Critical Separation Between Subcritical Clusters of Low Enriched UO₂ Rods in Water with Fixed Neutron Poisons

Measurement data have been obtained from a series of criticality experiments on systems simulating fuel element shipping containers and fuel storage pools. The conditions investigated involved clusters of low enriched UO₂ fuel rods immersed in water. The number of rods required for criticality near optimum neutron moderation and the critical separation between three subcritical clusters of these rods aligned in a row was determined for 2.35 wt% ²³⁵U-enriched rods and for 4.29 wt% ²³⁵U-enriched rods. The effect that the following fixed neutron absorbers had on the critical separation between these clusters of rods was also measured:

• Boral
• Type 304L stainless steel
• Type 304L stainless steel with 1.1 wt% boron
• Type 304L stainless steel with 1.6 wt% boron
• Copper
• Copper with 1 wt% cadmium
• Aluminum
• Zircaloy-4
• Cadmium.

Both types of fuel rods were ∼12 mm in diameter, aluminum clad, and ∼914 mm long. The neutron absorbers were in the form of plates and were positioned between the fuel clusters such that there was no line of sight between the clusters. Measurements were made with each absorber plate located approximately midway between the fuel clusters and with each plate near the cell boundary of the center fuel cluster. Calculations with KENO IV and ENDF/B cross-section data resulted in k_eff values 1 to 2% high, irrespective of neutron poisoning material or lack thereof.