Formation and Growth Process of Dislocation Loops in Zircaloys under Electron Irradiation

We have investigated the formation and growth process of dislocation loops in Zircaloys (Zrys) under electron irradiation using a high voltage electron microscope (HVEM). Dislocation loops are of great importance to degradation phenomena in fuel claddings, such as irradiation growth and reduced ductility in light water reactors. TEM specimens of three kinds of Zircaloys (Zry-2, Zry-4 and improved Zry-2) were irradiated with 1MeV electrons at temperatures from 320 K to 970K in the HVEM. Interstitial-type dislocation loops with the Burgers vector b = 1/3(1120) were formed on the pyramidal or the prismatic planes at the beginning of irradiation. It was found that the nucleation and growth process of those loops follows the kinetics based on the &-interstitial model where di-interstitials act as the nuclei of interstitial dislocation loops. Based on this model, migration energies of interstitials and vacancies were determined to be 0.15–0.22eV and 1.0–1.2eV, respectively, from the irradiation temperature dependence of the density and the growth rate of loops.