In-Situ Imaging of Corrosion Processes in Nuclear Fuel Cladding Spent nuclear fuel in the UK 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 (AGR) 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. Highspeed atomic force microscopy (HS-AFM) with nanoscale resolution, enabled precipitates and pit initiation in stainless steel to be imaged. Magnox is a magnesiumaluminium 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 (SVET).

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Aluminium extrusions used for heat transfer applications are exposed to corrosive environments, which can eventually lead to perforation of the tube by pitting. In this study, the effect of microstructure on the corrosion behaviour of extruded AA3XXX series alloys has been investigated. It was revealed that pits developed were purely crystallographic and a drop testing procedure was developed to simulate the seawater acetic acid test in a laboratory environment. The drop testing experiment has been successfully employed to study the early stages of pitting for different surface finishes. Selected pits were studied with in-SEM nanotomography. Stable pits initiated on areas free of second-phase particles, in the vicinity of grain boundaries. When the corrosion front reached a grain boundary plane, pit propagation was interrupted in the direction perpendicular to the plane and the corrosion front followed grain boundaries, corroding one grain body more than its neighbour. A mechanistic understanding of crystallographic pitting corrosion has been developed.

Abstract. Worldwide, a variety of approaches to the management of spent fuel have been adopted. A review of approaches adopted internationally was undertaken to inform decision making on spent fuel management in UK. The review surveyed spent fuel storage and disposal practices, standards, trends and recent developments in 16 countries and carried out more detailed studies into the evolution of spent fuel storage and disposal strategies in four countries. The review highlighted that: (1) spent fuel management should be aligned to the national policy for final dispositioning of the fuel; (2) national spent fuel storage arrangements should deliver efficiency across all spent fuel management activities; (3) commercial and financial arrangements should ensure that spent fuel management decisions do not unnecessarily limit future fuel handling, packaging and disposal activities; (4) extended storage of spent fuel prior to packaging provides increased flexibility in the design of future packaging and disposal concepts. Storage of spent fuel over 100 years or more using existing technologies is technically feasible and operationally credible. Local factors such as existing infrastructure, approach to fuel cycle management, existing experience/capability and short-term cash flow considerations all influence technology selection. Both wet and dry storage systems continue to receive regulatory approval and are acceptable.