Examination of uranium and plutonium solution criticality in high-concentration solutions

It all comes down to physics. This paper describes calculations using the discrete-ordinates codes PARTISN and DANTSYS as well as the stochastic code MCNP and various cross section sets to examine the critical mass of highly enriched uranium and high-grade plutonium solutions in the concentration range of up to a few hundred grams per liter. When considering these solutions, uranium solutions have a slightly smaller critical mass in some concentration regions; a key goal is therefore to discern the underlying reasons for the differences in these solutions’ critical masses. The differences in the critical masses between these two materials are primarily the result of parasitic absorption by Pu-240 in the plutonium solutions. However, even with solutions of pure U-235 and Pu-239 isotopes, the U-235 solutions still have a slightly smaller critical mass than Pu-239 solutions. This possibly counterintuitive result (at least from the non-practitioner’s point of view) is primarily because U-235 is a more efficient neutron producer than Pu-239 in epithermal systems as demonstrated by an analysis of the parameter η. Thus, these results serve as a reminder to the criticality safety community and as a point of interest to those outside of the community to avoid preconceptions when considering criticality safety in solutions of U-235 and Pu-239; these results also provide the criticality safety community with a review of the key parameters that drive neutronic behavior as found in the latest ENDF/B cross section sets and how these results compare to other cross section sets.