DOE-HDBK-1019/2-93SAMARIUM AND OTHER FISSION PRODUCT POISONSReactor Theory (Nuclear Parameters)The xenon-135 and samarium-149 mechanisms are dependent on their very large thermal neutronFigure 7 Behavior of Samarium-149 in a Typical Light Water Reactorcross sections and only affect thermal reactor systems. In fast reactors, neither these nor anyother fission products have a major poisoning influence.OtherNeutronPoisonsThere are numerous other fission products that, as a result of their concentration and thermalneutron absorption cross section, have a poisoning effect on reactor operation. Individually, theyare of little consequence, but "lumped" together they have a significant impact. These are oftencharacterized as "lumped fission product poisons" and accumulate at an average rate of 50 barnsper fission event in the reactor.In addition to fission product poisons, other materials in the reactor decay to materials that actas neutron poisons. An example of this is the decay of tritium to helium-3. Since tritium hasa half-life of 12.3 years, normally this decay does not significantly affect reactor operationsbecause the rate of decay of tritium is so slow. However, if tritium is produced in a reactor andthen allowed to remain in the reactor during a prolonged shutdown of several months, a NP-03Rev. 0Page 46
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