Up-to-date growth results on Zr-2.5Nb pressure tube material from the DIDO reactor at AERE Harwell are presented. The observed temperature dependence and magnitude of growth are significantly different from the representations in a recently published model for in-reactor deformation of pressure tubes. These differences are discussed in terms of (1) the effects of stress on growth, (2) the comparability of test reactor and power reactor conditions, and (3) the relative contributions of creep and growth to pressure tube elongation.

A preliminary estimate of the anisotropic creep component of the in-service deformation suggests that the creep anisotropy may be different from that in the model (derived from small specimen stress relaxation tests). Biaxial in-reactor creep tests on specimens with microstructures and textures similar to pressure tubes are required to support this view. This would also help to resolve an apparent discrepancy in the temperature dependence of growth and pressure tube elongation.