The Nirex Coherent Approach to Achieving Criticality Safety

The role of Nirex is to provide the UK with safe, environmentally sound and publicly acceptable options for the long-term management of radioactive materials. This will include all intermediate-level waste and some low-level waste (ILW and LLW). Nirex has developed a Phased Disposal Concept based on conditioning the wastes then isolating them deep underground. Based on this phased disposal concept, Nirex advises waste producers on the conditioning and packaging of radioactive waste. The ILW and LLW which comprise the ‘reference volume’ of the Phased Disposal Concept contain approximately 5 t Pu-239 and 28 t U-235. Nirex’s work is based on a number of standards for safety and environmental protection set by the Government. Three different Regulators ensure application of these standards in the transport, operation and post-closure phases. Therefore a coherent approach to criticality safety is required. The approach to criticality safety begins by avoiding criticality resulting from the way in which the waste is packaged, by controlling the package design including the level of fissile material. The concept safety assessments will support a screening level of approximately 5 g Pu-239 equivalent. In specific cases where waste producers indicate that their proposed packages will not comply with the screening level, package-specific criticality safety assessments can be incorporated into the concept through the change control process. Engineering measures are available to prevent criticality for such time as the waste packaging affords a high level of containment. In the long term, however, after deterioration of the physical containment provided by the waste packages, there would be the possibility of movement of fissile material out of the waste packages and subsequent accumulation into new configurations which could in principle lead to a criticality. It is conceivable that a criticality could adversely impact on the performance of a repository after closure because, for example, of the heat that would be produced affecting the engineered barriers. It is therefore necessary to consider the post-closure criticality safety of the repository concept. Nirex is currently undertaking a programme of work on ‘understanding criticality under repository conditions’. The aim of the programme is to obtain a better understanding of the processes that would control the nature and magnitude of a criticality under the particular conditions of the concept repository. An essential component of demonstrating criticality safety is to ensure waste packagers develop operating arrangements, and provide objective evidence in the form of criticality compliance assurance documentation, to demonstrate how fissile material will be controlled to meet levels defined in concept criticality safety assessments. This paper will describe the coherent approach and the processes by which it is applied.