Microbiological effects on the Cavern Extended Storage (CES) repository for radioactive waste—A quantitative evaluation

The recently developed Cavern Extended Storage (CES) repository design for radioactive waste combines some of the advantages of the flexibility provided by long-term interim storage with the safety of deep geological disposal. It is also of particular interest where demands are made for long periods of institutional control and monitoring (or staged implementation) of repositories or where a desire exists to keep options open for possible retrieval with regard to spent fuel reprocessing. In this concept, large steel casks containing vitrified High Level Waste (HLW) and/or spent fuel are stored in large rock caverns at depths 500 to 1000 m below surface for approximately 300 years before backfilling to transform this ‘store’ into a final ‘repository’. However, the extreme environmental conditions of a HLW repository cannot guarantee sterility. As a result, the humid, ventilated caverns could provide many potential niches for microbial growth, which could influence both the integrity of the storage casks (and any associated infrastructure) and the subsequent long-term performance of the sealed repository. The storage period is likely to provide the optimum period for microbial colonisation and growth, with the casks and rock walls exposed to constantly circulating air, which would provide oxygen for aerobic microbial processes. In addition, nutrients and energy sources will be introduced from the surface during operational and monitoring procedures. The duration of this storage period may be extended or shortened, depending on changing institutional requirements and it is thus particularly important to know the effects of microbial activity during this time. Simple models of the constraints on such growth can be used to scope the potential for activity to influence repository performance, particularly in relation to changing storage times, and to compare the CES design with conventional repository concepts.