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

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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.