Iodine pit, also called iodine hole and xenon pit, is a temporary disabling of a nuclear reactor due to buildup of short-lived nuclear poisons in the core of a nuclear reactor. The main isotope responsible is xenon-135, mainly produced by natural decay of iodine-135. Iodine-135 is a weak neutron absorber, while xenon-135 is the most powerful known neutron absorber. When xenon-135 builds up in the fuel rods of a reactor, it significantly lowers their reactivity, by absorbing a significant amount of the neutrons which provide the nuclear reaction.
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 tellurium-135, which undergoes beta decay with half-life of 19 seconds to iodine-135. Iodine-135 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. Iodine-135 undergoes beta decay with half-life of 6.57 hours to xenon-135. The yield of 135Xe for uranium fission is 6.3%; about 95% of xenon-135 originates from decay of iodine-135.
135Xe has a huge cross section for thermal neutrons, 2.6×106 barns, so it acts as a neutron absorber or "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 American Manhattan Project for plutonium production. Fortunately the designers had 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). 135Xe reactor poisoning played a major role in the Chernobyl disaster.
Xenon-135 is the most powerful known neutron absorber. Its buildup in the fuel rods significantly lowers reactivity of the reactor core. By a neutron capture, Xe-135 is transformed ("burned") to xenon-136, which is 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 on twice the power will have twice the xenon burn rate.
Xenon-135 beta-decays with half-life of 9.2 hours to caesium-135; a poisoned core will spontaneously recover after several half-lives. For some reactors, the 135Xe concentration will be equal to its equilibrium concentration at full power. After about 3 days of shutdown, the core can be assumed to be free of 135Xe, without it introducing errors into the reactivity calculations.
The increase of the 135Xe concentration during lowering the reactor power can lower the reactivity enough to effectively shut down the reactor. As there are not 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 135Xe decays. 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 valuable weapon plutonium-239, and cause an investigation by a committee and punishment of the operators.
The interdependence of 135Xe 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 the period of about 15 hours. A local variation of neutron flux causes increased burnup of 135Xe and production of 135I, depletion of 135Xe increases the reactivity in the core region. The local power density can change by 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 5×1016 neutrons/(m2·s), as the 135Xe 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 5×1017 neutrons/(m2·s) to avoid restart problems after shutdown.
The concentration changes of 135Xe in the reactor core after its shutdown is determined by the short-term power history of the reactor (which determines the initial concentrations of 135I and 135Xe), and then by the half-life differences of the isotopes governing the rates of its production and removal; if the activity of 135I is higher than activity of 135Xe, the concentration of 135Xe 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 135I is proportional to the neutron flux φ. The equilibrium concentration of 135Xe however depends very little on neutron flux for φ>1017 neutrons/(m2·s).
Increase of the reactor power, and the increase of neutron flux, causes a rise in production of 135I and consumption of 135Xe. At first, the concentration of xenon decreases, then slowly increases again to a new equilibrium level as now excess 135I decays. During typical power increases from 50 to 100%, the 135Xe concentration falls for about 3 hours.
Decrease of the reactor power lowers production of new 135I, but also lowers the burn rate of 135Xe. 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. The peak concentration of 135Xe 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.
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 135Xe begins to absorb neutrons and be transmuted to 136Xe. The reactor burns off the nuclear poison. As this happens, the reactivity increases and the control rods must be gradually re-inserted or reactor power will increase. The time constant for this burn-off transient depends on the reactor design, power level history of the reactor for the past several days (therefore the 135Xe and 135I concentrations present), and the new power setting. For a typical step up from 50% power to 100% power, 135Xe concentration falls for about 3 hours.
Reactors with large physical dimensions, e.g. the RBMK type, can develop significant nonuniformities of xenon concentration through the core. Control of such nonhomogeneously poisoned core, especially at low power, is a challenging problem. The Chernobyl disaster resulted from an attempt to recover the reactor from a nonuniformly poisoned state.
The iodine pit effect has to be taken in account for reactor designs. High values of power density, leading to high production rates of fission products and therefore higher iodine concentrations, require higher amount and enrichment of the nuclear fuel used to compensate. Without this reactivity reserve, a reactor shutdown would preclude its restart for several tens of hours until 135I/135Xe sufficiently decays, especially shortly before replacement of spent fuel (with high burnup and accumulated nuclear poisons) with fresh one.
Fluid fuel reactors cannot develop xenon inhomogeneity because the fuel is free to mix. Also, the Molten Salt Reactor Experiment demonstrated that spraying the liquid fuel as droplets through a gas space during recirculation can allow xenon and krypton to leave the fuel salts. However, removing xenon-135 from neutron exposure also means that the reactor will produce more of the long-lived fission product caesium-135.
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