Cladding behaviour of high enrichment and high burn-up nuclear fuel: determining the impact of radiation damage on storage and corrosion.

There is an economical drive to use uranium oxide fuel with higher enrichment and burn-up in the current UK nuclear reactor fleet. Higher burn-up will result in physical changes within stainless steel fuel rods, potentially impacting their performance at the end of the fuel cycle, specifically during post-discharge handling and storage prior to final disposal in a Geological disposal facility (GDF). Higher burn-up will result in larger quantities of helium generated due to the alpha-decay of the minor actinides in the fuel, and greater in-line temperatures during storage. Within the stainless steel system, these temperatures may be sufficient for helium atom diffusion and agglomeration at neutron-induced defect clusters, dislocations and grain boundaries, possibly resulting in features such as cracking, surface exfoliation, hardening and disintegration, especially during post-discharge handling for final storage. Similarly, these agglomerations may act as hydrogen getters, which will become problematic in later life. This affect may also be applicable to zircaloy cladding to some extent.

The overarching aim of this project is to enhance our understanding of the fundamental mechanisms which influence cladding behaviour in storage, focusing on the UK system. If successful, the summation of the project will allow proposal of a model for bubble formation, bubble agglomeration and distribution within irradiated stainless steels.
Specifically, this project will:
1. Determine post-discharge microstructures within ex-service AGR material, using transmission and scanning electron microscopies (TEM and SEM), and ex-service zircaloy if available.
2. Simulate post-discharge microstructures using cold work, ion-implantation and thermal annealing to produce non-active analogue samples,
3. Determine the impact of post-discharge microstructure on mechanical and corrosion properties, to assess the impact of high burn-up fuels on long-term storage and handling.

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