Graphite chamber with diamond windows to look at molten salts

Molten salts, and fluoride salts in particular, hold promise for multiple roles in next-generation nuclear power. Due to their stability under irradiation, minimal parasitic neutron absorption and low melting points and vapor pressures, their proposed uses range from fuel and coolant in fission reactors to a tritium breeding blanket in fusion.

To help realize the full potential of fluoride salts, Sean Robertson and Michael Short designed an optical system for measuring their thermophysical properties.

Due to the high corrosivity and operating temperatures of fluoride salts, existing measurement techniques struggle to accurately characterize them. The device created by Robertson and Short facilitates the use of transient grating spectroscopy, a technique that creates local changes in density and temperature within the salt through the absorption of two pulsed lasers. Monitoring the relaxation of these changes in density and temperature allows for determination of the salt’s thermal diffusivity and sound speed.

Their design uses a high-density graphite cell with two diamond windows that allow for laser transmission through the salt.

They tested the system’s capabilities by measuring the sound speed and thermal diffusivity in lithium chloride, which has well established measurement data, and found their device to perform well, with a temperature uncertainty of around 0.5 C.

Following this proof of concept, Robertson and Short have turned their attention to fluorides, focusing first on a commonly proposed coolant salt known as FLiNaK. They hope that by having more accurate salt property data, the safety and efficiency of next generation reactor designs can be optimized.

Source: “Design and performance of a molten fluoride salt-compatible optical thermophysical property measurement system,” by Sean G. Robertson and Michael P. Short, Review of Scientific Instruments (2021). The article can be accessed at http://doi.org/10.1063/5.0049727 .