BeH₂ as a Moderator in Minimum Critical Mass Systems

BeH₂ could serve as an effective moderator in reducing critical masses in minimum critical mass systems on account of its (n,2n) reactivity bonus and the possibly higher hydrogen atomic density. A parametric study of the variation of k_eff and critical mass with BeH₂ moderator density in the range of 600 to 1000 kg/m³ for the three main fissile nuclides of ²³⁵U, ²³³U, and ²³⁹Pu in small spherical thermal assemblies indicates that for 0.40-m-thick BeO-reflected spherical systems with BeH₂ moderator of density ≥680 kg/m³ the critical masses are lower than with any other known moderator (such as H₂O or CH₂). With a moderator of crystalline BeH₂ having a density of 780 kg/m³, critical masses are found to be lower than with CH₂ by about 15%.