Molecular dynamics simulation of cascade-induced ballistic helium resolutioning from bubbles in iron

Molecular dynamics simulations have been used to assess the ability of atomic displacement cascades to eject helium from small bubbles in iron. This study of the ballistic resolutioning mechanism employed a recently-developed Fe–He interatomic potential in concert with an iron potential developed by Ackland and co-workers. The primary variables examined were: irradiation temperature (100 and 600K), cascade energy (5 and 20keV), bubble radius (0.5 and 1.0nm), and He-to-vacancy ratio in the bubble (0.25, 0.5 and 1.0). Systematic trends were observed for each of these variables. For example, ballistic resolutioning leads to a greater number of helium atoms being displaced from larger bubbles and from bubbles that have a higher He/vacancy ratio (bubble pressure). He resolutioning was reduced at 600K relative to 100K, and for 20keV cascades relative to 5keV cascades. Overall, the results indicate a modest level of He removal by ballistic resolutioning. The results may be particularly relevant to fusion irradiation conditions which produces high levels of helium by transmutation. They can be used to provide initial guidance in selection of a “resolution parameter” that can be employed in kinetic models to predict the bubble size distribution that evolves under irradiation.