Prompt neutron decay constants at delayed criticality for the Oak Ridge Research Reactor with 20 and 93 wt% 235U enriched fuel

Abstract

Measurements of the prompt neutron decay constant at delayed criticality with water-reflected, unirradiated low- and high-enrichment fuel loadings and a beryllium-reflected low-enrichment fuel loading were performed for the Oak Ridge Research Reactor. The prompt neutron decay constants were obtained from nonlinear least-squares filling of the frequency-dependent cross-power spectral densities measured between two detectors placed adjacent to the core. The fitting function was derived from the point neutron kinetics equations, ignoring delayed neutrons. The measurements with this water-moderated pool-type reactor determined that the prompt neutron decay constant (for the water-reflected cores) is about 15% higher for the low-enrichment fuel loading (194.9 ± 0.9 s −1) than for the high-enrichment fuel loading (169.1 ± 1.1 s −1). Measurements also showed that the prompt neutron decay constant is about 28% less for the Be-reflected low-enrichment fuel loading than for the water-reflected low-enrichment fuel loading.