Xenon-135 Reactor Poisoning — The Iodine Pit

This simulator models the Xenon-135 poisoning transient that governs how a nuclear reactor can and cannot be operated after a power change. Iodine-135, a fission product, beta-decays into Xenon-135 — by far the strongest neutron-absorbing nuclide a reactor produces. During steady operation the two nuclides sit at equilibrium, but a shutdown removes the neutron flux that normally burns Xenon-135 away, so it keeps building up from Iodine-135's ongoing decay for roughly 9–11 simulated hours before finally falling — the "iodine pit." Drive the 3D reactor core's power up and down, trip it with SCRAM, watch the control rod bank withdraw to compensate the growing negative reactivity, and try to restart into a pit deep enough that the rods run out of worth — the operational trap that real reactor crews are trained to respect.