Criticality Issues With the 30B Canister with Enrichments Greater Than 5 Wt% U-235

Calculations are performed showing that 6 wt% U-235 fuel can be safely stored in the standard 30B UF6 cylinder if, consistent with current cylinder criticality analysis, moderator exclusion is allowed. However, current USA and international regulations limit the use of moderator exclusion to 5 wt% U-235. A modification to the 30B UF6 cylinder is proposed using steel clad B4C rods traversing the length of the cylinder. Using 61 of these rods, it is shown that the cylinder is criticality safe for 6 wt% U-235 fuel. The rods reduce the capacity of the 30B cylinder by 13%. For higher enrichments, more B4C rods can be added with further reduction in capacity.