Providing a criticality warning system omission case for a legacy reactor facility at AWE

The subject of this paper concerns the issuing of a Criticality Warning System (CWS) Omission Case for a legacy fast reactor known as VIPER, that was operational at the Atomic Weapons Establishment site for 40 years prior to operations ceasing in 2007. During its operational years fuel was moved about within the facility, being present either inside the reactor cell, stored within safes, or in an active laboratory to enable preparation and analysis prior to/or after the excursion. Due to the nature of the operations the facility was fitted with a CWS, as is usually the default case for facilities processing large quantities of fissile material. Due to the reduced scope in operations and to maintain operational excellence, the feasibility of the VIPER facility operating without a CWS was reviewed. As part of the review, it was concluded that it was feasible to operate without a CWS, but a robust CWS omission case had to be presented first. This paper will provide an analysis of the criterion set out in Aspinall and Daniels, it will explore the criticality safety justification provided in the report arguing why one of the criteria can be met. The paper will provide an overview of the VIPER criticality safety case and why the judgement was made that a CWS omission case was deemed feasible. (author)