In situ imaging of corrosion processes in nuclear fuel cladding

Spent nuclear fuel in the U.K. is stored within ponds dosed with NaOH in order to inhibit corrosion and, to ensure the efficiency of storage regimes, there is a need to define and quantify the corrosion processes involved during immersion of fuel cladding. In this project, state-of-the-art characterisation techniques were employed to image the corroding surfaces of two nuclear fuel cladding materials: stainless steel and Magnox. Advanced gas-cooled reactor fuel cladding consists of 20Cr-25Ni-Nb stabilised stainless steel and during irradiation the microstructure of the cladding undergoes significant changes, including grain boundary element depletion and segregation. High-speed atomic force microscopy with nanoscale resolution, enabled precipitates and pit initiation in stainless steel to be imaged. Magnox is a magnesium-aluminium alloy and during irradiation in a reactor the outer metal surface oxidises, forming an adherent passive layer which subsequently hydrates when exposed to water. Corrosion processes encompass breakdown of passivity and filiform-like corrosion, both of which were imaged in situ using the scanning vibrating electrode technique.