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Secondary ion mass spectrometry (SIMS) and transmission electron microscopy (TEM) have been used to investigate composition and structure of oxides on pure zirconium and Zr-2.5Nb following both in and out-reactor exposures in aqueous and gaseous environments. Thin oxides formed in steam at 400°C on Zr-2.5Nb act as excellent hydrogen permeation barriers for CANadian Deuterium Uranium (CANDU) pressure tubes. Following up to 4350 effective full power days (EFPD) exposure in-reactor in the annulus gas, and out-reactor elevated exposures to deuterium gas, these oxides generally continue to show diffusional-type through-thickness deuterium concentration profiles, with negligible deuterium contents at the metal/oxide interface. Diffusion coefficients inferred from these profiles are as low as ∼2 × 10-22m2/s at 300°C. The structure of these thin oxides on Zr-2.5Nb consists of columnar grains with amorphous regions at grain boundaries and at the metal oxide interface, and non-interconnected porosity, which implies that deuterium permeation is likely controlled by solid state diffusion through the bulk oxide. At regions containing relatively high deuterium contents in the bulk metal of removed pressure tubes, outside surface oxides showed several regions with flat deuterium concentration profiles with higher deuterium concentrations at the metal/oxide interface. Examination of thicker oxides with interconnected porosity, on pure Zr, following exposures to pure deuterium gas, also showed the presence of flat deuterium concentration profiles. This would tend to suggest that regions of high deuterium concentration dissolved in the base metal of pressure tubes may also contain oxides with interconnected porosity.
zirconium, Zr-2.5Nb, hydrogen ingress, hydrogen diffusion, oxide structure, oxide composition, electron microscopy
University of Western Ontario, London, Ontario