The effect of cold work on irradiation creep of Zircaloy-2 alloys at low temperature

Internally pressurized capsules made from Zircaloy-2 tubing were irradiated in the National Research Universal (NRU) reactor at Chalk River Laboratories (CRL) at a temperature of approximately 57°C (330 K) in a fast neutron flux of about 3.37 × 10 ¹⁷ n·m ⁻²·s ⁻¹ (E>1 MeV). The purpose of this experiment was to study the effect of cold work and crystallographic texture on biaxial stress irradiation creep. The creep capsules were cold worked in both transverse and axial directions. The applied hoop stress during irradiation ranged from 0 to approximately 200 MPa. Two irradiation cycles were completed with a fast neutron fluence of 3.18 × 10 ²⁵ n·m ⁻² (E >1 MeV). Diametral and axial strains were measured after each irradiation cycle. This experiment has provided the following insights on Zircaloy-2 alloys irradiated at low temperature. The axial strain increases with fast neutron fluence, while the diametral strain either increases or decreases with fluence depending on the value of hoop stress and the amount of cold work. Both the diametral and axial strain rates increase non-linearly with the hoop stress. There is a weak dependence of the strain rate on cold work at low hoop stresses (50-150 MPa) suggesting that dislocation slip is not a dominant strain producing mechanism at irradiation temperature of about 57°C. The non-linear dependence of the strain rate on cold work that is observed at 200 MPa indicates that dislocation slip mechanisms start to dominate at this stress level. Dislocation densities of the creep capsules were measured by X-ray line broadening, and their structures were further analyzed using transmission electron microscopy (TEM). From these results, the weak dependence of the creep rate on cold work at low stresses can be explained using rate theory model formalism.