High hydrogen retention in the sub-surfaces of tungsten plasma facing materials: A theoretical insight

A high hydrogen concentration at the sub-surface layers of tungsten plasma facing materials has been previously observed by experiments. We have performed the first-principles calculations to understand this experimental phenomenon. The layer-dependent hydrogen dissolution energies, vacancy formation energies and hydrogen trapping energies are calculated for the W(110) surface. Our results show that hydrogen dissolving into the vacancy-free sub-surfaces is energetically unfavorable. A surface tungsten vacancy is easier to form than in the bulk and has a large trapping effect on hydrogen dissolving. The present work demonstrates the important role of surface vacancies on the hydrogen retention in the tungsten sub-surfaces.