Neutron poison

A neutron poison (also called a neutron absorber or a nuclear poison) is a substance with a large neutron absorption cross-section, in applications such as nuclear reactors. In such applications, absorbing neutrons is normally an undesirable effect. However neutron-absorbing materials, also called poisons, are intentionally inserted into some types of reactors in order to lower the high reactivity of their initial fresh fuel load. Some of these poisons deplete as they absorb neutrons during reactor operation, while others remain relatively constant.

The capture of neutrons by short half-life fission products is known as reactor poisoning; neutron capture by long-lived or stable fission products is called reactor slagging.

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.