Absorption and desorption of hydrogen, deuterium, and tritium for Zr–V–Fe getter

Abstract

Nonevaporable getters have wide applicability for developing the tritium handling techniques for thermonuclear fusion devices. From this viewpoint, mechanisms of the absorption and desorption of hydrogen isotopes and the isotope effects were investigated for a Zr–V–Fe alloy (St-707) by means of the mass analyzed thermal desorption spectroscopy. It was observed that the absorption rate was proportional to the first power of the pressure, indicating that the rate limiting step is the dissociative adsorption of hydrogen isotopes on the surface. The activation energy was very small, in the order of magnitude of a few tens of calories per mole in a temperature range from −196 to 200 °C. The desorption rate was proportional to the square of the amount of absorption, indicating that the rate limiting step is the associative desorption reaction of hydrogen atoms or ions diffused to the surface from the bulk. The rate constants for hydrogen and deuterium were determined as kd(H2)=(5.3+2.6−1.7)exp[−(28.0±0.7)×103/RT] and kd(D2)=(5.0+2.7−1.7)exp[−(28.6±0.8)×103/RT] in [1/Pa 1 s], respectively, where R is in [cal/mol deg]. With regard to tritium, the rate constant was evaluated as kd(T2)=(5.0+20−4.0)exp[−(29.3±3)×103/RT], however, the frequency factor will have to be corrected by knowing the relative sensitivity factor of the mass spectrometer for tritium (T2).