Critical Evaluation on the Use of Non-Destructive Assay of Nuclear Packages Through Destructive Breakdown and Inventory Recovery

Abstract

Two historic waste packages on the Sellafield Site with potentially high fissile content were destined for re-packaging. Prior to relocation and subsequent breakdown, each item underwent a campaign of non-destructive assay. The aim of the assay was to gather information that would assist with the production of a safety case. The assay work consisted of: conventional x-ray radiography to determine the identity of the contents; gamma imaging and three-dimensional tomographic re-construction to determine the location of the gamma emitting material; and neutron coincidence counting coupled with gamma spectrometry to assign a fissile mass. Most items on the Sellafield Site that undergo non-destructive assay normally remain intact or are re-packaged with minimal interference of the content. However, in this instance each item was dismantled and the fissile material recovered. This paper provides a comparison of the measured results with the actual results for each technique. The x-ray radiographic information was used to construct a three dimensional representation of the contents of each item. This information was useful in identifying the plant items contained within. The results were discussed with plant operators who were familiar with the historic plant. The operators were able to identify areas of likely accumulations of fissile material. The two-dimensional gamma survey and subsequent threedimensional re-construction revealed the location of the gamma emitting materials within the packages. It was assumed that areas of increased gamma activity indicated areas of increased fissile mass. The neutron coincidence counting, gamma measurements and subsequent modelling provided an estimate of fissile mass for each item. The fissile mass estimate is an essential component of the safety case planning. However, it does not provide all of the information required to plan an ALARP breakdown strategy. The combination of the information gathered through assay was essential to construct a safety case that considered: transport requirements; criticality risk; dose to operator and breakdown methodology. Subsequent studies suggest that improved combination of the data acquired from the assay programme and more post-assay discussion of the specific issues and implications between the various stakeholder groups (e.g. assay engineers and safety case authors) may have led to a fissile mass result with lower uncertainties, which given a more favourable example would have resulted in a significant cost savings through reduced criticality assessment burden and mitigation actions and reduced security control.