Reactor Xenon Poisoning & Iodine Pit Calculator
Reactor operational physics: Model steady-state Xe-135 poisoning, post-scram Iodine Pit transient, peak negative reactivity depth, and restart dead time.
Pre-Trip Operating Flux & Reactor Core
Xenon Reactivity & Iodine Pit
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Frequently Asked Questions
Why does Xenon-135 concentration increase after reactor shutdown before decreasing?
During full-power operation, Xenon-135 is produced from direct fission (γ_Xe = 0.3%) and primarily from Iodine-135 decay (γ_I = 6.1%, T1/2 = 6.7 h). At steady state, Xenon-135 is eliminated in two ways: radioactive decay (T1/2 = 9.2 h) and neutron burnout (Xe-135 + n → Xe-136, with σ = 2.65 million barns). When the reactor shuts down, the neutron flux drops to zero, abruptly halting neutron burnout. However, the large inventory of I-135 continues decaying into Xe-135 for several hours, creating a massive post-shutdown peak known as the "Iodine Pit".
What is the "Xenon Dead Time" in commercial nuclear power plants?
The Xenon Dead Time is the multi-hour or multi-day period following a reactor trip during which the negative reactivity worth of accumulated Xenon-135 exceeds the total excess reactivity of the control rods. Because the reactor cannot achieve criticality during this window, operators must wait 24 to 40 hours for the xenon to naturally decay before attempting a restart.
How did Xenon poisoning contribute to the Chernobyl disaster?
At Chernobyl Unit 4, operators throttled the reactor to low power, causing a massive Xenon-135 buildup that stalled the core in a severe xenon pit. Instead of allowing the xenon to decay over 24 hours, operators withdrew nearly all control rods beyond safe limits to overcome the xenon poisoning and force power back up. Once localized power began to rise, the high void coefficient ignited an uncontrollable prompt supercritical power excursion.