Measurement of tritium permeation in FLiBe (2LiF-BeF2)

Interrelated transport processes and chemical interactions characterize the behavior of hydrogen isotopes in molten fluoride salts. In fusion systems hydrogen species in the molten fluoride salts include H2, HT, T2, HF, and TF. Tritium is generated by neutron capture in Li, while hydrogen is present to control the fluorine potential in the gas phase or as a residual product of salt purification. In general, the transport processes are dependent on the following properties: solubility and diffusivity in the molten salt, dissociation at the surface and diffusion in contacting materials, mass transport and possibly recombination at gas-melt interfaces. The chemical behavior is determined to a great extent by the fluorine potential in the molten salt (REDOX condition), which is in turn affected by the radiation environment and nuclear transmutation reactions, by reactivity with container or other contacting materials, by reactivity with the gaseous environment over the salt and by interaction with impurities. This paper reports on the experimental investigation of tritium permeation in flibe (2LiF-BeF2) at the Safety and Tritium Applied Research facility of the Idaho National Laboratory in the frame of the US-Japan Jupiter-II collaboration. A stainless steel cell formed by two independent volumes separated by a 2mm thick nickel membrane is maintained at temperatures between 500 and 650 degrees Celsius. A controlled amount of T2 gas is flown in excess of argon in the source volume in contact with the bottom side of the nickel membrane, while a thin layer of molten salt is in contact with the top side. The salt has been purified and conditioned to minimize impurities following well-established procedure developed during previous corrosion control experiments, and is maintained in contacts only nickel containers during test and liquid-phase transfer operations. The tritium permeating above the liquid surface is carried by an argon flow to a diagnostic system comprised of a quadrupole mass spectrometer, a gas chromatographer and a proportional counter previously calibrated with D2. Tritium permeability in flibe as a function of temperature is determined by the measured permeation flow rates reached in steady-state conditions, while the diffusivity is determined by fitting the transient process with the analytical solution for the diffusion process. As a result, the solubility of tritium in flibe as a function of temperature is also determined. (orig.)