Behaviors Of Hydrogen,Helium And Transmutation Elements In Tungsten-based Materials

As the plasma facing material,W must withstand 14.1 MeV fusion neutron,and low energy H and He ions irradiation.Thus many point defects are produced in W material,such as H,He,transmutation elements,and Frenkel pairs.The interactions of those point defects in W would not only influence the microstructure and mechanical properties of W material,but also affect the retention of H isotopes in W.Therefore,first-principle calculations and the theory of atomic diffusion in metal were used to study the effects of transmutation elements Re,Os and Ta on the behaviors of H and He in W,the inhibiting effect of Re on the growth of H bubble in W,as well as the interactions of vacancy and Re,Os and Ta in W.The interstitial H and He still prefer the tetrahedral site in presence of Re,Os and Ta in W,and Os can trap He atom,Ta could weakly capture H and He atoms.The low concentrations of Re,Os and Ta have a little influence on the migration barrier of H in W,but 25 at.%Re can significantly decrease the migration barrier of H.The interaction between two H atoms is repulsive,therefore,the diffusion coefficient of H in W and W-Re alloy can be evaluated by first-principle calculations combined with harmonic transition state theory.The results show that the presence of Re would increase the diffusivity and decrease the solubility of H in W for the perfect crystals.In addiation,Re can decrease the trapping energy and number of vacancy on H atoms in W,thus Re has an inhibiting effect on the growth of H bubble.Those results may explain the experimental phenomenon that the size of H bubble in W-Re is smaller than in pure W after exposure to a low energy high flux D plasma.The substitutional Re and Os atoms in W can decrease the vacancy formation energy and facilitate the formation of divacancy.Furthermore,Os is more effective to increase the thermal equilibrium vacancy concentration than Re,the presence of only 1 at.%Os in W can increase the vacancy concentration by 2-6 orders of magnitude in the range of 300-900 K.It is found that vacancy plays a key role in interactions between transmutation elements.For instance,the presence of vacancy can increase the binding energies of Re-Re,Os-Os and Re-Os to above 0.2 eV,0.5 eV and 0.5 eV,respectively,although without a vacancy their binding energies are close to 0eV.