Criticality Experiments with Subcritical Clusters of Low Enriched UO₂ Rods in Water with Uranium or Lead Reflecting Walls

A series of criticality experiments with 2.35 and 4.31 wt% ²³⁵U enriched UO₂ rods in water has provided well-defined benchmark-type data showing that both depleted uranium and lead reflecting walls, submerged in the water reflector, are better neutron reflectors than water alone. For each fuel enrichment, the critical separation between three subcritical, near optimally moderated fuel clusters was observed to increase as either 77-mm-thick depleted uranium or 102-mm-thick lead reflecting walls were moved toward the fuel The maximum reactivity effect was observed for the depleted uranium with ∼20 mm of water between the reflecting walls and the fuel region, whereas for the lead, a maximum effect was obtained with essentially no water between the reflecting walls and the fuel region. This maximum reactivity effect was observed to occur at the same spatial separation between the fuel and reflecting walls for both fuel enrichments. However, the measurements indicated that the magnitude of this phenomenon is dependent on the ²³⁵U enrichment of the fuel The lead reflecting walls increased the critical separation between fuel clusters a maximum of 67% for the 2.35 wt% ²³⁵U enriched fuel and at least 152% for the 4.31 wt% enriched fuel Similar results were observed with the depleted uranium reflecting walls.