Flat Flux by Nonuniform Moderation

The production of flat thermal flux by the nonuniform distribution of the moderator is discussed within the framework of two group theory for two region reactors. Equations determining the moderator distribution are derived and a numerical solution is presented for a typical reactor system. The moderator density is found to increase with increasing distance from the center of the core. All combinations of core and reflector materials cannot be used in these flat flux systems, and the restrictions which determine allowability are discussed. In the special case of slab reactors in which the core and reflector are the same materials these systems have minimum critical mass.