Xenon-135 Poisoning
Xenon-135 is the strongest neutron poison in nature. After a power reduction, it builds up and can trap the reactor in a "xenon pit", forcing operators to withdraw control rods to dangerous levels.
Steady State at Full Power
Xenon-135 and Iodine-135 are at equilibrium. Xenon poisoning is compensated by control rod positions. The reactor is stable.
The Xenon Pit
When a high-power reactor reduces power, Iodine-135 (T½ = 6.57h) continues decaying into Xenon-135 (T½ = 9.17h). But the reduced neutron flux can no longer burn away the xenon fast enough. Xenon concentration rises far above its new equilibrium level, this overshoot is the "xenon pit."
The pit peaks around 6–10 hours after a power drop. During this period, the xenon poison is so strong that the reactor may not be able to return to full power even with all control rods withdrawn. The safe response is to wait 24–48 hours for the xenon to decay naturally.
The Chernobyl Connection
At 00:28 on April 26, 1986, Unit 4 power dropped unexpectedly to ~30 MWt (from the planned 700 MWt test level). This triggered xenon buildup. Instead of waiting for the xenon to decay, operators withdrew nearly all control rods to fight through the xenon pit, reducing the Operating Reactivity Margin (ORM) from the required minimum of 15 rods to just 6–8 equivalent rods.
By 01:00, the reactor was operating with dangerously few control rods, deep in the xenon pit, at very low power, the most unstable possible configuration for an RBMK. When they initiated the turbine run-down test at 01:23, this unstable starting point made the catastrophe inevitable.