Mechanism of deuterium pickup in Zr–2.5Nb alloy

Abstract

There are approximately 400 Zr–2.5Nb pressure tubes in a CANDU reactor. During operation, the pressure tubes contain heavy water at about 300°C, 10.3 Mpa with a room-temperature pD of 10.5. Corrosion of the pressure tubes leads to oxide formation and absorption of deuterium. Since deuterides are brittle, it is important to control the amount of deuterium absorbed to prevent hydride formation. A knowledge of how the deuterium passes through the oxide film to enter the metal is an important step in gaining control over ingress rates.Fresnel fringe imaging of cross-sectioned oxides grown on pressure tubes, combined with tilting in the electron microscope has revealed the true nature of porosity in the oxide films. Two primary types exist, flake and ribbon. The main vehicle for deuterium ingress is via ribbon porosity as shown by EIS (electrochemical impedance spectroscopy). The location of the ribbon porosity is along the boundary between the oxidized α-Zr and β-Zr phases. Modifications to reduce ribbon porosity are possible and this in turn, leads to significantly lower rates of deuterium absorption and extension of pressure tube lifetime.