Critical Experiments with Homogeneous PuO₂-Polystyrene at 5H/Pu

The results and analyses are presented from the latest series of experiments in a continuing program for determining the critical parameters of plutonium mixtures having concentrations that are typical of wet powders, precipitates, slurries, and polymers. The initial series of measurements in this program were made on 15 H/Pu fuel having ²⁴⁰Pu isotopic concentrations of 2.2 and 8.08 wt%. The latest experiments were conducted with fuel having a ²⁴⁰Pu isotopic concentration of 11.46 wt% and a H/Pu atom ratio of 5. Generally, these results indicate that the published values for the critical sizes and masses of plutonium should be increased for the highly concentrated systems. Additional data are needed, however, to better establish the criticality curves in this region. The 11.46 wt% ₂₄₀Pu isotopic concentration caused an increase of ∼30% in the spherical critical mass of a bare ²³⁹Pu-water system at 5 H/Pu. In the reflected system the increase was ∼43%.