Neutron Poison - Transient Fission Product Poisons

Transient Fission Product Poisons

Some of the fission products generated during nuclear reactions have a high neutron absorption capacity, such as xenon-135 (microscopic cross-section σ = 2,000,000 b (barns)) and samarium-149 (σ = 74,500 b). Because these two fission product poisons remove neutrons from the reactor, they will have an impact on the thermal utilization factor and thus the reactivity. The poisoning of a reactor core by these fission products may become so serious that the chain reaction comes to a standstill.

Xenon-135 in particular has a tremendous impact on the operation of a nuclear reactor. The inability of a reactor to be restarted due to the build up of xenon-135 (reaches a maximum after about 10 hours) is sometimes referred to as xenon precluded start-up. The period of time in which the reactor is unable to override the effects of xenon-135 is called the xenon dead time or poison outage. During periods of steady state operation, at a constant neutron flux level, the xenon-135 concentration builds up to its equilibrium value for that reactor power in about 40 to 50 hours. When the reactor power is increased, xenon-135 concentration initially decreases because the burn up is increased at the new higher power level. Thus, the dynamics of Xenon poisoning represents a positive reactivity feedback with importance for the stability of the flux pattern and geometrical power distribution especially in physically large reactors.

Because 95% of the xenon-135 production is from iodine-135 decay, which has a 6 to 7 hour half-life, the production of xenon-135 remains constant; at this point, the xenon-135 concentration reaches a minimum. The concentration then increases to the equilibrium for the new power level in the same time, roughly 40 to 50 hours. The magnitude and the rate of change of concentration during the initial 4 to 6 hour period following the power change is dependent upon the initial power level and on the amount of change in power level; the xenon-135 concentration change is greater for a larger change in power level. When reactor power is decreased, the process is reversed.

Because samarium-149 is not radioactive and is not removed by decay, it presents problems somewhat different from those encountered with xenon-135. The equilibrium concentration and (thus the poisoning effect) builds to an equilibrium value during reactor operation in about 500 hours (about three weeks), and since samarium-149 is stable, the concentration remains essentially constant during reactor operation. Another problematic isotope that is building up is gadolinium-157, with microscopic cross-section of σ = 200,000 b.