Iodine pit
The iodine pit, also called the iodine hole or xenon pit, is a temporary disabling of a nuclear reactor due to buildup of short-lived nuclear poisons in the reactor core. The main isotope responsible is Xe, mainly produced by natural decay of I. I is a weak neutron absorber, while Xe is the strongest known neutron absorber. When Xe builds up in the fuel rods of a reactor, it significantly lowers their reactivity, by absorbing a significant amount of the neutrons that provide the nuclear reaction. The presence of I and Xe in the reactor is one  of the main reasons for its power fluctuations in reaction to change of control rod positions. The buildup of short-lived fission products acting as nuclear poisons is called reactor poisoning, or xenon poisoning. Buildup of stable or long-lived neutron poisons is called reactor slagging. Fission products decay and burnup. One of the common fission products is Te, which undergoes beta decay with half-life of 19 seconds to I. I itself is a weak neutron absorber. It builds up in the reactor in the rate proportional to the rate of fission, which is proportional to the reactor thermal  power. I undergoes beta decay with half-life of 6.57 hours to Xe. The yield of Xe for uranium fission is 6.3%; about 95% of Xe originates from decay of I. Xe is the most powerful known neutron absorber, with a cross section for thermal neutrons of 2.6×10 barns, so it acts as a "poison" that can slow or stop the chain reaction after a period of operation. This was discovered in the earliest nuclear reactors built by the Manhattan Project for plutonium production. As a result, the designers made provisions in the design to increase the reactor's reactivity  (the number of neutrons per fission that go on to fission other atoms of nuclear fuel). Xe reactor poisoning played a major role in the Chernobyl disaster. By neutron capture, Xe is transformed ("burned") to Xe, which is effectively stable and does not significantly absorb neutrons. The burn rate is proportional to the neutron flux, which is proportional to the reactor power; a reactor running at twice the power will have twice the xenon burn rate. The production rate is also proportional to reactor power, but due to the half-life time of I, this rate depends on the "average  power over the past several hours. As a result, a reactor operating at constant power has a fixed steady-state equilibrium concentration, but when lowering" reactor power, the Xe concentration can increase enough to effectively shut down the reactor. Without enough neutrons to offset their absorption by Xe, nor to burn the built-up xenon, the reactor has to be kept in shutdown state for 1–2 days until enough of the Xe decays. Xe beta-decays with half-life of 9.2 hours to Cs; a poisoned core will spontaneously recover after several half-lives. After about 3 days of shutdown , the core can be assumed to be free of Xe, without it introducing errors into the reactivity calculations. The inability of the reactor to be restarted in such state is called xenon precluded start up or dropping into an iodine pit; the duration of this situation is known as xenon dead time, poison outage, or iodine pit depth. Due to the risk of such situations, in the early Soviet nuclear industry, many servicing operations were performed on running reactors, as downtimes longer than an hour led to xenon buildup that could keep the reactor offline for significant time, lower the  production of Pu, required for nuclear weapons, and would lead to investigations and punishment of the reactor operators. Xenon-135 oscillations. The interdependence of Xe buildup and the neutron flux can lead to periodic power fluctuations. In large reactors, with little neutron flux coupling between their regions, flux nonuniformities can lead to formation of xenon oscillations, periodic local variations of reactor power moving through the core with a period of about 15 hours. A local variation of neutron flux causes increased burnup of Xe and production of I, depletion of Xe increases the reactivity in the core region. The local power  density can change by a factor of three or more, while the average power of the reactor stays more or less unchanged. Strong negative temperature coefficient of reactivity causes damping of these oscillations, and is a desired reactor design feature. Iodine pit behavior. The reactivity of the reactor after the shutdown first decreases, then increases again, having a shape of a pit; this gave the "iodine pit" its name. 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. Iodine pit behavior is not observed  in reactors with neutron flux density below 5×10 neutrons ms, as the Xe is primarily removed by decay instead of neutron capture. As the core reactivity reserve is usually limited to 10% of Dk/k, thermal power reactors tend to use neutron flux at most about 5×10 neutrons ms to avoid restart problems after shutdown. The concentration changes of Xe in the reactor core after its shutdown is determined by the short-term power history of the reactor (which determines the initial concentrations of I and Xe), and then by the half-life differences of the isotopes governing the rates  of its production and removal; if the activity of I is higher than activity of Xe, the concentration of Xe will rise, and vice versa. During reactor operation at a given power level, a secular equilibrium is established within 40–50 hours, when the production rate of iodine-135, its decay to xenon-135, and its burning to xenon-136 and decay to caesium-135 are keeping the xenon-135 amount in the reactor constant at a given power level. The equilibrium concentration of I is proportional to the neutron flux φ. The equilibrium concentration of Xe, however, depends very little on neutron flux for φ  > 10 neutrons ms. Increase of the reactor power, and the increase of neutron flux, causes a rise in production of I and consumption of Xe. At first, the concentration of xenon decreases, then slowly increases again to a new equilibrium level as now excess I decays. During typical power increases from 50 to 100%, the Xe concentration falls for about 3 hours. Decrease of the reactor power lowers production of new I, but also lowers the burn rate of Xe. For a while Xe builds up, governed by the amount of available I, then its concentration decreases again to  an equilibrium for the given reactor power level. The peak concentration of Xe occurs after about 11.1 hours after power decrease, and the equilibrium is reached after about 50 hours. A total shutdown of the reactor is an extreme case of power decrease. Design precautions. If sufficient reactivity control authority is available, the reactor "can" be restarted, but a xenon burn-out transient must be carefully managed. As the control rods are extracted and criticality is reached, neutron flux increases many orders of magnitude and the Xe begins to absorb neutrons and be transmuted to Xe. The reactor "burns off