2.16.1 Introduction

Burnable poisons or burnable absorbers perform a very valuable function in nuclear fuels. They are materials, such as boron, gadolinium, erbium, or dys­prosium, which in their unirradiated state have a high propensity for neutron absorption, but which trans­form into ones that absorb very little after capturing a neutron. By this means, the neutron multiplication factor can be controlled in the reactor throughout the lifetime of the fuel with less reliance on insertion of neutron-absorbing control rods or other control mechanisms.

The first application of burnable poisons was actu­ally for an early submarine reactor design. It was discovered that control rods would be insufficient on their own to control the neutron multiplication factor for the specified refueling cycle time of the core and burnable poisons were used for supplementary control. Today, burnable poisons are used extensively in commercial light water reactors (LWRs), comprising western pressurized water reactors (PWRs) and the Russian equivalent denoted VVERs and boiling water reactors (BWRs). LWRs are by far the most numerous of the 436 commercial reactors currently (2009) in opera­tion. The UK’s advanced gas-cooled reactors (AGRs) also rely heavily on burnable poisons. The other reactor types in commercial operation (Canadian designed heavy water cooled and moderated reactor (CANDU), Russian designed graphite moderated water cooled reactors (RBMKs), and the UK Magnox reactors) do not normally use burnable poisons at present, though there is an advanced fuel bundle design available for CANDU reactors that do use them.