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A series of experiments was performed at the Battelle-Pacific Northwest Laboratories on water-flooded arrays of Fast Test Reactor fuel elements (PuO/sub 2/-UO/sub 2/) intermixed with solid neutron absorbers. The objective of these experiments was to provide reliable experiment data that could be used to benchmark computer codes and calculational schemes commonly used in criticality analysis of such systems. The neutron absorbers used were cadmium and Boral plates and gadolinium cylindrical rods. Critical array sizes were determined for square lattice pitches ranging from 9.7 to 24.9 mm, corresponding to water-to-fuel volume ratios of 3.5 to approx. 31 (hydrogen atom to fissile atom ratios of 58 to 473). For both systems, poisoned and unpoisoned, the minimum number of rods required for criticality occurred at a lattice pitch of approx. 20.5 mm, which also corresponds to a water-to-fuel volume ratio of 20.5.

Fourteen different critical systems of plutonium-uranium homogeneous mixtures are arranged in a format amenable for use in validating calculational techniques and checking nuclear data. Each experi...

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...