Wettability of molten salts on structural materials: implications for drain, flush, and freeze-valve operations in molten salt reactors

Several operational procedures in liquid-fuel molten salt reactors (MSRs) involve interfacial phenomena alongside geometry, thermal hydraulics, and phase change: gravity draining of the fuel salt, flushing of the circuit before maintenance, and operation of the freeze valve that seals and releases the drain line. This mini review synthesizes quantitative evidence on molten salt wettability, drawing on MSRE-era surface tension compilations, sessile-drop studies of alloys and graphite in fluoride and chloride melts, and concentrated solar power research on nitrate-salt wetting and creeping. The available evidence indicates condition-dependent wetting behavior: FLiBe is commonly non-wetting on nuclear graphite under controlled conditions, whereas other salt–substrate pairs include wetting exceptions. Wettability generally increases with temperature, while the direction and magnitude of impurity- and corrosion-related changes depend on the salt–substrate pair. Non-wetting raises the capillary pressure required for graphite infiltration, while alloy wetting may favor local film retention and crevice access. Contact-angle hysteresis and capillary, viscous, and gravitational forces provide a framework for interpreting these processes. The review relates these wetting regimes to MSRE operating experience while distinguishing fuel carryover from measured wall-film inventory and liquid-state wetting from frozen-plug adhesion. This assessment identifies four priorities for predictive drainage, flushing, and freeze-valve models: standardized corrosive-melt goniometry, additional FLiBe data, dynamic-wetting measurements, and independent characterization of solidified salt–wall interfaces.