Minimum critical mass in a heterogeneous thermal system

In well-moderated thermal systems with continuous fuel distribution the minimum critical mass is realized by flattening the incore thermal flux distribution. This so-called Goertzel theorem is extended for the case of heterogeneous systems with fissile foils. In this case, in addition to thermal flux equality at the foils, so-called current balance is required for minimum critical mass. Results for optimal foil spacings and foils strength distributions are presented for systems with up to 6 foils.

► The Goertzel theorem for minimization of critical mass (MCM) is extended.
► Equal thermal flux and current balance at all fuel foils is required for MCM.
► Fuel foils are not evenly distributed over space in the MCM case.
► Fuel mass per foil decreases from core centre to core edge.
► In case of limited core dimension, fuel mass in outer foils must be increased.