Molecular dynamics study of interstitial He clusters in nickel

This study presents a molecular dynamics analysis focusing on the behavior of interstitial helium (He) clusters in nickel (Ni), examining their formation, stability, and migration energetics. We found that the binding energies of interstitial He with a He cluster are positive and increase with the cluster size, indicating a preference for He atoms to cluster together. While the formation energy increases monotonically with cluster size, binding energy shows non-monotonic. trend Importantly, small He clusters were found to be thermally unstable at reactor operational temperatures (approximately 600 K), with the He-2 cluster exhibiting instability even at room temperature. With a binding energy of 0.44 eV for a He-4 cluster, we hypothesize that for He bubbles to form via homogeneous nucleation (i.e., through trap mutation) at reactor operating temperatures, the He concentration must be high enough to facilitate the formation of He clusters of at least size 4 or larger. At finite temperatures, He clusters of size 7 and larger trap mutate immediately. However, clusters of size 10 and larger will trap mutate even at 0 K. As expected, interstitial He and small He clusters are highly mobile, and found to be mobile even at temperatures as low as 200 K. Furthermore, the mean squared displacement method has been utilized to determine the activation energies and the corresponding prefactors for clusters ranging from He-1 to He-6