Development of a Generic Environmental Safety case for the Disposal of Higher Activity Wastes in the UK

The UK is committed to implementing geological disposal for the long-term, safe management of higher activity radioactive wastes. Higher activity waste includes low-level waste not suitable for near-surface disposal, intermediate-level waste and high-level waste. As yet, no site has been selected for a geological disposal facility (GDF) in the UK, but it has been agreed that a site will be sought using a consent-based approach, preceded by a national geological screening process. Radioactive Waste Management Ltd (RWM) is responsible for the delivery of the GDF. RWM also has a role to support the ongoing packaging of radioactive wastes and to provide disposability assessments for waste producers to provide confidence that packaged wastes will be suitable for eventual disposal in the GDF. To underpin its role, it is essential that RWM can demonstrate its confidence in the safety of a GDF. To this end, RWM maintains a generic Disposal System Safety Case that addresses all safety issues concerning the transport of radioactive wastes to a GDF, the construction and operation of a GDF and the long-term, environmental safety of a GDF after it has been sealed and closed. It is a challenge to present a meaningful safety case when the location and hence the design of the GDF are not known. This is particularly pertinent for the long-term, environmental safety case (ESC), which depends significantly upon an understanding of the geological setting of the GDF and its evolution. This paper explains how RWM has developed a generic ESC based on an understanding of the environmental safety functions provided by a multi-barrier disposal system and the features, events and processes (FEPs) that support them. It explains how an understanding of the basic physics and chemistry underpinning generic GDF concepts can be used to develop 'insight models' to build understanding of the long-term performance of a GDF, to support a safety narrative. The paper also explains the role of probabilistic total system models in providing illustrative calculations to support the generic ESC and RWM's approach to addressing the inevitable uncertainties associated with the long timescales that need to be considered within an ESC. (author)