Deuterium diffusion along the three principal directions in anisotropic Zr-2.5Nb

Pressure tubes of cold-worked Zr-2.5Nb material are used in the CANDU (CANada Deuterium Uranium is a registered trademark of Atomic Energy of Canada Ltd.) reactors to contain the fuel bundles and the heavy water coolant. The pressure tube microstructure consists of α-Zr platelets, with an aspect ratio of 1:5:50 in the radial, transverse and longitudinal directions, surrounded by a thin layer of β-Zr. The diffusivity of hydrogen in β-Zr is higher than in α-Zr. As a result, the diffusivity of hydrogen in the pressure tube material is enhanced compared to unalloyed α-Zr. Due to the anisotropic microstructure of the pressure tube with respect to the β-Zr network, the diffusivity of hydrogen is thought to be different in the three principal pressure tube directions. Measurements made using specimens machined from an 8 mm thick Zr-2.5Nb plate, with a microstructure similar to that of a CANDU pressure tube, suggest the difference between the diffusion constants for deuterium in different directions of an as-installed pressure tube may be as high as 25%.