XENON DOE-HDBK-1019/2-93 Reactor Theory (Nuclear Parameters) N_I(eq)                      g I  S fuel f f lI Since the equilibrium iodine concentration is proportional to the fission reaction rate, it is also proportional to reactor power level. The rate of change of the xenon concentration is equal to the rate of production minus the rate of removal.  Recall that 5% of xenon comes directly from fission and 95% comes from the decay of iodine.   The rate of change of xenon concentration is expressed by the following equations. rate  of  change  of xenon  135  concentration xenon  135  yield from  fission iodine  135 decay xenon  135 decay xenon  135 burnup dN_Xe dt g Xe  S fuel f f    lI  NI     lXe  NXe     s Xe a N_Xe   f where: N_Xe = ¹³⁵Xe concentration gXe = fission yield of ¹³⁵Xe S f f uel = macroscopic fission cross section of the fuel f = thermal neutron flux lI = decay constant for ¹³⁵I N_I = ¹³⁵I concentration lXe = decay constant for ¹³⁵Xe = microscopic absorption cross section¹³⁵Xe s Xe a The  xenon  burnup  term  above  refers  to  neutron  absorption  by  xenon-135  by  the  following reaction. 135 54 Xe 1 0 n     136 54 Xe     g Xenon-136 is not a significant neutron absorber; therefore, the neutron absorption by xenon-135 constitutes removal of poison from the reactor.  The burnup rate of xenon-135 is dependent upon the neutron flux and the xenon-135 concentration. The equilibrium concentration of xenon-135 is designated N_Xe(eq), and is represented as shown below. N_Xe  (eq)                                    g Xe  S fuel f f    lI  NI lXe     s Xe a f NP-03 Rev. 0 Page 36