A new approach to the criticality safety assessment of PCM at BNFL Sellafield

Plutonium Contaminated Material (PCM) arises as a solid waste on the Sellafield Site and is packaged into 200 litre drums which are placed into interim surface storage arrays. These wastes may also contain 235U. The traditional approach to criticality safety has been based on ''worst-case'' reactivity modelling. This has recently led to a number of difficulties by implying that the 230 g (Pu + 235U) drum limit is very important for criticality safety and the assay instruments used to demonstrate compliance with the limit need a high level of safety reliability. Also, the reliability and accuracy of the assay results of historical or legacy PCM became an issue. The new focus on substantiation of safety related equipment in BNFL has highlighted reliability shortfalls for the assay instruments. To overcome these shortfalls, additional operational practices on the PCM handling regimes were introduced to give increased confidence in the fissile assay results. These practices significantly delayed processing PCM waste stocks and resulted in significant additional operator dose uptake. Thus there were strong reasons to improve the existing approach. This paper describes a new approach to the criticality modelling of PCM. (author)