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A series of criticality experiments with 2.35 and 4.31 wt% 235U enriched UO2 rods in water has provided well-defined benchmark-type data showing that both depleted uranium and lead reflecting walls...

The results and analyses presented are from the latest series of experiments in a continuing program for determining the critical parameters of plutonium mixtures having concentrations typical of wet powders, precipitates, slurries, and polymers. Previous measurements in this program were made on 15 H/Pu fuel having 240Pu isotopic concentrations of 2.2 and 8.08 wt% and on 5 H/Pu fuel having a 240Pu isotopic concentration of 11.46 wt%. This latest series of experiments was conducted with fuel in which the 240Pu isotopic content has been increased to 18.35 wt% and the H/Pu atomic ratio decreased to essentially zero. The minimum critical slab thickness for a water-reflected homogeneous PuO2-water system of this composition was determined to be 2.31 ± 0.06 cm as compared to only 1.15 ± 0.03 cm for Pu-water. Thus, having the plutonium in an oxide form at this degree of moderation results in an increase of about a factor of 2 in the critical thickness. For spherical geometry, the difference in critical mass bet...