Exploitation of Bragg Cutoff Phenomenon for Improved Albedo in Beryllium-Reflected Minimum Critical Mass Systems

It has been shown that the “minimum” achievable spherical critical masses for the three main fissile isotopes of ²³⁵U, ²³⁹Pu, and ²³³U at normal temperature and density with BeH₂ as moderator and with a thick ⁹Be reflector is lower than for any other system reported so far. In this context the feasibility of decreasing the critical masses further by exploiting the Bragg cutoff phenomenon in cooled beryllium reflectors was investigated. The reactivity gain obtainable in cooling part (or whole) of the beryllium reflector of a BeH₂-moderated homogeneous ²³³U system to liquid nitrogen temperature (78 K) is explored. Transport theory calculations show that a 50-cm two-zone beryllium reflector with a cooled inner zone of optimum thickness (∼15 cm) at 78 K has an improved albedo and results in a further reduction of 6 to 8% in the critical mass.