The Development of Advanced Technologies and Modelling Capabilities to Improve the Safety and Performance of Nuclear Fuel

The main factors that limit the performance of nuclear fuels are related to cladding failure due to interactions with the fuel pellet and reactor coolant. Improvements to our understanding of the cladding failure mechanisms will enhance our ability to predict their effects, leading both to improvements in the safety and operation of current fuels, and to technological developments that will produce improved fuel, cladding, and coating materials. The research tasks below seek to address each of these issues in turn, from the perspective of both current and advanced fuel designs.Aomistic modelling research will focus on the input that atomic scale simulations make in describing the behaviour of micro-structural defects. This will refine our understanding of the fundamental physical processes that degrade fuel performance, and will result in improvements to current semi-empirical fuel performance models. The simulations will focus upon the interaction of fission products, radiation damage and dislocations, processes responsible for macroscopic observables such as fission gas release and irradiation induced creep.Fuel and cladding dimensional changes resulting from thermo-mechanical and irradiation conditions produce complex pellet-clad mechanical interactions (PCMI) that are known to cause fuel failure, especially under accident conditions. Models for PCMI failure based on ramp-test data have been developed, but these are highly empirical and therefore of limited applicability. However, advances in finite element (FE) modelling now permit the development of detailed models, and techniques such as the extended FE method can be applied to model crack growth and crack tip stresses and strains accurately whilst taking into account residual and applied stress redistribution. This research will investigate the development of pellet crack patterns and pellet-clad interface stresses under both normal and off-normal conditions. Mechanistic models for pellet failure and cladding damage will be developed.Research into composite cladding will investigate the potential for silicon carbide composites to provide significantly better performance compared with existing cladding materials. A new approach will be investigated, based on a solid SiC inner tube wrapped with SiC fibers and bonded using SiC vapour infiltration. The research will address fundamental aspects of this new concept, including: characterisation of the relationship between both design and manufacturing parameters and mechanical strength; ability of the tube to remain impermeable against fission products; and resistance to oxidation and fission product attack at high temperatures. Although UO2 has been used for many years as a fuel material, promising new materials have ben developed that could offer advantages in terms of safety and performance. The objective of this research is to identify alternative fuel materials and fuel forms; to evaluate their physical properties such as thermal conductivity; assess their reactivity with water using autoclave testing; and to assess industrially-feasible manufacturing routes. Candidate materials include alloys such as U3Si2, U-Mo, U-Zr and covalent compounds such as carbides and nitrides (in the latter case with additives to reduce the reaction rate in water). TRISO coated fuel particles manufactured by chemical vapour deposition (CVD) have demonstrated remarkable performance, but are known to be susceptible to attack by fission products such as Pa. This research will provide a fundamental understanding of these issues and will investigate alternative materials and processes to provide improved performance. The high temperature mechanical properties of coatings will be examined to understand the effects of manufacturing conditions. The mechanisms of fission product transport will be studied with a view to introducing materials and microstructural changes that will improve performance in this respect.

Private sector beneficiaries of the research will include designers and suppliers of nuclear fuels; suppliers of intermediary products such as fuel cladding and components; providers of specialist licensing services such as consultancy groups; and operators of nuclear power plants. In the longer term, improvements in coated particle technology will assist implementation of the HTR system, that can supply high temperature heat to a range of industries. All of the proposed research is linked with improvements in the performance and safety of nuclear fuel, which is expected to translate directly into reduced risk, providing a direct benefit to the public. The nuclear regulator will also benefit from access to improved fundamental understanding and modelling methods. The private sector will benefit through improvements in the competitiveness of the products that can be manufactured at the UK's nuclear fuel fabrication facility at Springfields, which will also benefit the supply chain for this business. Specialist consulting organisations will benefit from the analysis that will be required to validate and license any potential new fuel products that arise as a result of the proposed work. Nuclear power plant operators will benefit from improvements to the fuel product capabilities (by extending fuel lifetimes and reducing failure rates, for example), and also from improvements in fundamental understanding of life-limiting phenomena, which will allow better fuel models to be developed, potentially enhancing operational flexibility. The public will benefit from improvements in fuel capability and hence improved safety margins, and may also realise some economic benefit flowing from improvements in plant operations. The proposed work will also enhance the capability of UK academia and result in the training of young researchers who would be of immediate value to both private industry and public sctor bodies. Both Universities have excellent relationships with the nuclear industry and public bodies associated with nuclear energy in the UK. Agreements are in place with a majority of the UK's key nuclear industry and public sector organisations (EDF, British Energy, Westinghouse, Rolls-Royce, National Nuclear Lab, AMEC, Serco, etc.), and many of these have been involved in formulating the proposed research. Accordingly, robust arrangements are already in place for industry collaboration in the proposed research, as evidenced by the attached letters of support for the proposal, and channels for dissemination of the research results are already well established. Engagement will be ensured by working closely with the partner organisations, and providing opportunities for researchers to spend periods of time working with industry partners. The results of the research will be disseminated both at industry-focussed events such as workshops, and at wider events such as seminars. The partners will establish a web-site to engage a wider audience in the outcomes of the research activities, and will utilise their respective public affairs departments to ensure that the research is afforded a high visibility within their respective nuclear-focussed activities. As mentioned, both Universities have extensive collaborative links with private and public sector organisations, including other Universities in the UK and overseas. Both have collaborated together in previous and current EPSRC programmes such as KNOO and DIAMOND. The UK nuclear industry is well placed to exploit the benefits from the proposed research. Both Universities have established mechanisms for ensuring that the benefits of academic research are properly exploited and protected. Impact activities will be co-ordinated by the PI, but undertaken by all academic staff involved as appropriate, and the partners will benefit from the advice and involvement of Dr John Roberts, the External Liason Manager at the Dalton Nuclear Institute.