Effect of microstructure on irradiation creep and growth of zircaloy pressure tubes in power reactors

Abstract

Cold-worked zircaloy pressure tubes fabricated by eight different routes and irradiated in four power reactors have diametral strain rates varying from −1 to 11 × 10 −29 m 2/n (>1 MeV) and elongation rates varying from 5 to 16 × 10 −29 m 2/n (>1 MeV). X-ray diffraction and optical metallography show large differences in crystallographic texture, dislocation density and grain shape amongst the eight types of tubes. A quantitative correlation of the elongation and diametral rates of seven types of tubes with their texture, grain shape and dislocation density is developed assuming additive creep and growth components. The derived magnitudes of the creep and growth components in the pressure tube agree with published uniaxial creep and growth data for cold-worked zircaloy. The growth rate is approximately proportional to dislocation density and increases in a given direction as the grains become more elongated in that direction and as the proportion of basal plane normals in that direction decreases. The creep rate is insensitive to dislocation density. For internally pressurized tubes the diametral creep rate increases and the axial creep rate decreases as the proportion of basal plane normals in the radial direction increases. The model developed to describe the total long term dimensional changes of the pressure tubes successfully predicts the diametral and longitudinal rates for tubes which experienced end loads during operation.