Performance Assessment and Development of Mineral-Based Cements at High Pressure and Temperature for Deep Borehole Disposal of HLW and SNF

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The need to secure a low-carbon energy source to meet the ever-increasing demand for electricity makes the increasing use of nuclear power a virtual certainty. Nuclear power, like all other forms of energy, generates waste, including high-level radioactive waste and spent fuel (SF). No environmentally and politically satisfactory solution has yet been implemented anywhere in the world for the disposal of spent fuel. The main problem with spent fuel is the high radiogenic heating. Disposal in a mined, engineered repository such the Swedish KBS-3 concept or similar designs proposed for the UK and some other European countries would require prolonged periods of post-reactor cooling and even then would place severe constraints on the engineered barriers. Over the last decade, we (Gibb, Travis, McTaggart & co-workers at the University of Sheffield) have developed an alternative concept for dealing with SF, particularly the high burn-up SF likely to be removed from GEN III reactors in new-build power stations. This alternative is based on very deep disposal in geological boreholes. Deep borehole disposal which utilises an order of magnitude increase in the geological barrier (over and above a mined repository) is potentially a safer option for SF, could be implemented faster and at a fraction of the cost of a repository. The proposed research programme is to further develop borehole disposal such that we greatly extend its applicability to enable the safe, efficient long term disposal of a much wider range of SFs from very young hot fuel to older, legacy SFs.Our current borehole disposal concept for high heat-generating SFs would utilise a special lead-based alloy, employed as a fine shot. This material is designed to support the load of an overlying stack of waste containers and, through radiogenic heating by the waste, becomes fluid and fills any remaining crevices in and around the borehole, forming a permanent seal, and extra barrier upon subsequent cooling. This system is not a universal catch-all; for some waste loadings, insufficient heat will be generated to melt the shot, e.g. during disposal of older fuel . We are seeking to extend the flexibility of our disposal scheme to all SFs by developing a geothermal cement as an alternative to the lead-based alloy support matrix. The proposed research will identify a range of candidate geothermal cements based on heat flow modelling of typical borehole disposal scenarios. Our experienced team which includes: an international expert in geothermal cements, Dr Neil Milestone; the originator of our deep borehole concept for spent fuel disposal, Professor Fergus Gibb; and an expert in multiscale modelling, Dr Karl Travis, will conduct a programme of experimentally based research to reduce the list of candidate cements by measuring important properties such as viscosity, setting time, durability and geochemistry. If no suitable material is found, an attempt will be made to use the results of the investigation to develop one that is fit for purpose. It is highly probable that a successful outcome would yield a product with applications in other areas of nuclear waste packaging and disposal as well as the hydrocarbon and geothermal drilling industries.