Performance Characterisation of AGR Fuel Cladding Relevant to Long-term in-Pond Storage in pH-moderated Aqueous Environment

Fuel cladding of the UK’s advanced gas-cooled reactors (AGR) is made of 20%Cr/25%Ni–Nb stabilised austenitic stainless steel. The research reported in this paper aims to enhance the mechanistic understanding of AGR fuel cladding performance. Thermal heat treatments were applied to simulate a range of cladding microstructure, including a sensitised and stabilised condition. Oxalic acid metallographic etching and Electron Back-Scattered Diffraction (EBSD) were used to characterise microstructure, augmented by electro-chemical assessments to provide a quantitative measure of the degree of sensitisation. Anodic polarisaiton tests in 500ppm chloride containing aqueous environments at neutral pH and pH 11.5 indicated differences in microstructure corrosion resistance. In neutral environment pitting corrosion is observed in as-received microstructure, inter-granular corrosion initiating at triple junctions in the sensitised microstructures, and localised pitting corrosion at Nb-rich precipitates interfaces in the stabilised microstructure. Whereas at high pH a suppression of corrosion was achieved, supporting the strategy to pH-control storage pond environments. However, microstructure analysis of the sample surfaces after exposure to pH 11.5 solution indicated dissolution of Nb-containing precipitates, which is currently subject of further investigations. The relationship between microstructure conditions, corrosion morphology, with relevance to currently applied corrosion mitigation strategies for in-pond storage of AGR fuel cladding will be elaborated.