Effects of Displacement Cascades on Helium and Hydrogen Near Grain Boundaries in Tungsten*

We conduct molecular dynamics (MD) simulations of displacement cascades consistent with secondary neutron collisions to study the effect of neutron damage on hydrogen and helium in plasma-facing tungsten. We focus on 10–30 keV cascades—energies below the threshold at which electron–phonon coupling is particularly important—that result from primary knock-on atom (PKA) trajectories that interact with helium or hydrogen atoms. In the case of helium, the size and shape of the helium bubbles near the area of peak damage can change substantially, changing the local stress and occasionally resulting in the bubble bursting and venting of the bubble's helium content back to the plasma. For hydrogen, we focused on bicrystals in which the grain boundary is parallel to the surface—this results in a layer of hydrogen forming along the grain boundary. We then introduced PKA trajectories within 30 degrees of the grain boundary plane. The overall distribution of hydrogen is not significantly different before and after the cascade: though some PKA trajectories imparted enough energy to desorb hydrogen atoms from the surface, very few of them significantly altered the distribution of hydrogen beneath the surface, particularly the distribution on the grain boundary. Our results suggest that helium bubbles are likely to deform, coalesce, or burst in the presence of neutrons, and that hydrogen near grain boundaries—prior to the formation of blisters—may be relatively unaffected by the energy and damage from secondary neutron damage cascades.