Computational study of critical mass curves for various ratios of ²³⁸Pu to ²³⁹Pu

A MCNP study for a water-reflected sphere of plutonium metal water pseudo-solution was performed for a various ratios of ²³⁸Pu to ²³⁹Pu to develop minimum critical mass curves. The ²³⁸Pu atom percent (at. %) was varied from 35 to 100 percent. It was demonstrated that at ²³⁸Pu ratios of ≥ 65 at. % the high absorption cross section of ²³⁸Pu in the thermal energy region dominates the system reactivity for water moderated systems resulting in a minimum critical mass in the metal rather than solution regime. However, due to the lack of available benchmark experiments to validated use of the ²³⁸Pu cross-section data for quantities above gram-level, the models fell outside the area of applicability (AOA) of the Los Alamos National Laboratory's (LANL) Nuclear Criticality Safety Division's (NCSD) MCNP validation report. An additional of margin of subcriticality (MOS) is needed for ²³⁸Pu systems. This additional MOS will be dependent on the amount and significance of the ²³⁸Pu, which varies in the MCNP study. Whisper is a computational software package designed to assist with validation studies for MCNP. Whisper uses sensitivity/uncertainty (S/U) methods to select relevant benchmarks to a particular application model. Therefore, Whisper 1.0.0 was used on the curve minima in the solution regime for the 65 at. % ²³⁸Pu curve to estimate an appropriate additional margin of subcriticality and create a subcritical curve based on the suggested upper subcritical limits (USL)