In-reactor deformation of internally pressurized Zr-2.5 wt% Nb tubes at 570 K

Small tubes of cold-worked Zr-2.5 wt% Nb, which were internally pressurized to give hoop stresses from -25 to 495 MPa have been irradiated at 571 K for up to 23400 h. Diameters and length have been measured periodically. The resulting circumferential and axial strain rates have been compared to those calculated from an equation derived earlier from CANDU pressure tubes which takes account of texture, grain shape, dislocation density, stress, fast neutron flux and temperature; 80% prism slip and 20% basal slip is assumed for creep. This comparison shows that axial and circumferential irradiation growth is well calculated by the equation but creep of the small tubes is greater than predicted and the anisotropy between circumferential and axial creep strains is not well predicted. Since an unrealistically high proportion of basal slip is required to account for this anisotropy, another factor, e.g. distribution of active dislocations, is influencing anisotropy. The lower creep strength of the small tubes compared to pressure tubes is probably due to the absence of a β-network around the α-grains. (orig.)