Criticality Experiments with Subcritical Clusters of 2.35 and 4.31 wt% ²³⁵U-Enriched UO₂ Rods in Water with Steel Reflecting Walls

A series of criticality experiments with 2.35 and 4.31 wt%o ²³⁵U-enriched UO₂ rods in water was performed to provide well-defined benchmark-type data on thick steel reflecting walls. For each fuel enrichment, the critical separation between three subcritical fuel clusters was observed to increase as 178.5-mm-thick reflecting walls of reactor-grade steel was moved toward the fuel. This increase was observed for fuel clusters having an undermoderated water-to-fuel volume ratio of 1.6 and for fuel clusters having near optimum neutron moderation (2.92 for the 2.35 wt% ²³⁵U-enriched fuel and 3.88 for the 4.31 wt% ²³⁵Uenriched fuel). In all cases the critical separation between fuel clusters increased to a maximum as the steel walls were moved toward the fuel clusters. This maximum effect was observed with ∼10 mm of water between the fuel clusters and the steel reflecting walls. As this water gap was decreased, the critical separation between the fuel clusters also decreased slightly. Measurement data were also obtained for each enrichment with neutron absorber plates between the fuel clusters having the 1.6 water-to-fuel volume ratio. During these measurements, the steel reflecting walls were at the near optimum distance from the fuel clusters. The fixed neutron absorbers for which data were obtained include Type 304L stainless steel, borated Type 304L stainless steel, copper, copper containing 1 wt% cadmium, cadmium, and two tradename materials containing boron (Boral and Boroflex).