Criticality challenges in the characterisation of historic waste items

Various historic containers holding plutonium contaminated materials (PCM) are stored at the Sellafield Site. As part of a project to allow some of these to be moved to new facilities for long-term storage, they were assayed to provide an estimate of their fissile content. The assay results for one item, originally assigned a fissile mass of '>370 g Pu' in the 1980's, yielded a nuclear safety mass significantly in excess of this which meant it fell outside the requirements for long-term storage in the new facilities. This presented a significant challenge to complete the project of removal of these legacy items. (authors)