Xenon-135

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

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 to 7 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 6.7 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 manage xenon poisoning and 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

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.

See also

• Isotopes of xenon

References

Further reading

• "Xenon Poisoning" or Neutron Absorption in Reactors