Speaker
Description
The goal of the U.S. Nuclear Criticality Safety Program’s (NCSP) Thermal/Epithermal eXperiments (TEX) is to design and conduct new critical experiments to address high priority nuclear data needs from the nuclear criticality safety and nuclear data communities. The first set of five TEX experiments, using layers of plutonium-aluminum Zero Power Physics Reactor (ZPPR) plates stacked with varying thicknesses of polyethylene (PE) moderators to tune the neutron spectrum of the assembly, was benchmarked as PU-MET-MIXED-002 (PMM-002) in the 2020 International Criticality Safety Benchmark Evaluation Project (ICSBEP) Handbook. Two of the cases, Cases 4 and 5 of PMM-002, had thermal fission fractions above 50% and provided useful validation cases for testing PE thermal scattering laws (TSLs) and showed that the new PE TSL provide as part of the ENDF/B-VIII.0 library improved agreement between the calculated and experimental benchmark results as compared to ENDF/B-VII.1. Case 5 is one of the most sensitive benchmarks to polyethylene thermal scattering, due to the general lack of appropriate integral experiments in ICSBEP. Based on these results, two additional TEX-Pu experiments were executed in 2021 that incorporate thicker PE moderators to provide greater sensitivity to the PE TSLs. Two similar experiments were also executed that incorporated thick polymethyl methacrylate (C5O2H8, commonly called Lucite or Plexiglass) moderators.
Initial experimental results indicate that the PE cases were over predicted by approximately 0.5% in keff when compared to the predicted critical configurations calculated with MCNP6 and ENDF/B-VIII.0. The temperature of the stack was seen to have a large impact on reactivity of the critical configuration, as a 1-2 °C change produced reactivity differences of 25 pcm (0.00025) in keff. The two Lucite cases also appeared to be overpredicted based on the design calculations, and to a much higher degree than polyethylene. The critical Lucite calculations calculate approximately 1-1.5% too high in keff.