Cluster formation and eventual mobility of helium in a tungsten grain boundary

An exhaustive analysis based on density functional theory (DFT) simulations of the accumulation of several He atoms has been performed at the vicinity of a non-coherent W $〈110〉$ /W $〈112〉$ interface. The He impurities have been placed both at interstitial positions along the grooves present at each side of the interface and inside the most stable interfacial single vacancy. At such areas, the electronic charge density is lower and the repulsion with the metallic atoms is minimized. Our results show much lower formation energies at both positions studied here as compared to the equivalent bulk cases, confirming the effective great attraction exerted on helium by this kind of interfaces. The most stable groove is completely filled before the system prefers to promote the He atoms to other alternative groove. On the other hand, the vacancy can admit at most seven He atoms, but the successive ones find the best accommodation in the surrounding sites thereafter. This result corroborates the well-known picture of vacancies as efficient sinks for He atoms in W. The binding energy estimation suggests a larger attraction between the He atoms and the vacancy. From the low values obtained at the interface and the energy barriers estimated, we can infer a decreasing mobility of the He clusters along the interface for a given temperature. This situation could favor their accumulation in the stable grooves until they are filled and the outgassing process could subsequently take place. Therefore, a tungsten system with many interfaces, the so-called nanostructured W, can be considered as a good candidate for plasma facing material in a future nuclear fusion reactor.