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Title: Corrosion of thermally-aged Advanced Gas-Cooled Reactor fuel cladding
Author: Phuah, Chin Heng
ISNI:       0000 0004 2732 5323
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2012
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The microstructure of Advanced Gas-Cooled Reactor (AGR) fuel cladding that underpins its corrosion behaviour has been established, contributing to an understanding of long-term control, monitoring practice and storage decisions for this fuel. AGR fuel cladding specimens sourced from Sellafield Ltd, cut and individually heat treated at temperatures from 400 to 800°C for 24 to 192h were attempts to approximate irradiated AGR fuel cladding and characterised both in terms of their corrosion behaviour and of microstructure. Niobium carbide (NbC) second phases are the primary local corrosion sites. Bulk austenite-γ cladding metal (50.3±1.7 at% Fe, 21.0±1.1 at% Cr and 21.0±0.4 at% Ni) around NbC precipitates exhibited extensive corrosion even though the precipitate themselves appear unchanged. Corrosion observed from the specimen surface took the form of lacy covers around an NbC precipitate at the cover centre (~10 to 25 μm dia. depending on the site) and in the subsurface were voids (~0.1 μm pin-holes), cavities (~2 to 5 μm), an envelope of dissolved-metal region along NbC peripheries (~1 μm thick with austenite-γ composition decreased by on average 20% Fe, 21% Cr and 17% Ni) or a large, smooth concave pit bottom comparable to the cover dimension. These observations collectively suggest that AGR cladding corrosion is a diffusion-controlled phenomenon where the NbC precipitate may act as the cathode in a local galvanic couple and the adjacent austenite-γ metal is the anode that undergoes preferential oxidation. The primary contributing factors to NbC-induced AGR cladding corrosion are high NaCl concentration of the electrolyte solution, large NbC precipitates, small austenite-γ grains and presence of stress in the microstructure. Specifically, corrosion potential measurements in the 0.001M electrolyte NaCl are ~800mV (v.s. Ag|AgCl reference electrode) more noble than in the 0.1M electrolyte, suggesting that cladding wet storage requires maintenance with lowest chloride concentration practically achievable. Specimens with comparatively large NbC precipitates (~5 μm) and small austenite-γ grains (~10 μm) that result from heat treatment are ~810mV more corrosion susceptible than the as-received specimens with ~0.1 dia. NbC precipitate and ~25 μm austenite-γ grains. Increased dislocation densities were observed adjacent to the grown-NbC precipitate, imparting a stress-corrosion effect on the AGR cladding corrosion.
Supervisor: Ryan, Mary ; Lee, Bill Sponsor: Engineering and Physical Sciences Research Council ; Imperial College London
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral