Applicability of ORCEF UF4/CF2 Experiments to Validation of 30-Inch UF6 Cylinders

Critical experiments were performed at the Oak Ridge Critical Experiments Facility (ORCEF) in the 1940s and 1950s combining blocks of UF4 and Teflon (CF2) with plastic moderators. These experiments were performed with UF4 and Teflon specifically to mimic the neutronic behavior of UF6. The results of these experiments were used to help define criticality safety limits at the K-25 plant. The experiments incorporated varying amounts of polymethyl methacrylate moderator to create a wide range of neutron energy spectra. The uranium was enriched to 37.5 wt% ²³⁵U, and in some experiments, depleted UF4 was also used. The ratio of enriched and depleted uranium was varied to create average enrichments ranging from 12.5 to 30 wt% ²³⁵U. The evaluations of these experiments in the International Criticality Safety Benchmark Evaluation Project (ICSBEP) Handbook are IEU-COMP-THERM-001, IEU-COMP-INTER-003, and IEU-COMP-MIXED-002, and document a total of 52 experiments. These experiments are of interest in validating criticality safety calculations for UF6 cylinders, and the enrichment of these experiments is also relevant to high-assay low-enriched uranium. Studies have shown a limited number of critical experiments with significant similarity to the limiting analysis conditions for these cylinders. The limiting scenarios involve intermediate neutron energy spectra, making the ORCEF experiments likely candidates for validation. Unfortunately, only a limited number of the experiments show significant similarity to representative 30-inch cylinder models. This paper investigates these results and discusses the impact of neutron energy spectrum and fissile form on similarity.