Molecular Dynamics Simulations of Radiation Damage Effects in Disordered Waste Forms

The nature of the amorphous state has been notably difficult to ascertain at the microscopic level. In addition to the fundamental importance of understanding the amorphous state, potential changes to amorphous structures as a result of radiation damage have direct implications for the pressing problem of nuclear waste encapsulation. Here, we develop new methods to identify and quantify the damage produced by high-energy collision cascades that are applicable to amorphous structures and perform large-scale molecular dynamics simulations of high-energy collision cascades in a model zircon system. We find that, whereas the averaged probes of order such as pair distribution function do not indicate structural changes, local coordination analysis shows that the amorphous structure substantially evolves due to radiation damage. Although we do not reach the point of convergence where the changes of the amorphous structure saturate, our results imply that the nature of this new converged amorphous state will be of substantial interest in future experimental and modeling work [1].<br/>We subsequently address radiation damage effects in two important systems, zirconolite and zircon. We find that the amorphous structure of zirconolite responds to radiation damage differently from the crystal. Amorphous zirconolite is found to be "softer" than crystalline zirconolite with a much larger number of atoms becoming displaced and changing coordination during a 70 keV cascade. The local coordination and connectivity analysis shows that the amorphous structure continues to evolve as a result of repeated radiation damage, changes which cannot be identified from globally averaged properties such as pair distribution functions. We also find large density inhomogeneities at the local level which we suggest may play an important role for future developments in nuclear waste storage. Our results raise an interesting possibility of whether an evolution of the amorphous structure due to radiation damage can converge to a new equilibrium amorphous regime, posing the fundamental question of what that structure may be [2].<br/>In zircon, we study the effects of radiation damage on helium diffusion [3]. We observe an increase in activation energy for helium diffusion as a result of radiation damage and increasing structural disorder. The activation energy in a heavily damaged region is smaller than in a completely amorphous system which is correlated with remaining order in the cation sublattices of the damaged structure not present in the fully amorphized system. The increase in activation energy is related to the disappearance of fast diffusion pathways that are present in the crystal. Consistent with the change in activation energy, we observe the accumulation of helium atoms in the damaged structure and discuss the implications of this effect for the formation of helium bubbles.<br/>1. A Diver et al, J Phys: Condens Matt 32, 415703 (2020)<br/>2. A Diver et al, J Nucl Mater 544, 152654 (2021)<br/>3. A Diver et al, J Mater Research 36, 3239 (2021)