Availability of Zr80Ni20-alloy for tritium Extraction from tritiated methane

Decomposition of methane by gettering materials is one of promising methods to extract tritium atoms from tritiated hydrocarbon species in the exhaust gases of thermonuclear fusion devices. Basic properties of the powered Zr80Ni20-alloy on the decomposition kinetics of methane and the absorption-desorption characteristics of hydrogen isotopes have been investigated. The Zr80Ni20-alloy mainly consisted of Zr and Zr2Ni crystals. The decomposition rate obeyed the first order kinetics with respect to the pressure of methane, and most of methane(>99.7%) was decomposed within a few minutes at 723K. The activation energies for the decomposition were determined as 37.2 and 58.3 kJ/mol for temperatures of Region I and Region II, respectively. Pressure changes of hydrogen in the temperature range from 373 to 673K indicated the existence of two hydrides. ΔHo and ΔSo evaluated from temperature dependence of the equilibrium dissociation pressure at a low hydrogen concentration were determined as-168 kJ/mol-H2 and -124J/K·mol-H2, respectively. These values were close to those of Zr-hydride. On the other hand, the pressure changes at a high hydrogen concentration indicated the existence of Zr2Ni-hydride. It was revealed, therefore, that hydrogen atoms liberated by methane decomposition are absorbed by Zr and Zr2Ni crystals, depending on the hydrogen concentration in the Zr80Ni20-alloy. (author)