Flow measurement in high-temperature liquid metals and molten salts: principles, recent advances, and deployment...

Next-generation energy systems-including sodium- and lead-cooled fast reactors, molten salt reactors, concentrating solar power, and fusion breeding blankets-rely on high-temperature liquid metals and molten salts operating at approximately 400 °C-700 °C, conditions under which conventional flow instrumentation becomes increasingly constrained. This Mini Review examines electromagnetic, ultrasonic, optical, differential-pressure, vortex, Coriolis, and thermal approaches to flow measurement, with an emphasis on the temperature-sensitive physical properties and sensor-fluid interfaces that govern measurement validity and practical deployment. Differences of several orders of magnitude in electrical conductivity between liquid metals and molten salts favor electromagnetic techniques in liquid metals, particularly sodium, while reducing the signal margin of conductivity-dependent inductive and Lorentz-force approaches in molten salts. Ultrasonic techniques are independent of electrical conductivity and have been demonstrated in high-temperature fluoride-salt loops, although acoustic coupling, thermal-interface stability, and long-term qualification remain important limitations. Across all principles, the central challenge is maintaining measurement validity in the presence of thermal drift, corrosion, changes in wetting, material degradation, and limited opportunities for recalibration. Future high-temperature flow instrumentation will therefore require not only thermally robust sensors but also measurement systems that incorporate condition diagnostics, reduced calibration dependence, and cross-validation between independent physical principles.