Dynamic processes during displacement cascades in oxide glasses: A molecular-dynamics study

Management of long-lived radioactive nuclear waste implies understanding its structural behavior when subjected to irradiation. This paper presents molecular-dynamics simulations about the effect of a recoil nucleus on a simplified nuclear glass containing ${\mathrm{SiO}}_2,$ ${\mathrm{B}}_2{\mathrm{O}}_3,$ ${\mathrm{Na}}_2\mathrm{O},$ ${\mathrm{Al}}_2{\mathrm{O}}_3,$ ${\mathrm{ZrO}}_2$ and a few heavy ions of uranium. A statistic on displacement cascades at energies ranging from 300 eV to 7 keV in glass compositions with and without alkali metals revealed the influence of the latter on the thoroughness of structural restoration. More generally, following a depolymerization peak, the glass structure is reconstructed by local readjustments, facilitated by the presence of alkali metal atoms. If the cascade energy is sufficiently high, the initial structure is completely restored. The large majority of the atom displacements occur during the first instants of the cascade, during the thermal peak; displacements during the initial structure restoration phase account for only a small fraction of the total. Several types of displacements were identified, ranging from jumps by individual atoms to collective displacements of an atom and its neighbors. Individual displacements and ${\mathrm{O}}_{\mathrm{bridging}}-{\mathrm{O}}_{\mathrm{nonbridging}}$ transitions were the most numerous during the first instants of the cascade, but were quickly superseded by collective displacements and local break-and-rebranch processes. The structure volume was observed to remain stable or increase after irradiation, but never to diminish. Coincidental evidence was noted between the coordination numbers, the ring size distributions, the Voronoï volumes, and the cell expansion, to provide some lightening about the swelling origin observed experimentally. Finally, we observe the ease with which the nuclear glasses withstand displacement cascades, and this is an important result in regard to the long term storage of the oxide matrices.