The formation energies of various defect configurations of He and H atoms in W were estimated based on the density functional theory. A special consideration was given to the coexistence of the He and H atoms at the presence of the vacancy and vacancy cluster in W. A single He atom favours a substitutional site, while a H atom spontaneously incorporates at an interstitial site with the negative formation energy. When He and H were present close to each other, they form an interstitial pair, occupying relaxed tetrahedral sites. When He, H and a vacancy coexist within a unit cell of W, however, He occupies the vacancy site then the Hesub–Htet pair was predicted to be the lowest energy configuration. At the presence of a nearby vacancy cluster, He atoms occupy the vacant space while H atoms move slightly toward W.

Energetics of He and H Atoms with Vacancies in Tungsten - First-Principles Approach. S.C.Lee, J.H.Choi, J.G.Lee: Journal of Nuclear Materials, 2009, 383[3], 244-6