Hydrogen retention in graphite and carbon materials under a fusion reactor environment

Bulk hydrogen retention and hydrogen diffusion in graphite and carbon materials have been studied to estimate hydrogen recycling and tritium inventory under a fusion reactor environment. Two kinds of hydrogen trapping sites may exist. The first will be one of lined carbon dangling bonds located at the edge surface of a crystallite with an adsorption enthalpy of 2.6 eV, the second will be a solitary carbon dangling bond, such as an interstitial cluster loop edge, with an enthalpy of 4.4 eV. The correlation between hydrogen retention and the microstructure should refer to the edge surface area of a crystallite for an unirradiated sample and to lattice spacing along the c axis for an irradiated sample. The diffusion process is the rate-determining step for hydrogen absorption into graphite, and detrapping dominates the hydrogen desorption process due to the high trapping energy.