Minimum critical mass in coupled core systems

Minimum critical mass configurations of well moderated coupled core systems are studied for the moderators heavy water, beryllium and graphite. Characteristic for these systems are the delta functions in the fuel distributions at the core boundaries facing each other and the occurrence of minima in the core size, both depending on the moderator properties and the inter core distance. For an ideal moderator like pure heavy water, the minimum critical mass configuration of the fuel consists of two delta functions for a range of inter core distances.