Theiodine pit, also called theiodine hole orxenon pit, is a temporary disabling of anuclear reactor due to the buildup ofshort-livedneutron poisons in thereactor core. The main isotope responsible is135Xe, mainly produced bybeta decay of135I.135I is a weakneutron absorber, while135Xe is the strongest known neutron absorber. When135Xe builds up in thefuel rods of a reactor, it significantly lowers theirreactivity, by absorbing a significant amount of the neutrons that provide the nuclear reaction.
The presence of135I and135Xe in the reactor is one of the main reasons for its power fluctuations in reaction to change ofcontrol rod positions.
The buildup of short-livedfission products acting as nuclear poisons is calledreactor poisoning, orxenon poisoning. The buildup of stable or long-lived neutron poisons is calledreactor slagging.
One of the commonfission products is135Te, which undergoesbeta decay withhalf-life of 19 seconds to135I.135I 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.135I undergoes beta decay with half-life of 6.57 hours to135Xe. The yield of135Xe for uranium fission is 6.3%; about 95% of135Xe originates from decay of135I.
135Xe is the most powerful knownneutron absorber, with across section forthermal neutrons of 2.6×106 barns,[1] so it acts as a "poison" that can slow or stop thechain reaction after a period of operation. This was discovered in the earliest nuclear reactors built by theManhattan Project forplutonium production. As a result, the designers made provisions in the design to increase the reactor'sreactivity (the number of neutrons per fission that go on to fission other atoms ofnuclear fuel).[2]135Xe reactor poisoning played a major role in theChernobyl disaster.[3]
Byneutron capture,135Xe is transformed ("burned") to136Xe, which is effectively[a] stable and does not significantly absorb neutrons.
The burn rate is proportional to theneutron flux, which is proportional to the reactor power; a reactor running at twice the power has twice the xenon burn rate. The production rate is also proportional to reactor power, but due to the half-life time of135I, this rate depends on theaverage power over the past several hours.
As a result, a reactor operating at constant power has a fixed steady-state equilibrium concentration, but whenlowering reactor power, the135Xe concentration can increase enough to effectively shut down the reactor. Without enough neutrons to offset their absorption by135Xe, nor to burn the built-up xenon, the reactor has to be kept in shutdown state for 1–2 days until enough of the135Xe decays.
135Xe beta-decays with half-life of 9.2 hours to135Cs; a poisoned core will spontaneously recover after several half-lives. After about 3 days of shutdown, the core can be assumed to be free of135Xe, without it introducing errors into the reactivity calculations.[4]
The inability of the reactor to be restarted in such state is calledxenon precluded start up ordropping into an iodine pit; the duration of this situation is known asxenon dead time,poison outage, oriodine 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 of239Pu, required for nuclear weapons, and would lead to investigations and punishment of the reactor operators.[5]
The interdependence of135Xe 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 ofxenon 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 of135Xe and production of135I, depletion of135Xe 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 negativetemperature coefficient of reactivity causesdamping of these oscillations, and is a desired reactor design feature.[4]
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 theneutron flux before the shutdown. Iodine pit behavior is not observed in reactors with neutron flux density below 5×1016 neutrons m−2s−1, as the135Xe 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 5×1013 neutrons m−2s−1 to avoid restart problems after shutdown.[4]
The concentration changes of135Xe in the reactor core after itsshutdown is determined by the short-termpower history of the reactor (which determines the initial concentrations of135I and135Xe), and then by the half-life differences of the isotopes governing the rates of its production and removal; if the activity of135I is higher than activity of135Xe, the concentration of135Xe will rise, and vice versa.
During reactor operation at a given power level, asecular 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 of135I is proportional to the neutron flux φ. The equilibrium concentration of135Xe, however, depends very little on neutron flux for φ > 1017 neutrons m−2s−1.
Increase of the reactor power, and the increase of neutron flux, causes a rise in production of135I and consumption of135Xe. At first, the concentration of xenon decreases, then slowly increases again to a new equilibrium level as now excess135I decays. During a typical power increase from 50 to 100%, the135Xe concentration falls for about 3 hours.[6]
Decrease of the reactor power lowers production of new135I, but also lowers the burn rate of135Xe. For a while135Xe builds up, governed by the amount of available135I, then its concentration decreases again to an equilibrium for the given reactor power level. The peak concentration of135Xe occurs after about 11.1 hours after power decrease, and then equilibrium is reached after about 50 hours. A total shutdown of the reactor is an extreme case of power decrease.[7]
If sufficientreactivity control authority is available, the reactorcan be restarted, but a xenon burn-outtransient must be carefully managed. As thecontrol rods are extracted andcriticality is reached,neutron flux increases many orders of magnitude and the135Xe begins to absorb neutrons and be transmuted to136Xe. The reactorburns 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 the135Xe and135I 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.[6]
The first time135Xe poisoning of a nuclear reactor occurred was on September 28, 1944, in Pile 100-B at the Hanford Site. TheB Reactor was a plutonium production reactor built by DuPont as part of the Manhattan Project. The reactor was started on September 27, 1944, but the power dropped unexpectedly shortly after, leading to a complete shutdown on the evening of September 28. Next morning the reaction restarted by itself. The physicistsJohn Archibald Wheeler, working for DuPont at the time, andEnrico Fermi were able to identify that the drop in the neutron flux and the consequent shutdown was caused by the accumulation of135Xe in the reactor fuel. The reactor was built with spare fuel channels that were then used to increase the normal operating levels of the reactor, thus increasing the burn-up rate of the accumulating135Xe.[8]
Reactors with large physical dimensions, e.g. theRBMK type, can develop significant nonuniformities of xenon concentration through the core. Control of such non-homogeneously poisoned cores, especially at low power, is a challenging problem. TheChernobyl disaster occurred after recovering Reactor 4 from a nonuniformly poisoned state. Reactor power was significantly reduced in preparation for a test, to be followed by a scheduled shutdown. Just before the test, the power plummeted in part due to the accumulation of135Xe as a result of the low burn-up rate at low power. Operators withdrew most of the control rods in an attempt to bring the power back up. 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.
The iodine pit effect has to be taken in account for reactor designs. High values ofpower density, leading to high production rates of fission products and therefore higher iodine concentrations, require higher amount and enrichment of thenuclear fuel used to compensate. Without this reactivity reserve, a reactor shutdown would preclude its restart for several tens of hours until135I/135Xe sufficiently decays, especially shortly before replacement of spent fuel (with highburnup and accumulatednuclear poisons) with fresh fuel.
Fluid fuel reactors cannot develop xenon inhomogeneity because the fuel is free to mix. Also, theMolten 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.[b]