Oxide microstructure and deuterium ingress in Zr-2.5Nb CANDU® pressure tubes

The microstructures of oxides grown on the inside surface (water-side) of Zr-2.5Nb pressure tubes removed from CANDU(R)* reactors were characterised by TEM and correlated with deuterium ingress. Oxide cross-sections consisted of two structurally distinct regions: a columnar oxide region next to the metal/oxide interface, and an outer coarse equiaxed oxide region. Near the metal/oxide interface, the microstructure consisted of columnar grains without overt porosity. Away from the interface, the oxide consisted of coarsened equiaxed grains with (100)(m)-twins, grain-boundary cracks and nanopores. The oxide microstructures on various pressure tubes differ in the proportion of equiaxed grains. Electron micrographs suggest that a larger proportion of equiaxed grains is associated with higher deuterium uptake. The predominance of grain-boundary cracks in equiaxed oxides indicates that they are likely more permeable to water than columnar oxides. Energy dispersive X-ray analyses revealed substantial amounts of Fe-K-alpha (and Mn-K-alpha) in the equiaxed oxide grains at the outermost surface. Energy Dispersive X-ray mapping of Fe-K-alpha and detection of the Mn-K-alpha (produced by neutron activation of Fe-54 and subsequent decay of Fe-55) in the absence of external excitation, unequivocally demonstrated that the iron had accumulated in the oxide during reactor operation. The Fe concentration was highest near the outermost region, and decreased inwards towards the metal/oxide interface. These results are consistent with permeable equiaxed oxides picking up considerable amounts of Fe at the outermost region from the heavy water coolant.