Critical Experiments with Homogeneous Mixtures of Plutonium and Uranium Oxides Containing 8,15, and 30 wt% Plutonium

Data are presented from critical experiments with mixed PuO₂-UO₂ fuels containing 30.0, 14.62, and 7.89 wt% Pu and having H/X (H:Pu + U) atomic ratios of 47.4, 30.6, and 51.8, respectively. In addition to the experimental results, which can be used directly as integral benchmark checkpoints, derived critical sizes are presented for homogeneous mixtures, at theoretical density, of ²³⁹PuO₂-U(0.71)O₂-water in slab, spherical, and cylindrical geometries at the three experimental H/X atomic ratios. These types of data provide the bases for establishing criticality safety control limits.Critical thicknesses of 10.80 ± 0.11, 11.56 ± 0.09, and 14.83 ± 0.60 cm were determined, respectively, for slabs of the 30.0, 14.62, and 7.89 wt% Pu-enriched fuels infinite in two dimensions and fully reflected with 15 cm of Plexiglas. Values of k_eff within 8 mk of unity were calculated for these three critical systems using either the diffusion theory code, HFN, or the transport theory code, DTF-IV, with the original GAMTEC-II cross-section data previously used at the Critical Mass Laboratory in correlating plutonium critical experiments with theory. Similar calculations with ENDF/B-II cross-section data yielded k_eff values within 12 mk of unity for these three one-dimensional slab assemblies. Except for the more highly moderated 8 wt% Pu-enriched fuel (H/Pu = 659), calculations with ENDF/B-II data resulted in higher keff values for the critical assemblies than did like calculations using the original GAMTEC-II cross-section library. In the case of the 8 wt% Pu enriched fuel, the computed values for were essentially the same for either of the cross-section sets used.