Hydrogen release behaviors from the 8Cr2W stainless steel (RAF/M) around RT are examined by using tritium tracer techniques, and trapping effects of bulk and surface are discussed. In the overall release, three different release stages are clearly distinguished giving three different diffusion coefficients and release amounts which indicate the existence of different kinds of trapping. In addition, the appreciable amount of hydrogen (tritium) is trapped on the surface and/or surface oxides of RAF/M, but they are hardly released and show no influence on the overall hydrogen release behavior.

At very low hydrogen concentration, almost all hydrogen atoms are trapped at the deepest trapping site, probably M23C6, and the sites are easily saturated. With increasing the hydrogen concentration, the shallower trapping sites are occupied. Remaining hydrogen atoms seem to be in normal (interstitial) sites, whose amount increases with the square root of the hydrogen loading pressure, but they are still influenced by trapping with lattice imperfections and/or grain boundaries.