Long-term control of excess reactivity by burnable poison in reflector regions

An analysis is presented of the feasibility of the use of burnable poisons in reflector regions of graphite moderated reactors. This is in fact an application of the phenomenon of so-called burn up waves in diffusive media. It is shown that this special case of heterogeneous poisoning offers some degrees of freedom in design that can be used to ʹtailorʹ the excess reactivity as a function of core burn up. Two cases are used for illustrative purposes: a cylindrical core with outer reflector and an annular core with both inner and outer reflector