Radiation-enhanced exhalation of hydrogen out of stainless steel

Abstract

The state and diffusion of hydrogen in solids have been studied extensively^1,2, and the transport of hydrogen and its isotopes in metals has been found to involve classical diffusion and quantum effects such as tunnelling migration and small polaron hopping^3,4. The permeability and diffusion of hydrogen isotopes, particularly tritium (T), have been studied in a D–T fusion device⁵. Diffusion of tritium in the first wall of fusion reactor is expected to occur under high radiation fields due to plasma radiation, self β rays from the held-up tritium and neutron-induced radioactivity⁶. Heinrich et al. observed α-ray enhanced hydrogen permeation in an iron foil due to activated adsorption of the ionized hydrogen gas on the metal surface⁷. The γ-ray irradiation from a 1 mCi source, however, caused no appreciable change in permeation in 316 stainless steel⁵. No detectable loss of tritium was observed from a niobium sample containing 1% tritium⁸: the effect of the self β rays on tritium diffusion in niobium seems to be negative. We report here an experimental study of radiation-enhanced diffusion of hydrogen in hydrogen-charged stainless steel using direct observation of hydrogen exhalation and lattice dilatation.