Ab initio static and molecular dynamics studies of helium behavior in beryllium

作者：Vladimirov P V^*; Moeslang A

来源：Journal of Nuclear Materials, 2013, 442(1-3): S694-S698.

DOI：10.1016/j.jnucmat.2013.04.041

Beryllium is an effective neutron multiplier material widely exploited in nuclear applications. It will be used in the helium-cooled beryllium pebble bed of fusion reactor blankets for increasing the efficiency of tritium production. Macroscopic effects of irradiation (e. g., swelling) on beryllium are greatly influenced by accumulated transmutation helium. Atomic scale simulations of beryllium behavior under irradiation are necessary for understanding the basic mechanisms and reliable prediction of microstructural changes. %26lt;br%26gt;In this study, we investigate the behavior of interstitial and substitutional helium, its diffusion pathways and interaction with point defects present in irradiated beryllium by means of static and molecular dynamics ab initio simulations. It was shown that a mixed dumbbell consisting of self-interstitial and helium atoms represents the ground-state configuration for interstitial helium. At low temperatures, the mixed dumbbell migrates in the basal plane through a series of in-basal-plane rotations, while at higher temperatures it jumps also between adjacent basal planes. %26lt;br%26gt;It was revealed that, as in many other metals, interstitial helium atoms are bound to each other (E-b similar to 1eV). In beryllium, two mixed dumbbells meet each other so that helium atoms are the nearest neighbors, whereas the helium pair can be oriented either in-or out-of-basal plane. Diffusion modes of the pair are discussed. %26lt;br%26gt;In addition, it was found that helium is very strongly bound to vacancies: the binding energy of more than 3 eV found by static ab initio calculations suggests that helium is unlikely to be released from a monovacancy at temperatures below the melting point. As has been shown previously, vacancy clusters are unstable in beryllium. This study shows that the addition of helium stabilizes di-vacancies. Thus, it is confirmed that the presence of transmutation gas is necessary for the development of a porous microstructure in beryllium.

• 出版日期2013-11