Molecular dynamics study of structural changes versus deposited energy dose in a sodium borosilicate glass

Assessing the long-term behavior of nuclear glass implies evaluating the impact of cumulative alpha decay induced by the minor actinides it contains. When subjected to alpha decay (244Cm-doped glass specimens) or to external ion irradiation, some macroscopic properties vary appreciably with the dose. Above a given dose level, the properties do not evolve any more. To improve our understanding of these modifications, studies are carried out on simplified glass compositions (three oxides SiO2, B2O3, Na2O), modeled by molecular dynamics in which irradiation effects are simulated by accelerating uranium projectiles. Accumulation of displacements cascades have been performed up to 4.5*1020 keV/cm3 nuclear energy deposited in the glass. The density variations observed in actinide-doped materials is qualitatively reproduced. At high doses, the swelling tends to stabilize. Marples model is used to fit the glass swelling versus the deposited energy dose, giving the volume damaged per projectile. This volume approximates the cascade core volume, suggesting that the underlying mechanisms of volume expansion are contained in the cascade core and are thus related to the highest energy events: atom ejection and thermal quenching. On the contrary, the volumetric parameter of the Marples model applied to the other structural properties is related to a volume corresponding to the core + periphery of the cascades. (authors)