Trapping of hydrogen and helium at dislocations in tungsten: an ab initio study

Retention of plasma gas components such as hydrogen (H) isotopes and helium (He) is one of the limiting factors in selection of plasma facing materials for future thermonuclear fusion devices. Tungsten (W) is one of the promising candidates for such materials and was chosen for the divertor armor for International Thermonuclear Experimental Reactor (ITER) and the first wall material for the design of the demonstrational fusion power plant - DEMO. For the analytical estimation of accumulation of H/He components in tungsten, it is important to understand the relevant physical mechanisms of their trapping in the material and thoroughly parameterize them numerically.
Experiments involving high flux plasma exposures of tungsten at temperature below 500 K conclude on significant amount of retained hydrogen, which unlike helium, does not agglomerate in the form of clusters in the bulk defect-free material. The observed hydrogen isotope trapping and deep diffusion is conventionally attributed to the trapping at the natural lattice defects such as dislocations and grain boundaries.
Computational assessment of trapping strength and capacity of the dislocations is the subject of this work. Here the electronic structure calculations using density functional theory (DFT) are done to evaluate the affinity of hydrogen and helium to the screw and edge dislocations. For this, we calculate the interaction energy map around the dislocation core for hydrogen and helium atoms. The energetically favorable positions are rationalized on the basis of charge density distribution and local stress concentraion. The results obtained help to refine the input parameters of the macro-scale models of retention of plasma components, such as mean field rate theory methods.
The additional molecular statics simulations are also performed to analyze whether the contempory atomistic models using the recently developed interatomic potentials for W-H-He system can grasp adequately the interaction of H and He with dislocations.