The Effect of Stress on Orientation of Hydrides in Zirconium Alloy Pressure Tube Materials

The presence of radial-axial hydride in pressure tubes in CANDU reactors is to be avoided since it can result in significant reductions in fracture toughness. Fabrication procedures produce tubes which have little radial-axial hydride when hydrided without stress. A quantitative technique for evaluating the susceptibility of pressure tube material to the formation of radial-axial hydride when hydrided under stress has been developed and applied to four types of Zr-2.5 wt % Nb material and to Zircaloy-2. The results indicate that the threshold stress for producing such hydride increases with material strength. This probably reflects grain to grain residual stresses built into the material during fabrication.