Physics and engineering review of gas sparging in molten salt reactors

• Gas sparging can remove noble gas and, to a lesser extent, tritium and noble metals from the salt. • Two-film mass-transfer and flotation models describe removal of gases and metals. • Removal efficiency depends on engineering factors, such as bubble size, gas flow rate, and mass transfer coefficient. • Earlier MSRE and MSBR prototypes inform sparging system designs for modern MSRs. • Necessary experiments include salt-based tests to replace water-glycerin data and validation of noble metal migration models. The operation of molten salt reactors (MSRs) generates noble gases, tritium, and noble metals from fission reactions, neutron activation of salt constituents or impurities, and decay of other fission products. The accumulation in the salt of these products creates substantial operational and safety challenges associated with high neutron capture cross-sections (for xenon and krypton), high permeability across metal structures (for tritium), and precipitation on reactor internals (for noble metals). Since the early concepts of MSRs, inert gas sparging has been proposed as a strategy to remove noble gases from the salt and has shown potential to affect the distribution of tritium and noble metals. Gas sparging accelerates the transport of salt-dissolved gases to the cover gas by increasing the salt-gas interfacial area and providing an alternative to bubble nucleation. At the same time, the introduction of bubbles enables transfer of noble metals via flotation. The transfer rate of these species from the salt is affected by chemical properties (e.g., solubility, diffusion coefficient), operating conditions (e.g., temperature, pressure, salt velocity), and engineering parameters (e.g., size of the sparging bubbles, liquid–gas separator design). In this paper, we present a review of the physics and engineering of gas sparging and discuss its potential towards the removal of noble gases, tritium, and noble metals, highlighting fundamental and applied gaps towards its development in new generation MSRs.