Additional Remarks on BeH₂ as a Moderator in Minimum Critical Mass Systems

It was shown earlier that nonsolvated crystalline BeH₂ could serve as an effective moderator in reducing nuclear critical masses below minima achievable in CH₂-moderated systems on account of its (n, 2n) reactivity bonus and higher hydrogen number density. The ⁹Be cross sections used in these calculations were found to overestimate the (n, 2n) multiplication. The precise (n, 2n) contribution to system reactivity and critical mass in the light of the latest ⁹Be cross-section data are evaluated. The results show that in the case of BeH₂-moderated and BeO-reflected systems, five additional neutrons are born in the reaction multiplication  in beryllium per 100 fission neutrons released in the core, resulting in a reactivity gain of ∼4%. The corresponding reduction in critical mass is ∼16%. The critical masses calculated with corrected ⁹Be cross sections show that the crystalline BeH₂-moderated and BeO-reflected systems apparently have the smallest possible theoretical critical masses, namely, 0.180, 0.137, and 0.105 kg for ²³⁵U, ²³³U, and ²³⁹Pu, respectively.