The neutron-absorbing fission products xenon-135 and samarium-149 have particular operational importance. The isotope with a short half-life (e.g., xenon-135) is known as reactor poison, while the isotope that is long-lived or even stable is known as reactor slag. From this point of view, we distinguish between neutron poisons, which causes reactor poisoning, and neutron poisons, which cause reactor slagging. These isotopes are described in the following table.Īs can be seen, most of these isotopes are stable, and some undergo decay. Most fission products have a low absorption cross-section, but there are very important exceptions. After several half-lives, the rate of decay will equal the production rate. Equilibrium is eventually reached during reactor operation if a radioisotope is produced at a constant rate. For that reason, these nuclei are known as neutron poisons.Įach poison’s significance depends on the yield, decay rates, and absorption cross-section. Their buildup in the reactor tends to reduce the multiplication factor. Fission products are of concern in reactors primarily because they become parasitic absorbers of neutrons and result in long-term sources of heat (so-called decay heat). Many of the resulting fission products have measurable thermal absorption cross-sections. Plenty of isotopes, usually neutron-rich isotopes, are produced by fission. The most probable fragment masses are around mass 95 (Krypton) and 137 (Barium). Fission fragment yield for different nuclei. Most of these fission fragments are highly unstable (radioactive) and undergo further radioactive decays to stabilize themselves. It is much more probable to break up into unequal fragments, and the most probable fragment masses are around mass 95 (Krypton) and 137 (Barium). The average of the fragment mass is about 118, but very few fragments near that average are found. Nuclear fission fragments are the fragments left after a nucleus fissions. Neutron poisons are fission products that are of concern in reactors primarily because they become parasitic absorbers of neutrons and result in long-term sources of heat (so-called decay heat).
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