Development of a small in-situ ultraviolet-visible absorption spectrometer for molten salt in thorium-based molten salt reactors

BackgroundFluoride molten salt is a key coolant and fuel carrier for thorium-based molten salt reactors (TMSR). Its microstructure directly determines the physicochemical properties at high temperatures, and ultraviolet-visible (UV-Vis) absorption spectroscopy is an effective method to detect the oxidation states and coordination environments of metal ions in molten salts. Limited by extreme conditions such as high temperature (>500 ℃), strong corrosion, high volatility and sensitivity to trace H2O/O2, conventional instruments are difficult to realize in-situ characterization of fluoride molten salts.PurposeThis study aims to develop an in-situ high-temperature UV-Vis absorption spectroscopy system specifically designed for corrosive fluoride molten salts.MethodsThe core of in-situ high-temperature UV-Vis absorption spectroscopy system for corrosive fluoride molten salts was a high-temperature optical furnace with optical paths, integrated with inert atmosphere protection, high-precision temperature control, efficient thermal insulation and corrosion-resistant structure. Performance verifications for this developed spectrometer included temperature calibration, mechanical repeatability and room-temperature benchmark comparison. Using this system, experiments were conducted on FLiNaK and FLiBe molten salts, where the salts were heated to elevated temperatures under an inert atmosphere, and the spectrometer was employed to collect absorption spectra and to monitor the dynamic evolution of chromium species during the reaction process.ResultsExperimental results show that the system operates stably up to 800 ℃ without optical window degradation or baseline drift. At 650 ℃, FLiNaK-CrF3 exhibits two well-resolved absorption peaks at approximately 450 nm and 700 nm, corresponding to the characteristic d-d transitions of Cr3+. FLiBe-UF4 shows a characteristic absorption spectrum consistent with literature reports. These peak positions deviate from literature values by less than ±2 nm. During real-time monitoring of Cr3+ in FLiBe at 650 ℃, the absorbance at 450 nm decreases from 1.28 to 0.39 within 120 min.ConclusionsThe developed UV-Vis absorption spectroscopy system in this study has the ability of in-situ characterization of molten salt species and redox processes in high-temperature and strong corrosion environments, providing an experimentally verified key test platform for the research of molten salt chemistry and corrosion mechanism of TMSR.