Hydrogen trapping in neutron-irradiated graphite

Bulk hydrogen retention and the analysis of absorption kinetics have been studied on three brands of graphite irradiated with neutrons at various fluences. Two kinds of hydrogen trapping sites may exist and be additionally produced during irradiation: interstitial cluster loop edge sites (trap 1) and carbon dangling bonds at edge surfaces of crystallites (trap 2). Neutron irradiation preferably creates trap 2 sites at lower fluences and trap 1 sites at a higher fluence above 0.017 dpa. Trap 2 tends to be annealed out at high temperatures, although trap 1 is hardly decreased even at 1873 K. Diffusion coefficients of hydrogen are reduced for 1–2 orders of magnitude after neutron irradiation at 0.047 dpa, although the activation energy of hydrogen diffusion is kept nearly the same level to unirradiated samples.