The unique ability of hydrogen isotopes to rapidly permeate through heated palladium membranes has been recognized for over 130 years. Palladium's infinite selectivity for hydrogen relative to other gases makes Pd alloy permeators an attractive technology to use in fusion fuel purification and exhaust cleanup operations, either as a stand-alone device or as an integral component of a palladium membrane reactor. Virtually all applications of Pd membranes for hydrogen purification, though, have been at relatively high pressures (>1 atm.) where the thermodynamic driving force for permeation is large. Under these conditions, it is well established that the mass flux through the metal obeys the Richardson equation, in which the flux varies with the difference between the square-roots of the hydrogen partial pressures on either side of the membrane. However, in fusion applications it is necessary to purify exhaust and waste streams to extremely low levels of tritium, on the order of ppm, where the driving force for permeation is small. The permeability of hydrogen in Pd alloys is poorly understood under these conditions, with conflicting literature reports ranging from a continuation to follow the classical P^1/2 scaling, to a shift in the flux from a square-root to a linear dependence on partial pressure, to possibly no permeation at all.

This paper reports results on the permeability of tritium, deuterium, and protium through a Pd/25% Ag membrane at partial pressures of 10^-4 - 20 torr and temperatures of 625 - 825K. Experiments were conducted at the Tritium Systems Test Facility (TSTA). The experimental apparatus consisted of a 12 x 1/4" Pd/Ag tube, sealed at one end and encased in an evacuated 4" pipe, which was surrounded by a clamshell heater. Hydrogen isotopes, mixed with helium or argon, were metered into the interior of the Pd/Ag tube and permeated outward into the enclosing pipe. Unpermeated gases were withdrawn through at 1/16" tube centered coaxially within the palladium membrane. Gas compositions were monitored using ionization chambers and gas chromatography. Results for the pressure, temperature, and isotope dependence are presented and interpreted in terms of a mechanistic model which accounts for adsorption, desorption, and diffusion limitations in the metal, as well as axial and radial diffusion in the gas phase. Results for permeation isotopic mixtures are also presented and compared to existing and new theories for multicomponent permeation.