The behaviour of helium in neutron irradiated beryllium: A molecular dynamics study

An atomic scale study is presented of the trapping of helium atoms by vacancies and di-vacancies in beryllium. Constraint molecular dynamic methods are used to estimate formation energies, helium-vacancy binding energies, migration energies as well as for direct evaluation of Helmholtz free energy differences. The beryllium cohesion is described with a model derived from the second moment tight binding approximation, while the helium-beryllium interaction is based on the embedded atom model and a mean field approximation. It is found that up to ten helium atoms may be bound to one single vacancy and up to fourteen to a di-vacancy. The possible trapped helium configurations are identified. The thermodynamic stability of such helium-vacancy clusters is found to be only little dependent on the vibrational entropy in a temperature range from 0 K to 1270 K, which shows that 0 K energetic calculations already provide reasonable free energy estimates. This suggests that the method can be used to study helium retention in large systems with extended defects at temperatures relevant to fission and fusion technologies.