Lead Void Reactivity Worth in Two Critical Assembly Cores with Differing Uranium Enrichments

To validate lead (Pb) nuclear cross sections, a series of integral experiments to measure lead void reactivity worths was conducted in a high-enriched uranium (HEU)/Pb system and a low-enriched uranium (LEU)/Pb system using the Comet Critical Assembly at the National Criticality Experiments Research Center. There is a follow-on experiment to measure the lead void reactivity worths in a plutonium/Pb system that is currently under investigation. The critical experiments in the two uranium systems were designed to provide complementary data sets having different sensitivities to scattering cross sections of lead. The larger amount of the ²³⁸U present in the LEU/Pb core increases the neutron importance above 1 MeV compared with the HEU/Pb core. Since removal of lead from the core shifts the neutron spectrum to the higher energy region, positive lead void reactivity worths were observed in the LEU/Pb core while negative values were observed in the HEU/Pb core. This technical note is a preliminarily report of the experimental analysis results for the lead void reactivity worths with the Monte Carlo calculation code MCNP^® version 6.1 together with nuclear data libraries JENDL-4.0 and ENDF/B-VII.1. The calculation values were found to overestimate the negative reactivity worths for the HEU/Pb core while being consistent for the LEU/Pb core.