Molecular dynamics simulation of energetic uranium recoil damage in zircon

Abstract : Defect production and amorphization due to energetic uranium recoils in zircon (ZrSiO$_4$), which is a promising ceramic nuclear waste form, is studied using molecular dynamics simulations with a partial charge model. An algorithm that distinguishes between undamaged crystal, crystalline defects and amorphous regions is used to develop a fundamental understanding of the primary damage state. The damage consists of under-coordinated Zr and polymerised Si leading to amorphization and phase separation on a nanometer scale into Zr-and Si-rich regions. This separation could play an important role in the experimentally observed formation of nanoscale ZrO$_2$ in ZrSiO$_4$ irradiated at elevated temperatures. The observed amorphisation and the associated volume expansion have the potential to enhance leaching of cations by providing diffusion pathways for cations and water molecules.