Adam Burak

Post-critical heat flux (post-CHF) heat transfer corresponds to a highly complex two-phase flow regime with important implications for the safety assessments of light water reactors. However, the available data for the dispersed flow film boiling (DFFB) regime remain limited, both in terms of the range of flow conditions studied and the availability of measurements for critical parameters, such as void fraction, vapor superheat, and droplet temperature. Furthermore, the complexity of the phenomenon, particularly due to the presence of entrained droplets, contributes to significant uncertainties in system-level safety analyses for loss of coolant accidents (LOCAs). To improve our understanding of the DFFB regime and reduce code and model uncertainties in predicting DFFB heat transfer, a sequence of experiments was conducted to achieve quasi-steady-state DFFB conditions. The experiments covered a mass flux range of 60–150 kg/m2-s and a system pressure range of 0.138–0.414 MPa. The measured wall heat transfer exhibits a clear dependence on the thermodynamic equilibrium quality, indicating a transition in the prevailing mechanism. Specifically, this transition reflects a shift from droplet–vapor interfacial heat transfer to convection-dominated heat transfer within the vapor phase. To further investigate this shift in heat transfer behavior, local flow parameters, including the vapor Reynolds number and void fraction, were analyzed using a one-dimensional two-fluid film boiling model. The model predictions were validated through comparison with void fraction measurements obtained using an X-ray radiography system. Furthermore, existing DFFB heat transfer correlations were subsequently benchmarked against the acquired experimental data to evaluate their predictive capacity and identify any inherent limitations. The benchmark results indicate that these correlations exhibit limited predictive performance, primarily due to lack of the regression robustness and insufficient incorporation of the physical effects associated with liquid droplets. To address these deficiencies, a new wall heat transfer correlation was developed based on the experimental database and the Reynolds analogy, incorporating the variation of the interfacial area concentration as a function of the droplet size. The proposed model demonstrates improved predictive performance for wall heat transfer in the DFFB regime, successfully capturing variations in the Nusselt number with respect to the vapor Reynolds number, system pressure, and mass flux.

Laser-induced breakdown spectroscopy (LIBS) is a versatile analytical technique with applications in several fields of science and technology. It requires minimal or no sample preparation and can provide information about the composition of the analyte, with limits of detection typically at the ppm level. There is growing interest in the scientific community in improving the sensitivity and specificity of LIBS. In this work, we explore the combination of LIBS with two-photon laser-induced fluorescence (TALIF) for the enhancement of atomic oxygen spectra signatures originating from a nanosecond laser-produced plasma on a solid target. The line intensity of the O I transition at 844.67 nm was enhanced up to 29.3 times by exciting an oxygen atomic resonance using a second UV nanosecond pulse at 225.59 nm, with a focused intensity of ∼2.4×107 W/cm2, produced with an optical parametric oscillator. The enhancement did not significantly increase the background, leading to a comparable increase in the signal-to-noise ratio. The dependence of the enhancement on the UV excitation wavelength and on the inter-pulse delay is presented. The demonstrated LIBS-TALIF combination permits the exploration of high-lying gaseous transitions not accessible with classical laser-induced fluorescence and unlocks the pathway to applications where increased sensitivity and specificity are imperative.

A study was conducted to determine how variables during metallothermic purification of chloride salts affect the oxygen content of the final product. 100 g batches of ternary MgCl₂–NaCl-KCl mixture (50–20–30 wt %) were purified for 3 h with flowing sparge gas and 0.25 wt % Mg addition as a reducing agent. After slag removal and remelting, oxygen content was measured via inert gas fusion analysis (IGFA). The as-received, melted, and baseline-purified salts had oxygen concentrations of 3466 ± 513, 889 ± 135, and 436 ± 151 ppm, respectively, with purification showing statistical significance (>99% confidence). A parametric study with 12 trials identified processing time as the most influential factor, as extending it to 6 h further reduced oxygen concentration to 143 ppm (>99% confidence). Variations in sparge and flush gas had no significant effect, and all tests used excess Mg. Uncertainty analysis of the IGFA technique estimated a 9% measurement uncertainty, primarily due to calibration standard variability.

A significant challenge in sodium-cooled fast reactors is controlling impurities, in particular oxygen impurities, within the sodium coolant, as they can accelerate corrosion and indicate leaks. Optical methods offer the potential to rapidly detect small concentrations of both gaseous and metal impurities that accelerate corrosion, plug coolant channels, and lead to increased activation of isotopes in the coolant. We present the design and performance of an apparatus designed to enable the application of multiple optical analytical techniques, such as laser-induced breakdown spectroscopy, to detect elemental impurities in the sodium melt with high sensitivity. We experimentally demonstrate the detection of characteristic sodium and oxygen spectral lines in liquid sodium, which sets the stage for the optimization of its analytical sensitivity. A robust sensor of this type integrated with the sodium cooling loop has the potential to significantly improve the safety and operational efficiency of generation IV nuclear reactors.

A reliable high-temperature molten salt pump is critical for the development of Fluoride-salt-cooled, High-temperature Reactors (FHRs). By supporting the rotating journal, the appropriate journal bearing can ensure that the high-temperature molten salt pump runs smoothly and efficiently in the high-temperature fluoride salt over a long period of time. However, many bearing candidates served well for only a short period and experienced several issues. Moreover, the alignment of the molten salt pump journal bearings is a key factor for the molten salt pump's long-term steady running. In the long-term operation, a misalignment in the journal bearing can result in vibrations and excessive wear on the bearing surface of the molten salt pump. The journal bearing dynamic characteristics can be used to accurately assess the journal misalignment. Therefore, it is necessary to investigate the detailed journal-bearing dynamic behavior under the high-temperature hydrodynamic fluoride salt lubrication conditions for FHR applications. In this study, a small amplitude vibration is superimposed on a steady-running journal bearing to simulate the molten salt operating conditions. A Fortran 90 code has been written for the journal-bearing dynamic behavior analysis. The code was verified using the numerical data reported in the literature. The code is then employed to predict the dynamic coefficients of high-temperature fluoride salt hydrodynamic lubricated journal-bearing with various Sommerfeld numbers. These journal-bearing dynamic coefficients can be used to provide guidelines in the design of molten salt pumps.