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 mum. In contrast, no deuterium was found even at a depth of 2 mum 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 similar to 10 nm thin carbon layers grown on the plasma facing surface can be a barrier for D penetration into the bulk of tungsten. (C) 2001 Elsevier Science B.V. All rights reserved. [References: 7]
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