Retention of D in neutron-irradiated W and desorption were examined after plasma exposure at 773 K. Deuterium was accumulated at a relatively high concentration up to a large depth of 50-100 m due to the trapping effects of defects uniformly induced in the bulk. A

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Pressure-composition isotherms of TiC 1Àx -H system were measured at 773, 873 and 973 K over a pressure range from 10 À2 to 10 5 Pa. The obtained isotherms could be divided in three concentration regions. In the low concentration region in which ½H=½TiC 1:4 Â 10 À3 , Sieverts' law held, and the solubility of hydrogen clearly increased with decreasing temperature. The enthalpy of solution was À70 AE 5 kJ mol À1 . In the region in which 1:4 Â 10 À3 < ½H=½TiC 8:0 Â 10 À3 , the dependence of hydrogen concentration, C H , on pressure and temperature weakened as pressure increased. Finally, C H was independent of temperature when it reached 8:0 Â 10 À3 . This C H was comparable with the concentration of carbon vacancies in TiC 1Àx . In the high concentration region in which ½H=½TiC ! 8:0 Â 10 À3 , Sieverts' law held again, but the solubility of hydrogen was almost independent of temperature. These observations indicated that hydrogen dissolved in carbon vacancies in the low and middle concentration regions with the above-mentioned value of enthalpy of solution, and also occupied much less stable sites in the high concentration region.

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