Optimising storage arrangements for low enriched uranic residues subject to limited slab loading control

One of the key activities undertaken at the National Nuclear Laboratory's Preston Laboratory is the clean-up of legacy residues from various UK sites. The main Facility undertakes a wide range of operations, some of which generate additional uranic residues that are temporarily stored in 210 litre drums whilst awaiting further processing. Materials with enrichments up to 5 w% ²³⁵U are routinely processed and the safe fissile mass per drum is derived as a function of enrichment. To aid operational flexibility in the Facility, a 'hybrid' approach is adopted where the drum loading limit for each enrichment band is set as the most restrictive of the limits derived for either an isolated single unit subject to safe mass or for a 1-high drum array based on infinite safe slab loading. This allows drums to be readily moved between areas with different modes of criticality control. This has proven to be operationally convenient, but the facility has recently been looking to provide additional storage capacity and to implement general improvements to its drum storage arrangements. To this end, it was proposed to provide additional capacity with 2-high drum loading in a number of Empteezy storage containers. The simplest approach from a criticality safety viewpoint would have been to halve the existing drum limits for drums in the 2-high array. However, it was recognised that this would be overly restrictive for plant operations. The challenge for the criticality safety assessors was therefore to accommodate 2-high drum loading whilst maintaining (if possible) the well-established criticality safety limits for the drums in the array. The paper shows how the criticality safety justification for 2- high drum loading was made successfully for the existing drum fissile mass limits. This important conclusion has supported enhanced and optimised storage of uranic residue drums in the facility. (authors)