XENONDOE-HDBK-1019/2-93Reactor Theory (Nuclear Parameters)Xenon-135Responseto ReactorPowerChangesDuring periods of steady state operation, at a constant neutron flux level, the xenon-135concentration builds up to its equilibrium value for that reactor power in about 40 to 50 hours.Figure 6 illustrates a typical xenon transient that occurs as a result of a change in reactor powerlevel. At time zero, reactor power is raised from 50% power to 100% power. When thereactor power is increased, xenon concentration initially decreases because the burnup isincreased at the new higher power level. Because 95% of the xenon production is fromiodine-135 decay, which has a 6 to 7 hour half-life, the production of xenon remains constantfor several hours. After a few hours (roughly 4 to 6 hours depending on power levels) the rateof production of xenon from iodine and fission equals the rate of removal of xenon by burnupand decay. At this point, the xenon concentration reaches a minimum. The xenon concentrationthen increases to the new equilibrium level for the new power level in roughly 40 to 50 hours.It should be noted that the magnitude and the rate of change of xenon concentration during theinitial 4 to 6 hours following the power change is dependent upon the initial power level and onthe amount of change in power level. The xenon concentration change is greater for a largerchange in power level.Figure 6 Xenon-135 Variations During Power ChangesNP-03Rev. 0Page 40
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