Effect of Hf doping on He behavior in tritium storage material ZrCo

An exhaustive analysis based on density functional theory (DFT) simulations of the effect of Hf doping on helium behavior has been performed in ZrCo. The He impurities have been placed both at interstitial positions and substitutional positions from the first nearest neighbor (1nn) of the Hf atom to the sixth nearest neighbor (6nn). In such areas, the electronic charge density is different, and therefore the formation and diffusion of He atoms vary in the surrounding of the Hf atom. The results show that Hf doping reduces the volume of the interstitial sites nearby, resulting in the weakening ability of the interstitial sites to accommodate He atoms. According to the results of formation energy, whether it is the substitutional He or the interstitial He atom, the formation is not only related to the distance of Hf, but more importantly, it is closely related to the unit cell where the He atom is located. In addition, Hf atoms promote the capture of He atoms by vacancies nearby and the migration of He atoms between the tetrahedral positions. The result also validates the well-known knowledge of vacancies as efficient sinks for He atoms in ZrCo. From the lower and lower migration energetic barriers along 3nn → 2nn → 1nn → 1nn pathways, we can infer an increasing mobility of the He atom from 3nn to 1nn. This situation could favor their accumulation surrounding an Hf atom, improving the ability of helium retention. These findings provide really indisputable evidence that the Hf dopant makes a difference in the behavior of He atoms in bulk ZrCo. Therefore, a ZrCo system with Hf doping can be considered as a good candidate for tritium storage material in a future nuclear fusion reactor.