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

Management of long-lived radioactive nuclear waste implies understanding it s structural behavior when subjected to irradiation. This paper presents mo lecular-dynamics simulations about the effect of a recoil nucleus on a simp lified nuclear glass containing: SiO2, B2O3, Na2O, Al2O3, ZrO2 and a few he avy ions of uranium. A statistic on displacement cascades at energies rangi ng from 300 eV to 7 keV in glass compositions with and without alkali metal s revealed the influence of the latter on the thoroughness of structural re storation. More generally, following a depolymerization peak, the glass str ucture is reconstructed by local readjustments, facilitated by the presence of alkali metal atoms. If the cascade energy is sufficiently high, the ini tial structure is completely restored. The large majority of the atom displ acements 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 wer e identified, ranging from jumps by individual atoms to collective displace ments of an atom and its neighbors. Individual displacements and O-bridging -O-nonbridging transitions were the most numerous during the first instants of the cascade, but were quickly superseded by collective displacements an d local break-and-rebranch processes. The structure volume was observed to remain stable or increase after irradiation, but never to diminish. Coincid ental evidence was noted between the coordination numbers, the ring size di stributions, the Voronoi volumes, and the cell expansion, to provide some l ightening about the swelling origin observed experimentally. Finally, we ob serve the ease with which the nuclear glasses withstand displacement cascad es, and this is an important result in regard to the long term storage of t he oxide matrices.