Breakaway Growth Modeling of Zirconium under Irradiation: The Importance of the Formation of <a>-Loop Layers

Abstract

We present object kinetic Monte Carlo simulations that have been developed to understand a number of experimentally observed facts related to the growth of high-purity recrystallized zirconium alloys under irradiation. In this modeling, the irradiation growth is the sum of the elemental deformations generated by defects resulting from irradiation. Such deformations were determined using atomic-scale (ab initio and empirical potential) calculations. According to our results, breakaway growth is strongly related to the vacancy diffusion anisotropy: in agreement with ab initio calculations, vacancies diffuse faster in the basal planes than in planes perpendicular to them. Conversely, the diffusion of interstitials is taken as almost isotropic, as shown by recent ab initio calculations. This combination of point-defect diffusion anisotropy leads to the formation of layers of <a> interstitial prismatic dislocation loops, which are parallel to the basal plane. These layers have been reported experimentally, but the rafts are made of <a> interstitial and vacancy loops. Their formation is also correlated with the growth of <c> vacancy loops that are introduced in the model by the collapse of stacking-fault pyramids. This collapse could explain why the diameter of the <c> loops has never been experimentally observed below a size of the order of 9 nm and before a certain threshold of fluence. Thus, the “breakaway” results from the development of <c> vacancy loops and the rafting of <a> prismatic loops. In a previous work these observations were reproduced, but rafts were only compounded of <a> interstitial loops in the simulation box.