Neutron absorbing materials for the control of reactors

My various reactivity control systems enable nuclear fission reactions in the core of a nuclear reactor to be kept under control at all times: power control, safe reactor shutdown, compensation for fuel wear through the core. These systems can take a variety of forms: gaseous (such as the helium 3 used in some experimental reactors), liquid (in PWRs, the coolant water is dosed with soluble boron to balance the reactor's reactivity), or most often solid, combined or not with other possibilities. In all cases, the constituent materials have neutron-absorbing nuclides, unlike fuel, which is a neutron-multiplying medium overall. The physico-chemical form, usually solid, in which these nuclides are present is referred to as the absorbing material, and the absorbing elements are the elementary components containing the absorbing material, called rods or bundles, or assemblies made up of rods (PWR), needles (RNR) or crosses (BWR) and grouped together in reactivity control devices.

The purpose of this dossier is to describe the absorbent materials used in the control rods of the main types of power reactors, as well as in the consumable poison rods of light water reactors.