Thermal/epithermal experiments with hafnium (TEX-Hf)
Critical experiment benchmarks are essential for validating calculational techniques used by criticality safety engineers to guarantee safe fissile material operations, yet there remain significant areas of need where existing benchmarks are insufficient or entirely absent. There is a particularly notable shortage of intermediate benchmarks (where the majority of fissions are caused by neutrons with energy between 0.625 eV and 100 keV). This work describes the design of a set of critical experiments (TEX-Hf) that address this gap by developing a highly enriched uranium (HEU) test bed into which various materials of interest (diluents) can be substituted. The first diluent to be investigated with this test bed is hafnium, a neutron poison used in nuclear applications for which there are no sufficiently sensitive benchmarks. Twenty-one critical configurations made up of stacked disks of HEU, polyethylene, and hafnium were designed and simulated in MCNP6 to calculate their fission energy spectra and the sensitivity of k{sub eff} to changes in various cross sections. It was shown that configurations spanned the energy range from thermal (below 0.625 eV), through the intermediate energy range (0.625 eV to 100 keV), to fast (above 100 keV). Furthermore, it was determined that this set of experiments is sensitive to the hafnium and {sup 235}U cross section data. These test bed experiments were designed to allow substitution of various diluent plates and can therefore be used to quickly and easily produce many more benchmarks. (authors)