Understanding the Effect of Strain on Microstructure Properties and Environmental Degradation of AGR Fuel Cladding

The backbone of the UK's nuclear energy is its fleet of Advanced Gas-cooled Reactors (AGR). The Uranium-oxide-based fuel within these reactors is safely encased in 20%Cr-25%Ni-Nb stainless steel cladding, to withstand the high operating temperatures and associated burn-up conditions. Radiation- and temperature-induced changes in cladding microstructure, however, occur during service exposure, which can render the material susceptible to localised corrosion, which has been observed during post-reactor storage. This PhD project aims to elucidate the effect of microstructure strain on the material performance and corrosion behaviour in post-reactor aqueous storage environment. The key objective of the research is to understand whether local microstructure strain and cold work plays a key role in AGR fuel pin failure, focusing primarily on aqueous storage conditions in pH-controlled environments.

The microstructure of cold worked, strained, and sensitised (as well as sensitised and then strained) samples will be characterised using a wide range of characterisation techniques, including SEM, AFM/SKPFM, EBSD, XRD and XPS. Nano-, micro-, and meso-scale corrosion measurements using novel electrochemical characterisation techniques will be employed to probe the corrosion response. Corrosion rate measurements will then be correlated to local microstructure strain characteristics using in-situ time-lapse imaging and Digital Image Correlation (DIC). An extension of the work to investigate dry storage conditions using similar characterisation techniques and methodologies will also be explored via exposure of strained AGR cladding to thin layer electrolytes under Co-60 irradiation. The proposed project draws on a previous PhD study supported by NDA, which has explored general corrosion mechanism in sensitised simulants of AGR fuel cladding, identifying the role of carbide inclusions and associated corrosion mechanism. The proposed work here focuses on more realistic microstructure conditions relevant to AGR fuel pins, including end caps, strained anti-stacking grooves and welds.

The student will have opportunity to spend several days @ NNL/Culham to discuss results/literature with NNL experts, and to Identify literature in NNL's data-base that may be of interest. A secondment into the Nuclear Decommissioning Authority (NDA) will be sought. (A Research visit to Swansea University to explore application of their in-situ time-lapse rigs.

The EPSRC Centre for Doctoral Training in Advanced Metallic Systems was established to address the metallurgical skills
gap, highlighted in several reports [1-3] as a threat to the competitiveness of UK industry, by training non-materials
graduates from chemistry, physics and engineering in a multidisciplinary environment. Although we will have supplied ~140
highly capable metallurgical scientists and engineers into industry and academia by the end of our existing programme,
there remains a demonstrable need for doctoral-level training to continue and evolve to meet future industry needs. We
therefore propose to train a further 14 UK based PhD and EngD students per cohort as well as 5 Irish students per
cohort through I-Form.

Manufacturing contributes over 10% of UK GVA with the metals sector contributing 12% of this (£10.7BN [4,5]) and
employing ~230,000 people directly and 750,000 indirectly. It is estimated that ~2300 graduates are required annually to
meet present and future growth [5]. A sizeable portion of these graduates will require metallurgical expertise and current
numbers fall far short. From UK-wide HESA data, we estimate there are ~330 home UG/PGT qualifiers in materials and
~35 home doctoral graduates in metallurgy annually, including existing AMSCDT graduates, so it is unsurprising that
industry continues to report difficulties in recruiting staff with the required specialist metallurgical knowledge and
professional competencies.

As well as addressing this shortfall, the CDT will also impact directly on the companies with which it collaborates, on the
wider high value manufacturing sector and on the UK economy as a whole, as follows:

1. Collaborating companies, across a wide range of businesses in the supply chain including SMEs and research
organisations will benefit directly from the CDT through:

- Targeted projects in direct support of their business and its future development and competitiveness.
- Access to the expertise and facilities of the host institutions.
- Involvement in the training of the next generation of potential employees with advanced technical and leadership skills
who can add value to their organisations.

2. The UK High-Value Manufacturing Community will benefit as the CDT will:

- Develop the underpinning science and advanced-level knowledge base required by future high technology areas, where
there is high expectation of gross added value.
- Provide an enhanced route to exploitation, by covering the full spectrum of technology readiness levels.
- Ensure dissemination of knowledge to the sector, through student-led SME consultancy projects, the National Student
Conference in Metallic Materials and industry events.

3. The wider UK economy will benefit as the CDT will:

- Promote materials science and engineering and encourage future generations to enter the field, through outreach
activities developed by the students that will increase public awareness of the discipline and its contribution to modern
life, and highlight its importance to future innovation and technologies.
- Develop and exploit new technologies and products which will help to maintain a competitive UK advanced
manufacturing sector, ensure an internationally competitive and balanced UK economy for future generations and
contribute to technical challenges in key societal issues such as energy and sustainability.

References:
1. Materials UK Structural Materials Report 2009
2. EPSRC Materials International Review 2008
3. EPSRC Materially Better Call 2013
4. The state of engineering, Engineering UK 2017
5. Vision 2030: The UK Metals Industry's New Strategic Approach, Metals Forum