Thermodynamic stability of structural materials in a fusion reactor environment

Compatibility of structural materials with high fluxes of low energy (50-100 eV) hydrogen isotopes, oxygen and carbon is investigated by a new approach of thermochemical analysis. As representative materials molybdenum and tungsten are chosen. The concentrations in a depth of up to 10 μm of hydrogen and oxygen resulting from inmplantation, diffusion and escape are determined using the theory of Doyle and Brice for tritium inventory calculations. The concentration of carbon is determined by diffusion into the bulk material from the infinite source at the surface built up by the condensation of graphite. Isothermal Ellingham diagrams of the systems Mo-O-H-C and W-O-H-C are calculated for the temperature range 700-1500 K and compared with the concentrations of H, C and O in Mo an W. From this analysis it is found that a very deep surface layer of Mo and W will precipitate quickly Mo2C, (W2C) together with MoO2, (WO2) eventually leaving important if precipitation occurs at voids or bubbles. Aspects of increased tritium inventory and recycling time are pointed out. The system graphite-oxygen-hydrogen is also calculated and discussed with respect to equilibria in the open pores of nuclear graphite. (orig.)