Iodine pit

The presence of 135I and 135Xe in the reactor is one of the main reasons for its power fluctuations in reaction to change of control rod positions.

This was discovered in the earliest nuclear reactors built by the Manhattan Project for plutonium production.

Without enough neutrons to offset their absorption by 135Xe, nor to burn the built-up xenon, the reactor has to be kept in shutdown state for 1–2 days until enough of the 135Xe decays.

After about 3 days of shutdown, the core can be assumed to be free of 135Xe, without it introducing errors into the reactivity calculations.

Strong negative temperature coefficient of reactivity causes damping of these oscillations, and is a desired reactor design feature.

The degree of poisoning, and the depth of the pit and the corresponding duration of the outage, depends on the neutron flux before the shutdown.

At first, the concentration of xenon decreases, then slowly increases again to a new equilibrium level as now excess 135I decays.

For a while 135Xe builds up, governed by the amount of available 135I, then its concentration decreases again to an equilibrium for the given reactor power level.

[7] If sufficient reactivity control authority is available, the reactor can be restarted, but a xenon burn-out transient must be carefully managed.

[6] The first time 135Xe poisoning of a nuclear reactor occurred was on September 28, 1944, in Pile 100-B at the Hanford Site.

The physicists John Archibald Wheeler, working for DuPont at the time, and Enrico Fermi were able to identify that the drop in the neutron flux and the consequent shutdown was caused by the accumulation of 135Xe in the reactor fuel.

[8] Reactors with large physical dimensions, e.g. the RBMK type, can develop significant nonuniformities of xenon concentration through the core.

Unbeknownst to the operators, these and other actions put the reactor in a state where it was exposed to a feedback loop of neutron power and steam production.

A flawed shutdown system then caused a power surge that led to the explosion and destruction of reactor 4.