Criticality Experiments with Subcritical Clusters of Light Water Reactor Type Fuel Separated by a Flux Trap

Fuel element storage racks in shipping casks or fuel basin storage pools are generally designed and built such that either structural materials and/or fixed neutron poisons create neutron flux traps between the fuel elements. To provide data for comparison with calculations on such systems, a series of criticality experiments has been performed in which flux traps were created between subcritical clusters of low ²³⁵U-enriched (2.35 and 4.31 wt%) UO₂ rods in water. The flux traps were created by attaching thin plates of either Boral or Type 304-L stainless steel to the opposing faces of the fuel clusters. For both ²³⁵U enrichments the number of fuel rods required in each fuel cluster for the assembly to be critical increased uniformly as the distance between the plates creating the flux trap increased from near zero (0.64 cm) to that approaching infinity. The measurement data indicate that as the thickness of the flux trap increases the type of material creating the trap becomes less important.