Thermodynamics of hydrogen in graphite at high temperature and the effects of oxidation, irradiation and isotopics

Abstract

The interaction between hydrogen isotopes and graphite at temperatures of 500–800 °C and a few Pa partial pressure is of importance to the management of tritium produced in fission and fusion reactors. Data are compiled on uptake and desorption from nuclear graphite above 500 °C and up to tens of kPa partial pressures. The enthalpies of reaction at the different types of reactive carbon sites (RCS) are reviewed and then used to discuss how temperature and pressure extrapolations could be performed in order to fill the gaps in available hydrogen uptake data. Chemisorption is the dominant pathway of hydrogen uptake at the conditions of interest. The importance of Trap 2 for reversible hydrogen uptake is emphasized. Neutron irradiation and pre-oxidation of graphite impact the abundance of available RCS and the relative distribution between Trap 1 and Trap 2. Isotopic effects will impact differently the occupancy fraction of Trap 1 and Trap 2.