A neutron absorber. Deliberate poisons include soluble boron (boric acid) in PWR coolant, burnable poisons such as gadolinia mixed into fuel pellets, and boron in spent fuel racks; unwanted ones include the fission products xenon-135 and samarium-149. In reactor talk, poisoning means reactivity lost to absorbers, and a standby liquid control system injects poison as a backup shutdown method in BWRs.
Burnable poison is mostly gone by mid-cycle, so the boron letdown curve flattens after that.
Also heard as
- neutron poison
- neutron absorber
Related terms
Xenon poisoning
The loss of reactivity caused by xenon-135, a fission product that absorbs neutrons with enormous efficiency.
Control rod
A movable rod or blade of neutron-absorbing material used to control reactor power and shut the reactor down.
Reactivity
A measure of how far a reactor is from criticality, positive when power will rise and negative when it will fall.
Criticality safety
The discipline of preventing an accidental chain reaction wherever fissile material is handled outside a reactor.