Hyungmo Kim

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.

In wire-type pool boiling, the wall temperature is conventionally inferred from the electrical resistance of the heater and is therefore a single spatially averaged value. Infrared thermography resolves surface-temperature fields on planar heaters but is impractical for a curved sub-millimeter heater. This study presents what is, to our knowledge, the first spatially and temporally resolved wall-temperature measurement on a wire-type boiling heater, covering nucleate boiling through the critical heat flux and burnout. A Rayleigh-OFDR fiber was loosely inserted into a Joule-heated stainless-steel capillary heater (outer diameter 0.4 mm, heated length 60 mm), which provides a strain-free, conduction-coupled measurement with a single-point calibration at saturation, and the temperature record was synchronized with high-speed imaging. The distributed data reveal the local structure hidden by the average. The time-averaged temperature varies by approximately 5 K along the heater. The temperature standard deviation identifies active nucleation regions (σT ≈ 0.9–1.1 °C versus 0.4 °C) that migrate along the heater as the heat flux increases. Individual bubble cycles produce heating–cooling excursions of 0.7–0.9 °C on average (up to 1.8 °C) at 5.9–8.5 Hz, and the apparent local heat-transfer coefficient varies by ±16 % about its mean. At the critical heat flux (1227 kW/m², 1.11 times the Zuber prediction), the spatial average remained near 121 °C while one site escalated at up to 700 °C/s to a last (indicative) reading of 174.02 °C within 0.13 s, until the tube burnout severed the fiber. Distributed Rayleigh-OFDR thermometry thus provides local boiling diagnostics in wire-type geometries that have been limited to averaged measurements.

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.

Structural materials remain a central obstacle to molten salt reactor (MSR) deployment. Hastelloy N, the historical reference alloy for fluoride-salt service, does not have an established Class A design basis under ASME Section III, Division 5, whereas industrially available 316H stainless steel does within the applicable limits but suffers chromium dealloying in halide melts. This mini review examines a bimetallic strategy in which 316H provides the structural function and a thin, corrosion-resistant, Ni-based clad protects salt-wetted surfaces. We summarize corrosion mechanisms in fluoride and chloride salts and compare candidate materials in terms of corrosion performance and the availability of a Division 5 Class A design basis and integrate evidence from three areas: (i) immersion tests showing that electroplated Ni protects 316H in FLiNaK and that deposited Ni clads outperform bare 316H in a molten chloride salt, (ii) weld-overlay and laser-directed-energy-deposition studies defining dilution windows, and (iii) a draft ASME Code Case-compatible design methodology for clad Class A components. Available evidence indicates that clad performance is specific to salt chemistry and operating conditions: electroplated Ni protects 316H in FLiNaK, while weld- and laser-deposited Ni clads outperform bare 316H in NaCl–MgCl ₂ ; however, wrought Ni-200 degrades severely in a separate chloride screening. Candidate clads must therefore be validated under the target salt chemistry and operating conditions. We conclude by identifying the main industrialization gaps: irradiation response of clad layers and diluted interfaces, volumetric inspection of anisotropic clad structures, joining at field welds, thermal-expansion mismatch, and specialty-alloy supply chains.

In wire-type pool boiling, the wall temperature is conventionally inferred from the electrical resistance of the heater and is therefore a single spatially averaged value. Infrared thermography resolves surface-temperature fields on planar heaters but is impractical for a curved sub-millimeter heater. This study presents spatially and temporally resolved wall-temperature measurements in a wire-type configuration. A Rayleigh-OFDR fiber was loosely inserted into a Joule-heated stainless-steel capillary heater (outer diameter 0.4 mm, heated length 60 mm), which provides a strain-free, conduction-coupled measurement with a single-point calibration at saturation, and the temperature record was synchronized with high-speed imaging. The distributed data reveal the local structure hidden by the average. The time-averaged temperature varies by approximately 5 K along the heater. The temperature standard deviation identifies active nucleation regions (σT ≈ 0.9–1.1 °C versus 0.4 °C) that migrate along the heater as the heat flux increases. Individual bubble cycles produce heating–cooling excursions of 0.7–0.9 °C on average (up to 1.8 °C) at 5.9–8.5 Hz, and the apparent local heat-transfer coefficient varies by ±16 % about its mean. At the critical heat flux (1227 kW/m², 1.11 times the Zuber prediction), the spatial average remained near 121 °C while one site escalated at up to 700 °C/s to a last (indicative) reading of 174.02 °C within 0.13 s, until the tube burnout severed the fiber. Distributed Rayleigh-OFDR thermometry thus provides local boiling diagnostics in wire-type geometries that have been limited to averaged measurements.

The non-uniform distribution of condensate in the piping systems of power plants and thermal supply systems can cause mechanical issues such as water hammer or turbine blade erosion, motivating the need for real-time monitoring of interface and local velocity in stratified two-phase flow. Conventional ultrasonic, differential-pressure, and capacitance-based flowmeters rely on cross-sectional averaged values and have limited ability to capture the local interfacial behavior inherent in stratified flow. This study proposes a measurement methodology based on a distributed optical fiber sensor, capable of multi-point sensing along a single fiber to determine interface location and local flow velocity and explores the feasibility of extending this approach toward phase-specific volumetric flow rate characterization. The thermal response of a directly heated sensor is governed by the phase-dependent difference in thermal capacitance during the early transient period and by that in the convective heat transfer coefficient at quasi-steady state; these two responses were used for interface detection and velocity estimation, respectively. Experiments in air-water stratified flow showed that the inflection point in the initial temperature rise rate closely matched with the observed interface location, and that continuous monitoring was achievable using a periodic heating-cooling cycle. The quasi-steady temperature varied with flow velocity, and combining the interface-identified local phase with this temperature yielded a phase-specific Nusselt number that exhibited a trend consistent with a power-law correlation with flow rate. These results demonstrate that interface and local velocity can be reliably characterized using the proposed distributed optical fiber sensor, and provide preliminary evidence supporting its potential extension toward phase-specific flow rate characterization in stratified two-phase flow.

Fluid flow is widely encountered in engineering applications such as industrial processes, pipeline transport, and nuclear systems, where accurate monitoring is essential for improving efficiency and ensuring operational safety. However, simultaneous co-located measurement of temperature and phase distribution in multiphase flows remains challenging due to spatial and temporal inconsistencies between separate sensing systems. In this study, an integrated sensing approach combining a wire-mesh sensor (WMS) and an optical fiber sensor (OFS) is proposed to achieve time-coordinated measurement of temperature and phase distribution. A controlled water–air two-phase flow experimental system was constructed to validate the sensing framework. A modified tube-mesh WMS structure was developed to physically integrate the distributed optical fiber sensor within the sensing region. The OFS measures distributed temperature information along the integrated optical fiber path, while the WMS captures the cross-sectional phase distribution. The results show that the integrated system successfully captured temperature and phase distribution variations under different operating conditions. The significantly opened condition increased the proportion of normalized conductivity values below 0.9 from 0.48% to 6.37%, indicating enhanced gas dispersion and stronger phase nonuniformity. The time-coordinated measurements demonstrated that the integrated sensing framework can distinguish gas injection conditions and localized heating behavior within the same sensing region. The integrated sensing framework enabled co-located visualization of reconstructed phase and temperature distributions within the same sensing region. Compared with conventional approaches, the proposed configuration reduces temporal mismatch and improves co-located multiparameter monitoring capability. The present study mainly demonstrates the feasibility of co-located multiparameter monitoring rather than establishing a quantitative thermal–hydrodynamic coupling mechanism.