DFT calculations of vacancy formation energies and interaction energies with hydrogen atoms in tungsten

Full text: Since tungsten will be used for plasma facing component materials in ITER and is a candidate material also for DEMO reactors, tritium retention in neutron damaged tungsten is a key issue. Recently, experimental works with fission neutrons and high-energy heavy surrogate ions have been performed to evaluate the radiation effects. These works show significant enhancement of the retention in the damaged tungsten and primary roles of radiation induced vacancies and/or vacancy clusters (voids). In divertor plasmas, the tungsten target would be exposed to low-energy and high-flux plasma particles as well as neutron. Strong enhancement of the hydrogen retention in single- and polycrystalline tungsten specimens has been observed at higher fluxes. A distinct feature of plasma irradiation from gas permeation obeying the Sieverts' law is that incident hydrogen atoms/ions from the plasma can readily overcome permeation barriers of the surface without high pressures. One can, therefore, assume that under the continuous high-flux implantation local concentration of hydrogen atoms can exceed the solubility limit of the tungsten. In such cases, the tungsten matrix may contain super-saturated hydrogen atoms and sustain extremely high stresses. Therefore, the blistering of tungsten surfaces may be induced by the exceedingly high local concentration of hydrogen atoms beyond the solubility limit of the tungsten. (author)