Dislocation loop banding in zirconium alloys under irradiation – DIBAZA

The main objective of the project is to give a physically sound basis to mesoscopic models of microstructure evolution in Zr alloys under irradiation in order to gain a better understanding of the longstanding problem of their macroscopic growth in such conditions. Given the strong non-linearity of the microstructure evolution equations, the parameters of the model have to be checked or constrained either with atomic scale calculations and/or with experiments. A particular attention will be paid to the study of (a) loop banding in the microstructure and its impact on the macroscopic growth. This work is of fundamental (microstructure evolution under irradiation in anisotropic materials) and technological interest (safety of Pressurized Water nuclear Reactors - PWRs). This is the reason why academic and industrial partners are associated with this project, all specialists in the behavior of Zr alloys under irradiation, including the two most important companies in the French nuclear industry (EDF and FRAMATOME).
The work planned includes the experimental study of precursors of (c) loops, (a) loops and Nb nanoprecipitates. Indeed, these microstructural defects are of particular importance for the growth of the Zr claddings but their formation mechanism is not well known, especially for the (c) loops, these can involve precursors of a different nature which must be identified. The alignment of the (a) loops parallel to the basal plane, although often observed, has never aroused particular interest in explaining the growth. However, recent object kinetic Monte Carlo simulations suggest that this alignment is correlated with the acceleration of growth. Further experimental study of this alignment as well as the proportion of interstitial and vacancy (a) loops is required to validate this growth model. Finally, the addition of Nb appears to improve resistance to growth. Is this linked to the formation of Nb nanoprecipitates under irradiation? Answering this question implies to better understand their interaction with the (a) and (c) loops which supposes a thorough characterization of these objects. This will be carried out experimentally but also by calculations at the atomic scale, in order to determine the degree of coherence of these nanoprecipitates with the matrix and consequently their stress field. In addition, since these precipitates behave like sinks for point defects, their sink strengths will also be calculated. This information will serve as input data for mesoscopic models used to simulate the formation of microstructures in irradiated Zr alloys. Object kinetic Monte Carlo type simulations will be carried out. This technique will in fact be chosen because it is the only one so far to have reproduced the banding of (a) loops parallel to the basal plane. It is therefore planned to consolidate the parameterization of this code for pure Zr and then to consider the effect of Nb in solution and in the form of precipitates. Finally, a complementary phase-field approach will be developed in order to study the elastic effects more precisely and to simulate the radiation induced segregation of Nb near (a) and (c) loops which may possibly lead to the formation of Nb nanoprecipitates.