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Xenon-135 (135Xe) is an unstable isotope of xenon with a half-life of about 9.2 hours. 135Xe is a fission product of uranium and it is the most powerful known neutron-absorbing nuclear poison (2 million barns[1]), with a significant effect on nuclear reactor operation. The ultimate yield of xenon-135 from fission is 6.3%, though most of this is from fission-produced tellurium-135 and iodine-135.

135Xe effects on restart[edit]

Main article: Iodine pit

During periods of steady state operation at a constant neutron flux level, the 135Xe concentration builds up to its equilibrium value for that reactor power in about 40 to 50 hours. When the reactor power is increased, 135Xe concentration initially decreases because the burn up is increased at the new higher power level. Because 95% of the 135Xe production is from decay of iodine-135, which has a 6.57 hour half-life, the production of 135Xe remains constant; at this point, the 135Xe concentration reaches a minimum. The concentration then increases to the new equilibrium level for the new power level in roughly 40 to 50 hours. During the initial 4 to 6 hours following the power change, the magnitude and the rate of change of concentration is dependent upon the initial power level and on the amount of change in power level; the 135Xe concentration change is greater for a larger change in power level. When reactor power is decreased, the process is reversed.[2]

Iodine-135 is a fission product of uranium with a yield of about 6% (counting also the iodine-135 produced almost immediately from decay of fission-produced tellurium-135).[3] This 135I decays with a 6.57 hour half-life to 135Xe. Thus, in an operating nuclear reactor, 135Xe is being continuously produced. 135Xe has a very large neutron absorption cross-section, so in the high neutron flux environment of a nuclear reactor core, the 135Xe soon absorbs a neutron and becomes stable 136Xe. Thus, in about 50 hours, the 135Xe concentration reaches equilibrium where its creation by 135I decay is balanced with its destruction by neutron absorption.

When reactor power is decreased or shut down by inserting neutron absorbing control rods, the reactor neutron flux is reduced and the equilibrium shifts initially towards higher 135Xe concentration. The 135Xe concentration peaks about 11.1 hours after reactor power is decreased. Since 135Xe has a 9.2 hour half-life, the 135Xe concentration gradually decays back to low levels over 72 hours.

The temporarily high level of 135Xe with its high neutron absorption cross-section makes it difficult to restart the reactor for several hours. The neutron absorbing 135Xe acts like a control rod, reducing reactivity. The inability of a reactor to be started due to the effects of 135Xe is sometimes referred to as xenon precluded start-up, and the reactor is said to be "poisoned out".[4] The period of time where the reactor is unable to override the effects of 135Xe is called the xenon dead time.

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 and neutron flux increases, and the control rods must be gradually reinserted to counter the loss of neutron absorption by the 135Xe. Otherwise, the reactor neutron flux will continue to increase, burning off even more xenon poison, on a path to runaway criticality. The time constant for this burn-off transient depends on the reactor design, power level history of the reactor for the past several days, and the new power setting. For a typical step up from 50% power to 100% power, 135Xe concentration falls for about 3 hours.[5]

Failing to anticipate and manage xenon poisoning and compensate for the subsequent burn-off properly was a contributing factor to the Chernobyl disaster; during a run-down to a lower power, a combination of operator error and xenon nuclear poisoning caused the reactor thermal power to fall to near-shutdown levels. The crew's resulting efforts to restore power, including the manual withdrawal of control rods not under the SKALA computer's automated control, placed the reactor in a highly unsafe configuration. A failed SCRAM procedure, resulting in the control rods being jammed at a level that actually increased reactivity, caused a thermal transient and a steam explosion that tore the reactor apart.

Reactors using continuous reprocessing like many molten salt reactor designs might be able to extract 135Xe from the fuel and avoid these effects. 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 causes the reactor to produce more of the long-lived fission product caesium-135.

Decay and capture products[edit]

135Xe that does not capture a neutron decays to Cs-135, one of the 7 long-lived fission products, while 135Xe that does capture a neutron becomes stable 136Xe. Estimates of the proportion of 135Xe during steady-state reactor operation that captures a neutron include 90%,[6] 39%–91%[7] and "essentially all".[8]

136Xe from neutron capture ends up as part of the eventual stable fission xenon which also includes 136Xe, 134Xe, 132Xe, and 131Xe produced by fission and beta decay rather than neutron capture.

133Xe, 137Xe, and 135Xe that has not captured a neutron all beta decay to isotopes of caesium. Fission produces 133Xe, 137Xe, and 135Xe in roughly equal amounts, but after neutron capture, fission caesium will contain more stable 133Cs (which however can become 134Cs on further neutron activation) and highly radioactive 137Cs than 135Cs.

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