr with a halflife of 1.53 million years which is one of the 7 major long-lived fission products.

In PUREX plants the zirconium sometimes forms a third phase which can be a disturbance in the plant. The third phase is the term in solvent extraction given to a third layer (such as foam and/or emulsion) which forms from the two layers in the solvent extraction process. The zirconium forms the third phase by forming small particles which stabilise the emulsion which is the third phase.

Molybdenum 95, 97, 98, 100

The fission product mixture contains significant amounts of molybdenum.

Technetium 99

⁹⁹Tc is produced at a yield of about 6% per fission; see also the main fission products page.

Ruthenium 101-106

Plenty of both stable ruthenium and radioactive ruthenium-103 is formed by the fission process. The ruthenium in PUREX raffinate can become oxidized to form ruthenium tetroxide which forms a purple vapour to appear above the surface of the aqueous liquor. The ruthenium tetroxide is very similar to osmium tetroxide, the ruthenium compound is a stronger oxidant which enables it to form deposits by reacting with other substances. In this way the ruthenium in a reprocessing plant is very mobile and can be found in odd places. Also at Chernobyl during the fire the ruthenium became volatile and behaved differently to many of the other metallic fission products. Some of the particles which were emitted by the fire were very rich in ruthenium.

In addition the ruthenium in PUREX raffinate forms a large number of nitrosyl complexes which makes the chemistry of the ruthenium very complex. The ligand exchange rate at ruthenium and rhodium tends to be long, hence it can take a long time for a ruthenium or rhodium compound to react.

It has been suggested that the ruthenium and palladium in PUREX raffinate should be used as a source of the metals [2][3].

Rhodium 103

While less rhodium than ruthenium and palladium is formed (around 3.6% yield), the mixture of fission products still contains a significant amount of this metal. Due to the high prices of ruthenium, rhodium and palladium some work has been done on the separation of these metals to enable them to be used at a later date. Because of the possibility of the metals being contaminated by radioactive isotopes, metals are not suitable for making consumer products such as jewellery but this source of the metals could be used for catalysts in industrial plants such as petrochemical plants.

Potential Applications of Fission Platinoids in Industry, Zdenek Kolarik, Platinum Metals Review, 2005, 49, April (2).[4]

A dire example of people being exposed to radiation from contaminated jewellery occurred in the USA where it is thought that the gold seeds which were used to contain radon were recycled into jewellery. The gold did contain radioactive decay products of ²²²Rn. Further details can be found at [5] and [6].

Palladium 105-110

A great deal of palladium forms during the fission process, during the fuel dissolution not all of the fission palladium dissolves. Hence the dissolver fines are rich in palladium, also some palladium dissolves at first and then comes out of solution again. The dissolver fines are often removed to prevent them disturbing the solvent extraction process through the stabilisation of the third phase.

The fission palladium can separate during the process in which the PUREX raffinate is combined with glass and heated to form the final waste form. The palladium forms an alloy with the fission tellurium. This alloy can separate from the glass.

Silver 109

Cadmium 111-116

Indium

Tin 117-126

Antimony 121,123

Tellurium 125, 128, 130

Tellurium-132 and its daughter ¹³²I are important in the first few days after a criticality. It was responsible for a large fraction of the dose inflicted on workers at Chernobyl in the first week.

The isobar forming ¹³²Te/¹³²I is: Tin-132 (half life 40 s) decaying to antimony-132 (half life 2.8 minutes) decaying to tellurium-132 (half life 3.2 days) decaying to iodine-132 (half life 2.3 hours) which decays to stable xenon-132.

Iodine 129, 131

¹³¹I and ¹²⁹I are made by the fission process. In common with ⁸⁹Sr, ¹³¹I is used for the treatment of cancer. A small dose of ¹³¹I can be used in a thyroid function test while a large dose can be used to destroy the thyroid cancer. This treatment will also normally seek out and destroy any secondary tumor which arose from a thyroid cancer. Much of the energy from the beta emission from the ¹³¹I will be absorbed in the thyroid, while the gamma rays are likely to be able to escape from the thyroid to irradiate other parts of the body.

Lots of ¹³¹I was released during an experiment named the 'Green run'[7]. The 'green run' was an experiment in which fuel which had only been allowed to cool for a short time after irradiation was reprocessed in a plant which had no iodine scrubber in operation.

Xenon 131-136

In reactor fuel, the fission product xenon tends to migrate to form bubbles in the fuel. As caesium 133, 135, and 137 are formed by the beta particle decay of the corresponding xenon isotopes, this causes the caesium to become physically separated from the bulk of the uranium oxide fuel.

Because ¹³⁵Xe is a potent nuclear poison with a large cross section for neutron absorption, the buildup of ¹³⁵Xe in the fuel inside a power reactor can lower the reactivity greatly. If a power reactor is shut down or left running at a low power level, then large amounts of ¹³⁵Xe can build up through decay of ¹³⁵I. When the reactor is restarted or the low power level is increased significantly, ¹³⁵Xe will be quickly consumed through neutron capture reactions and the reactivity of the core will increase. Under some circumstances, control systems may not be able to respond quickly enough to manage an abrupt reactivity increase as the built-up ¹³⁵Xe burns off. It is thought that xenon poisoning was one of the factors which led to the power surge which damaged the Chernobyl reactor core.

Caesium 133, 134, 135, 137

A lot of caesium is formed by the fission process, please see the section in the main fission product page for further details of caesium as a troublesome isotope in fall-out and nuclear wastes. This element is a key element which allows the fission products from a bomb to be distinguished from power reactor fission products.

A map of caesium-137 in the area around Chernobyl has been published by the IAEA.[8]

Barium 138, 139

A lot of barium is formed by the fission process, a short lived barium isotope was confused with radium by some early workers. They were bombarding uranium with neutrons in an attempt to form a new element. But instead they caused fission which generated a large amount of radioactivity in the target. Because the chemistry of barium and radium the two elements could be coseparated by for instance a precipitation with sulfate anions. Because of this similarity of their chemistry the early workers thought that the very radioactive fraction which was separated into the "radium" fraction contained a new isotope of radium. Some of this early work was done by Otto Hahn and Fritz Strassmann.

Lanthanides (lanthanum 139, cerium 140-144, neodymium 142-146, 148, 150, promethium-147, and samarium 149, 151, 152, 154)

A great deal of the lighter lanthanides (lanthanum, cerium, neodymium, and samarium) are formed as fission products. It is interesting to note that in Africa at Oklo where the natural nuclear fission reactor operated millions of years ago the isotopic mixture of neodymium is not the same as 'normal' neodymium, it has an isotope pattern very similar to the neodymium formed by fission.

A diagram showing the isotope signatures of natural neodymium and fission product neodymium from U-235 which had been subjected to thermal neutrons. Note that the Ce-142 (a long lived beta emitter) has not had time to decay to Nd-142 over the time since the reactors stopped working.

In the aftermath of criticality accidents the level of ¹⁴⁰La is often used to determine the fission yield (in terms of the number of nuclei which underwent fission).