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.