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

A series of criticality experiments with 2.35 wt% and 4.31 wt% {sup 235}U enriched UO{sub 2} rods in water were performed to provide well defined benchmark type data on the effects of 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 towards 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% {sup 235}U enriched fuel and 3,88 for the 4.31 wt% {sup 235}U enriched fuel). In all cases the critical separation between fuel clusters increased to a maximum as the steel walls were moved towards the fuel clusters. This maximum effect was observed with about 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 l .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 304-L steel, borated 304-L steel, copper, copper containing 1 wt% cadmium, cadmium, and two trade name materials containing boron (Boral and Boroflex), A comparison between these data and data from previous experiments indicates a slight reduction in the effectiveness of the absorber plates when the steel reflecting walls are present.