In reactors with discontinuous refuelling, or in the running-in phase of reactors with continuous refuelling, it is sometimes necessary to compensate the reactivity changes due to burn-up using the so-called burnable poisons. These poisons should burn in such a way as to compensate as nearly as possible the kc« changes due to burn-up.

Usually the cross-section of burnable poisons (l0B, Gd, etc….) is rather high and in order to avoid a too rapid burn-up these materials are lumped in rods, so that the effective cross-section is reduced by geometrical self shielding effects. The burn-up of these rods is calculated with methods similar to those used for the burn-up of control rods (see § 13.6).

The rods are subdivided in a certain number of regions whose depletion is calculated as in conventional burn-up codes with the difference that in this case diffusion theory cannot normally be employed because of the high localized absorption, and the flux distribution must be obtained with transport theory. Collision probability methods are usually employed for this purpose/24’25’