Role of grain boundaries and carbon deposition in deuterium retention behavior of deuterium plasma exposed tungsten

Data on deuterium retention in W single and polycrystals exposed to D plasma at 500 K are presented. The energy of D ions was well below the threshold of damage production. In a W polycrystal the deuterium spreads to a depth of about 4 μm. In contrast, no deuterium was found even at a depth of 2 μm in a W single crystal. This is an indication of significant grain boundary transport of deuterium at relatively low temperature. It has been found that ∼10 nm thin carbon layers grown on the plasma facing surface can be a barrier for D penetration into the bulk of tungsten.