Reactor Theory (Nuclear Parameters) DOE-HDBK-1019/2-93 XENON Production  and  Removal  of  Xenon-135 Xenon-135 has a 2.6 x 10⁶ barns  neutron absorption cross section.   It is produced directly by some 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%. 135 52 Te b 19.0  sec 135 53 I b 6.57  hr 135 54 Xe b 9.10  hr 135 55 Cs b 2.3x10⁶  yr 135 56 Ba    (stable) The half-life for tellurium-135 is so short compared to the other half-lives that it can be assumed that 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-135 produced comes from the decay of iodine-135.   Therefore, the half-life of iodine-135 plays an important 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 of removal.   This can be expressed in the equation below. rate of change of iodine concentration = yield from fission - decay rate - burnup rate or dN_I dt g I  S fuel f f      lI  NI      s I _aN_I   f where: N_I = ¹³⁵I concentration gI = fission yield of ¹³⁵I S f f uel = macroscopic fission cross section fuel f = thermal neutron flux lI = decay constant for ¹³⁵I = microscopic absorption cross section ¹³⁵I s I a Since the is very small, the burn up rate term may be ignored, and the expression for the rate s I a of change of iodine concentration is modified as shown below.      dN_I dt g I  S fuel f f      lI  NI When 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 for the equilibrium concentration of iodine can be determined from the preceding equation by setting the two terms equal to each other and solving for N_I(eq). NP-03 Rev. 0 Page 35