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

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.

The wider national & international impact is for the energy crisis and climate change through facilitating the continued and expanded use of carbon-free nuclear energy, to which the principle obstacle is how to deal with the wastes, especially spent fuel (SF) and reprocessing HLW.
The main impact is for the development of deep borehole disposal (DBD) as a potentially safer, more cost-effective and environmentally sound alternative to mined and engineered repositories for disposal of SF, HLW and possibly Pu. The impact is likely to occur soonest in the USA where, with the demise of the federal repository at Yucca Mountain, the problem of SF is acute and the Presidential Blue Ribbon Commission has focussed attention on DBD as a way forward. Our pioneering research in DBD has led to our involvement in a US DOE-funded consortium led by Sandia NL to research and develop DBD and take it through to a practical demonstration. A successful outcome to the proposed research on cement, while not part of this consortium's work, has the potential to feed into it and widen the options for the engineered barrier systems and for the range of wastes that could be managed by DBD.
In the UK, in response to a Government request to consider options for acceleration of the MRWS programme, NDA has acknowledged a possible role for DBD, especially for the early disposal of vitrified reprocessing HLW. The specific objective of the proposed research is to develop a fit-for-purpose cement grout aimed at DBD of the UK's inventory of reprocessing HLW (and possibly older SF) but, since DBD is, as yet, outside RWMD's remit the intended impact is more likely to occur in the longer term. However, as part of their brief to maintain awareness of new technologies, NDA and RWMD have already expressed interest. Given the scale and urgency of the UK's 'legacy waste' problem, the impact of the work could be considerable.
Secondary, but important, impacts could be elsewhere in the nuclear industry and in the hydrocarbon and geothermal energy industries where the problems of cementing and sealing wells are well known.
Currently, cements are used extensively in the nuclear industry for grouting wastes into their containers and it is proposed to use similar materials in proposed GDF concepts where elevated temperatures could prevail. While existing formulations are deemed satisfactory in the former context, they are not ideal in that their flow/void filling properties could be better and they do not perform well with reactive metal wastes. Clearly, the development of a cement grout with better flow, setting and performance characteristics (as required for DBD use) could spin off to applications in this area.
The main use of cements in the oil and gas industry is for grouting casing to seal wells. While commercially available well cements are generally satisfactory they can fail with serious consequences under the more extreme conditions being increasingly encountered as hydrocarbon exploration probes further and deeper, e.g., BP's Deepwater Horizon disaster, which has led to litigation against the cement supplier. Geothermal energy wells nearly always have to be sealed by casing and grouting to prevent fluid loss and erosion of the wall rock. Conventional geothermal cements usually prove adequate in the short term, but in the harsher and more aggressive chemical environments increasingly being exploited, e.g. in New Zealand, failure and loss of the well can occur within a few years. A successful outcome to the proposed research to develop a grout that can perform satisfactorily in the severe, high temperature and pressure conditions of DBD could lead to improved formulations to resolve serious problems in both the hydrocarbon and geothermal drilling industries as they seek to push the envelope ever further.