Reactor Theory (Nuclear Parameters)DOE-HDBK-1019/2-93XENONProductionand Removalof Xenon-135Xenon-135 has a 2.6 x 10^{6} barns neutron absorption cross section. It is produced directly bysome fissions, but is more commonly a product of the tellurium-135 decay chain shown below.The fission yield (g) for xenon-135 is about 0.3%, while g for tellurium-135 is about 6%.13552Teb19.0 sec13553Ib6.57 hr13554Xeb9.10 hr13555Csb2.3x10^{6} yr13556Ba (stable)The half-life for tellurium-135 is so short compared to the other half-lives that it can be assumedthat iodine-135 is produced directly from fission. Iodine-135 is not a strong neutron absorber,but decays to form the neutron poison xenon-135. Ninety-five percent of all the xenon-135produced comes from the decay of iodine-135. Therefore, the half-life of iodine-135 plays animportant role in the amount of xenon-135 present.The rate of change of iodine concentration is equal to the rate of production minus the rate ofremoval. This can be expressed in the equation below.rate of change of iodine concentration = yield from fission - decay rate - burnup rateordN_{I}dtg I Sfuelff lI NI sI_{a}N_{I f}where:N_{I}=^{135}I concentrationgI=fission yield of ^{135}ISffuel=macroscopic fission cross section fuelf=thermal neutron fluxlI=decay constant for ^{135}I=microscopic absorption cross section ^{135}IsIaSince the is very small, the burn up rate term may be ignored, and the expression for the ratesIaof change of iodine concentration is modified as shown below.dN_{I}dtg I Sfuelff lI NIWhen the rate of production of iodine equals the rate of removal of iodine, equilibrium exists.The iodine concentration remains constant and is designated N_{I}(eq). The following equation forthe equilibrium concentration of iodine can be determined from the preceding equation by settingthe two terms equal to each other and solving for N_{I}(eq).NP-03Rev. 0Page 35

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