Modeling Zirconium Under Neutron Irradiation: Interatomic Potentials, Displacement Cascades, and Electron-Ion Coupling

This thesis presents a comprehensive atomistic computational study on the effects of neutron irradiation on Zr, a material extensively used in CANada Deuterium Uranium (CANDU) reactors. Our goal is to improve the understanding of microstructural changes under irradiation, focusing on displacement cascades that produce defects. Point defects and their clusters influence the mechanical properties, dimensional stability, and service life of components in reactor environments. The key contributions are: 1. Reparameterization of Interatomic Potentials: Three embedded atom method (EAM) potentials were reparameterized to improve simulation accuracy. Using density functional theory (DFT) calculations, the two-body and embedding energy functions were refitted to better describe atomic interactions at short distances. The modified potentials maintained similar near-equilibrium properties such as defect formation energies and elastic constants but significantly improved threshold displacement energies (TDEs), particularly for [0001] direction. 2. Simulation of Displacement Cascades: Large-scale molecular dynamics (MD) simulations modeled high-energy collision cascades in hexagonal close-packed (HCP) α-Zr. The study examined defect generation, damage morphology and diffusion behaviour. For this purpose, a novel post-processing technique was developed. Self-interstitial atom (SIA) clusters displayed greater anisotropy than equilibrium clusters. 1D and 2D diffusion modes were observed. Findings were integrated into a rate-theory model that predicted radiation-induced growth strains. 3. Effect of Electron Stopping: MD simulations that incorporate ion-electron coupling were conducted using a calibrated version of the unified two-temperature model (UTTM). This approach allowed for a more accurate assessment of electronic effects on primary damage production in Zr, representing the first application of the UTTM to neutron irradiation in Zr. The results suggested that the electronic stopping and the assisted recovery altered the damage production. This thesis advances the atomic-scale understanding of radiation-induced damage in Zr. The reparameterized EAM potentials enhance the fidelity of future MD simulations. The insights into the anisotropy of defect diffusion and its role in radiationinduced growth, along with the novel UTTM-based simulations, contribute significantly to the field. These findings provide a stronger foundation for ongoing experimental, operational, and regulatory efforts to maintain the safety and performance of nuclear reactors.

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