Joanna McFarlane

LIBS enables real-time monitoring of noble gases and aerosols for molten salt systems as demonstrated on an engineering-scale pumped molten salt loop.

Molten salt reactors (MSRs) require reliable thermophysical and thermochemical property data for reactor design, safety analysis, licensing, and operational assessment. In particular, the speed of sound is important for describing molten salt compressibility and for completing equations of state for multiphysics analyses. This work presents the design, validation, and initial application of a high-temperature ultrasonic pulse-echo system for measuring the speed of sound in molten salts. The system uses a quartz rod as a guided wave transmitter and receiver. An ultrasonic pulse propagates through the rod, enters the molten salt, reflects off the bottom of the salt vessel, and returns to the transducer. The sound speed is calculated from the measured round-trip travel time and the propagation distance between the rod tip and container bottom. The method and system were first validated using room-temperature water (22 °C), for which a measured velocity of 1519 ± 38 m/s was compared with the accepted value of 1488 m/s. The system was then used to measure a LiCl–KCl mixture (60/40 mol %) at temperatures of 435°C, 520°C, and 644°C, yielding velocities of 2039 m/s, 1937 m/s, and 1829 m/s, respectively. The corresponding estimated standard uncertainties were 52 m/s, 49 m/s, and 47 m/s. The measured velocity decreased with increasing temperature and showed good agreement with available literature data. These results demonstrate the feasibility of the compact ultrasonic pulse-echo approach for high-temperature molten salt measurements and establish an infrastructure for future measurements of fluoride salts as well as actinide- and/or beryllium-containing salts.

The capture of volatile radioiodine from nuclear fuel reprocessing off-gas streams remains a critical challenge due to the high volatility, long half-life, and biological uptake of 129I from the environment. Although silver-based sorbents provide strong iodine chemisorption, their high cost and regulatory classification as mixed radioactive-hazardous waste motivate the development of alternative materials. Here, we report a silver-free Cu2O–Ti3C2Tx MXene hybrid for iodine gas capture at 150 °C. Structural and compositional analyses confirm the formation of Cu2O nanoparticles on Ti3C2Tx nanosheets and their subsequent conversion to thermodynamically stable CuI upon static iodine gas exposure. The nanohybrid achieves an iodine mass loading of up to 1115 mg/g. The iodine-loaded hybrid was further consolidated by spark plasma sintering into a mechanically self-supporting CuI–Ti3C2Tx monolith that retained the CuI phase and exhibited a seven-day cumulative normalized iodine loss of 1.64 g/m2, corresponding to only 0.076% of the initial iodine inventory during modified ASTM C1308–21 leach testing. These results demonstrate the potential of Cu2O–Ti3C2Tx MXene as a copper-based alternative to silver sorbents for elevated-temperature iodine gas capture.

Much of the radionuclide transport from a molten salt reactor (MSR) during operation may arise from aerosols generated in the pump bowl or from condensation of vaporized species. The Visualizing Insoluble Species and Transport of Aerosols (VISTA) apparatus was used to investigate such aerosol generation, bubble transport, and insoluble species behavior in a molten LiCl-KCl eutectic salt system. The VISTA apparatus provided an optically accessible quartz test cell coupled with simultaneous shadowgraph imaging, particle tracking velocimetry (PTV)/particle image velocimetry (PIV), laser-induced breakdown spectroscopy (LIBS), and high-resolution electrical low-pressure impactor (HR-ELPI+) measurements. Experiments examined the effects of sparging orientation, sparge gas flow rate, and cover gas sweep rate on bubble behavior and aerosol transport. Baseline tests were conducted with LiCl-KCl, followed by tests in which 2.6 µm Ni powder was introduced as an insoluble species. In a first-of-a-kind experiment, hybrid PIV-PTV measurements captured the motion of dispersed aerosols within the test cell cover gas region. These measurements led to velocity estimations. The HR-ELPI+ measurements indicated that the cumulative aerosol response generally increased with both sparge and sweep flow rate. Number-based particle size distributions were dominated by an ultrafine mode, with geometric mean diameters (GMDs) of approximately 0.024 µm to 0.027 µm, whereas the estimated mass distributions were dominated by larger particles with GMDs of approximately 4.7 µm to 4.8 µm. These results demonstrate that a relatively small population of coarse particles indicate aerosol mass even when ultrafine particles dominate the particle number distribution. LIBS detected Ni spectral features during sparging, with signal intensity increasing as the Ni concentration increased from 0.01 mol % to 0.03 mol %. Cascade impactor analysis further indicated that Ni-containing salt enabled the transport of greater mass than Ni-free salt. The increased aerosol mass collected was higher than what could be solely attributed to the higher density of Ni. These results support the use of combined optical, spectroscopic, and aerosol measurements for characterizing radionuclide transport and informing MSR off-gas and source term models. Experience gained from VISTA experiments will be used to prepare for the more challenging fluoride salt analyses, for which the use of quartz cells is not feasible. These experiments have started to address the challenge of measuring the transport of particles in molten salt media—a challenge relevant to the tracking of noble metal fission products in MSRs.

Two-dimensional transition metal carbo-chalcogenides (TMCCs) represent a novel class of layered materials with tunable electronic structures and high chemical versatility, making them promising candidates for energy and environmental applications. In this work, the structural and thermochemical properties of a series of TMCCs are investigated. High-temperature oxidative solution calorimetry, conducted at 800 °C in molten sodium molybdate (3Na2O·4MoO3), was used to determine the enthalpies of oxidation and formation from the elements at 25 °C, enabling a comprehensive thermodynamic assessment. Comparison of these enthalpies reveals distinct stability trends across the series. Cu0.67Nb2S2C exhibits the most exothermic oxidation enthalpy, indicating a strong thermodynamic driving force for oxidation and reduced resistance to oxidative degradation. Its positive enthalpy of formation from the elements suggests lower thermodynamic stabilization relative to the other compositions, consistent with the energetic influence of Cu and residual Fe within the Nb–S–C framework. In contrast, delaminated Nb2S2C exhibits poor oxidative stability but a highly exothermic enthalpy of formation, consistent with a defect-rich, metastable structural state rather than enhanced lattice stability. These findings establish correlations between crystal structure and energetic stability, providing insights into the design of robust TMCC-based materials for advanced energy and environmental technologies.

Oak Ridge National Laboratory (ORNL) has prepared a new set of iodine-bearing sorbents. The sorbent materials used for this activity were supplied by IONEX (NexSorb-99) or C&CS (AC6120-7%). Metal canisters were previously prepared by ORNL in 2022. They are 2 in. long by 1.5 in. in diameter and are made of 316 stainless steel.

Laser-induced breakdown spectroscopy (LIBS) is being considered for elemental analysis of radionuclides transported into the off-gas system of molten salt reactors. A mobile LIBS system was calibrated for gas-phase measurements of hydrogen (i.e., protium, denoted here as H) and deuterium (D) in argon (Ar). The system was then used to quantify the concentrations of H and D in the off-gas of an engineering-scale molten salt loop (∼200 kg salt) following the addition of H ₂ or D ₂ as a 4% blend in Ar. The limits of detection for H and D were determined to be 0.0013 and 0.0011 mol% (13 and 11 parts per million), respectively. This research was carried out at the U.S. Department of Energy’s Oak Ridge National Laboratory in the Facility to Alleviate Salt Technology Risks (FASTR) pumped salt loop. Previous molten salt LIBS research has predominantly been performed at the bench scale, thus this study represents a major stride toward deploying LIBS technology for monitoring the performance of molten salt reactors. During the FASTR testing, the LIBS system operated continuously for several days. This online monitoring campaign demonstrates advancement in the application of LIBS for online monitoring of off-gas in molten salt systems.

A novel flow cell allowing for multiple optical spectroscopy measurements on flowing molten salts was designed, and demonstrative calibrations of impurities in aqueous samples were performed. Online compositional measurements of molten salts are of high interest to monitor the state of relevant solar and nuclear systems. The Spectroscopic Configuration for Optical Real-Time Characterization of High-Temperature (SCORCH) fluids cell was designed to meet this need by providing optical access to a high-temperature molten salt sample stream without physical contact between the sample and window materials. Laser-induced breakdown spectroscopy (LIBS) was utilized to quantify Li, Cr, Fe, Ni, Sr, and Pr at concentrations ranging nominally from 0 to 315 mmol L ⁻¹ . Laser power, frequency, and plasma position were optimized to mitigate challenges associated with sample splashing. Univariate calibration models were built with R ² > 0.98, percent root mean square error of cross-validation (%RMSECV) as low as 2.7%, and limits of quantification (LOQs) down to 4.1 mmol L ⁻¹ . Simultaneously, absorbance calibrations were developed for the applicable analytes (Cr, Ni, and Pr) using Beer’s law with a pathlength of 4.41 ± 0.10 mm. These models provide excellent quantification performance with R ² > 0.999, %RMSECV as low as 0.6%, and LODs down to 0.08 mmol L ⁻¹ . Although these calibrations were performed for each spectroscopic technique separately, the two methods may be combined in the future through multivariate modeling and sensor fusion to provide more robust models with the benefits of both techniques (e.g., absorbance: oxidation state concentrations, LIBS: elemental concentration). Additionally, optimized spectrometers may be deployed to enhance sensitivity.