Simulated Used Nuclear Fuel Dissolution as a Function of Fuel Chemistry and Near Field Conditions

This research is a joint UK and US effort to understand the long-term safety of used nuclear fuel (UNF), the primary waste arising from the generation of electricity by nuclear fission. With more than 440 commercial nuclear power stations operating worldwide, a significant cumulative inventory of UNF has been produced, on the order of 300,000 metric tonnes. In the UK, several new nuclear reactors are planned for construction (e.g. Hinkley Point C), and the UNF reprocessing capability at Sellafield (ThORP) is due to close in 2018. Hence, the UK inventory, currently estimated at 3,500 - 8,000 tonnes, will continue to grow. The US currently (April 2016) has 80,150 metric tonnes of UNF, with a prediction of a total of approximately 140,000 metric tons by around 2050 when all currently operating reactors reach their designated life.

The UK and the US presently have no final disposal route for UNF; fuel is currently stored in cooling ponds (UK and US) or dry storage (US only), but this is not a sustainable final solution. Both countries agree that disposal in a deep (200 m - 1000 m) geological formation is the most suitable solution, since it will isolate the UNF from the biosphere and future populations for more than 100,000 years - the period of time for which this material will be highly radioactive. In such a Geological Disposal Facility (GDF), the release of radionuclides to the environment will be controlled by the interaction of the UNF with groundwater, and with the materials that have been built as an engineered barrier around the waste, particularly the fuel cladding and the metal canister. Fundamental mechanistic understanding of how UNF interacts with groundwater under GDF conditions is of paramount importance for UK and US waste management programs, which seek to satisfy citizens and regulators regarding the reliability of long-term degradation predictions for UNF originating from a variety of fuel designs, burn-ups, reactor operations, and storage conditions.

This research project is envisioned as a collaborative and joint enterprise between leading researchers from the UK and US who, collectively, bring mutually complementary and compatible skills, capabilities, and interests required to achieve a paradigm shift in our fundamental understanding of UNF dissolution in the presence of cladding and canister materials, and local groundwater conditions. This understanding will underpin the maturation of models for UNF evolution and interaction under different repository conditions, enabling reliable prediction of degradation and adjustment of repository conditions to achieve desired long-term performance and providing confidence in predicting behaviour for up to one million years.

Through the generation of much-needed fundamental science on dissolution mechanisms of UO2 and related used nuclear fuel materials, particularly in the presence of cladding materials and under different oxidising / reducing conditions, this research has the potential to impact the plans to deliver a geological disposal facility in the UK and the US. Importantly, the generation of improved understanding in the long-term behaviour of used nuclear fuel, and its interactions with the other materials in the engineered barrier, could influence public confidence in the disposal concepts under consideration. This is vital, particularly for the success of the UK voluntary site selection process that is currently underway. Once a site for geological disposal has been selected, there will be a significant reduction in the hazard arising from nuclear waste currently stored around the UK in above-ground facilities, and indications from Government are that future investment in new nuclear power will be made once a final disposal solution is in place. This will reduce the threat from the 'energy trilemma' and have far-reaching impacts to the economy, the environment and society.

The impacts will be realised through the primary beneficiaries, who include academic and non-academic researchers in the field of used nuclear fuel dissolution, and international waste management organisations responsible for implementing geological disposal solutions for used nuclear fuel, and also the early career researchers involved in the proposed project. The impacts to these groups include:

- New scientific understanding and data to support development of a post-closure safety case for geological disposal;
- Publications in peer-reviewed journals, conference proceedings and lay-person articles (e.g. The Conversation);
- Potential reduction of the estimated £18Bn cost of radioactive waste disposal in the UK, through more appropriate disposal facility design led by an improved understanding and confidence in spent fuel durability;
- Improved public confidence in UK radioactive waste disposal strategy, through provision of underpinning scientific understanding and data;
- Strengthened UK-US collaborative efforts in nuclear waste management and disposal.

The Pathways to Impact will focus on developing a forum for the exchange of knowledge with the primary beneficiaries; this will take the form of a final workshop in Y3 of the proposed project, with presentations on the research and round-table discussions about how this, and future research, can be incorporated into plans for US and UK geological disposal facility plans. The knowledge exchange forum will also extend to publicising the progress of the research to the wider public, and presenting key findings to the primary beneficaries, on the PI's website (funded through her EPSRC ECR Fellowship), the "Nuclear Waste Repository".