An innovative, multi-scale, real-time approach to the understanding of deformation and fracture in irradiated nuclear reactor core graphites

Graphite is one of the most fascinating materials used in the current UK reactors and is a candidate for the new generation of high temperature reactors (Gen IV) designed to operate for 60 to 100 years. Graphite has complex microstructure and behaviour under irradiation; it is a non-replaceable reactor core component in Advanced Gas-cooled Reactors (AGRs) and, hence, is life-limiting. This material has attracted extensive academic and industrial scrutiny to assist in underwriting the safe operation of nuclear fission reactors. Currently, the UK has 16 reactors generating about 20% of its electricity and all but one of these is scheduled to retire by 2023. However, life extension averaging 7 years for AGR units has been planned. There is 8 years before the earliest "end-of-life" scenarios for these AGRs is reached and this has set the horizon for this work programme on graphites. Lifetime extension of the AGRs is of strategic importance, not only for EDF Energy and its commercial interests but also for the UK's ability to meet electricity demand before the new generation of reactors are able to come online.
Further understanding of the graphite structure in the moderator components of AGRs continues to ensure their safety. Key challenges remain, and have to be addressed in terms of improving the fundamental mechanistic understanding of nuclear graphite. Although research in these areas is difficult and challenging, the present project proposal builds on the PI's expertise in this topic area, combined with the use of emerging novel techniques, to attack this critical problem.
1. Multi-scale characterisation of nuclear graphite
To generate microstructure-based descriptions at appropriate length-scales - with quantification of damage evolution - of the salient deformation, fracture mechanisms and general mechanical properties of irradiated nuclear reactor core graphites, a novel approach to investigate local damage has been developed by the PI at the University of Bristol. This approach, and combining the outcomes with computer modelling, has the advantage of establishing a solid fundamental base for structural integrity analysis and lifetime prediction of nuclear graphite.
2. Microstructure-based deformation and fracture of nuclear graphite at temperature
To provide three-dimensional, in situ, at-temperature (over 1000 deg. C for Gen IV reactors) characterisation of the deformation and fracture of graphites using computed synchrotron X-ray micro-tomography. No such tests have been undertaken on nuclear graphite. This objective will take into account the microstructural gradient created in AGR reactors in the UK and, hence, provide direct impact on life extension decision making. Part of this work will be undertaken with Prof. Robert Ritchie at the University of California, Berkeley, U.S.
3. Microstructure-based thermal creep in nuclear graphite under stress
To provide mechanistic understanding of the dimensional change of graphite over service life, i.e. to evaluate the thermal contribution to creep of virgin and irradiation graphite under load from ambient to reactor temperature (over 1000 deg. C for Gen IV reactors). Prof. Bryan Roebuck, of the National Physical Laboratory in the UK, will provide access to equipment that allows the realisation of these investigations.
4. Optimisation of project output
Inputs from the above three aspects will assist in generating a revised life evaluation methodology. On completion of the project with the above three key areas addressed, mechanistic understanding of the graphite, and the class of materials it represents, will directly benefit the related academic community. Dissemination of the results at the end of the project in the form of workshops will feed the input to industry and, thus, allow direct impact on the decision making for the continued safe operation of current reactors in the UK and validation for future reactors globally.

The UK's nuclear renaissance is underpinned by a high-quality, vibrant academic base. To maintain and further enhance the current national competitive advantage has been identified as a government strategic priority. In this context, the proposed project is anticipated to impact on the following aspects:
1. Impact on cross-disciplinary academic communities worldwide
The proposed research will benefit multidisciplinary research communities, including direct impact on nuclear fission, nuclear composites, damage tolerant porous ceramics, concretes, etc. The wide application of these materials and the importance of understanding deformation and fracture at multi-length scales for structural components set the importance and unbounded impact for the proposed project. The proposal is based on innovative methodologies that build on the PI's unique expertise and a combination of state-of-the-art technologies. The outcome therefore represents world-class advances.
2. Immediate and long term impact on nuclear fission in the UK and worldwide
The proposed research aims to investigate the fundamental mechanistic aspects of nuclear graphite, a material that has to be understood to ensure the continued safe operation of current AGR reactors in the UK and to directly impact upon the material qualification for the Gen IV reactors operating at very high temperatures across the world. Fundamental and novel research into nuclear graphite is key to maintain the world-leading position of the UK in nuclear fission in the face of strong and rapidly-growing international competition, providing unique input to the international academic community and influencing industrial decision making in critical areas. In addition, the methodology and characterisation of nuclear graphite will be transferred to other materials such as nuclear composites, concrete, porous ceramics, foams and biometric materials, either through conferences, new collaborations or joint publication with existing partners that have already had established reputation in composites, power plants and aerospace areas (for example, Prof. Robert Ritchie on biometric materials, Prof. James Marrow on nuclear composites and the PI's expertise on porous ceramic thermal barrier coatings).
3. Impact on the economy and society
Life extension of advanced gas-cooled reactors (AGR) is of significant importance for electricity demand to be met in the UK in the immediate future. With a strategic objective of seeking up to 7 years of life extension for all of the AGR stations, each operational site secures over 500 jobs, £300m of investment into the nuclear supply chain each year and helps to avoid ~1.2 million tonnes of CO2 emissions per year. Successful design and safe operation for the future Gen IV reactors worldwide has immeasurable benefit to secure energy supply and, in the meantime, to reduce the potential of adverse environmental impact from nuclear fission to minimum.
4. Impact on the host institution and public
The proposed research brings together a unique international team of experts to target tough problems encountered in nuclear graphites and the related class of materials. The host institution of this project, the University of Bristol, has been ranked to have the highest impact on nuclear research in the UK and has prioritised investment on nuclear related research. This proposed project will bring in a novel branch of work on nuclear graphites and will unarguably enhance the leading position of the University of Bristol in the UK on nuclear. In addition, the PI has rich experience interacting with the public in terms of school teaching and, most recently, additional funding from the University of Bristol was allocated to the PI to expand the recipients of this type of teaching to pupils from disadvantaged backgrounds. With this project, naturally, the PI will take the fundamental, exciting and newest concepts to a wider range of audience.