XENONDOE-HDBK-1019/2-93Reactor Theory (Nuclear Parameters)Xenon-135Responseto ReactorShutdownWhen a reactor is shutdown, the neutron flux is reduced essentially to zero. Therefore, aftershutdown, xenon-135 is no longer produced by fission and is no longer removed by burnup. Theonly remaining production mechanism is the decay of the iodine-135 which was in the core atthe time of shutdown. The only removal mechanism for xenon-135 is decay.dNXedtlI NI lXe NXeBecause the decay rate of iodine-135 is faster than the decay rate of xenon-135, the xenonconcentration builds to a peak. The peak is reached when the product of the terms lINI is equalto lXeNXe(in about 10 to 11 hours). Subsequently, the production from iodine decay is less thanthe removal of xenon by decay, and the concentration of xenon-135 decreases. The greater theflux level prior to shutdown, the greater the concentration of iodine-135 at shutdown; therefore,the greater the peak in xenon-135 concentration after shutdown. This phenomenon can be seenin Figure 5, which illustrates the negative reactivity value of xenon-135 following shutdown fromvarious neutron flux levels. Figure 5 Xenon-135 Reactivity After Reactor ShutdownNP-03Rev. 0Page 38
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