THE BEHAVIOR 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 bindin g energies, migration energies as well as for direct evaluation of Hel mholtz free energy differences. The beryllium cohesion is described wi th 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 heliu m atoms may be bound to one single vacancy and up to fourteen to a di- vacancy. The possible trapped helium configurations are identified. Th e thermodynamic stability of such helium-vacancy clusters is found to be only little dependent on the vibrational entropy in a temperature r ange from 0 K to 1270 K, which shows that 0 K energetic calculations a lready provide reasonable free energy estimates. This suggests that th e method can be used to study helium retention in large systems with e xtended defects at temperatures relevant to fission and fusion technol ogies. (C) 1997 Elsevier Science B.V.