Xenon-135

¹³⁵Xe is an isotope of xenon, and a fission product (yield 6.3333%) which is the most powerful known neutron-absorbing nuclear poison and has a significant effect on nuclear reactor operation.

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Xe-135 effects on restart

The inability of a reactor to be started due to the effects of Xe-135 is sometimes referred to as xenon precluded start-up. The period of time where the reactor is unable to override the effects of Xe-135 is called the xenon dead time. During periods of steady state operation, at a constant neutron flux level, the Xe-135 concentration builds up to its equilibrium value for that reactor power in about 40 to 50 hours. When the reactor power is increased, Xe-135 concentration initially decreases because the burn up is increased at the new higher power level. Because 95% of the Xe-135 production is from iodine-135 decay, which has a 6 to 7 hour half-life, the production of Xe-135 remains constant, at this point, the Xe-135 concentration reaches a minimum. The concentration then increases to the new equilibrium level for the new power level in again roughly 40 to 50 hours. The magnitude and the rate of change of concentration during the initial 4 to 6 hours following the power change is dependent upon the initial power level and on the amount of change in power level; the Xe-135 concentration change is greater for a larger change in power level. When reactor power is decreased, the process is reversed.

Reactors using continuous reprocessing like many molten salt reactor designs might be able to extract Xe-135 from the fuel and avoid these effects.

Decay and capture products

¹³⁵Xe that does not capture a neutron decays to Cs-135, one of only 3 long-lived fission products with high yields, along with Tc-99 and Zr-93. There are also 4 long-lived fission products with lower yields, Iodine-129, Pd-107, Se-79, and Sn-126.

¹³⁵Xe that does capture a neutron becomes stable ¹³⁶Xe, which joins several other xenon isotopes which are fission products. In continuous operation an estimated 10% to 50% of the ¹³⁵Xe produced undergoes neutron capture.

See also

• Isotopes of xenon

References

• "Xenon Poisoning" or Neutron Absorption in Reactors