Adam Burak

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.

Void fraction is one of the most important parameters that affect two-phase flow heat transfer and pressure drop. In this paper, a commercial gamma densitometer and a high-speed X-ray radiography system developed at the University of Michigan (UM) are used to measure the void fraction in two-phase boiling flows, with water as the working fluid, in a tubular test section. The test section is made of Incoloy 800H/HT with a total length of 1.589 m, an inner diameter of 12.95 mm, and a wall thickness of 3.05 mm. These two instrumentation systems are installed on a traversing platform that travels along the vertical test section to perform measurements at multiple elevations. Subcooled flow boiling and natural convection boiling experiments are performed to measure the void fraction in the test section. Flow visualization images are obtained for bubbly and slug flows from the X-ray radiography system. The wall temperature of the test section is measured at 17 elevations by thermocouples. In addition to the experiments, a multiphase computational fluid dynamics (MCFD) model is developed using ansysfluent to simulate the subcooled flow boiling. The measured wall temperature and void fraction from the experiments are compared with the MCFD simulation results. The root-mean-square (RMS) relative deviations are 3.6% and 16.1% for the wall temperature and void fraction, respectively, between the experimental data and MCFD simulations.

The Printed Circuit Heat Exchanger (PCHE) is considered promising as an intermediate heat exchanger for Molten Salt Reactors (MSRs) due to its highly compact construction, high heat transfer effectiveness, and capability of withstanding high pressures. In this study, thermal-mechanical simulations were performed using a two-channel unit-cell model with the attempt to investigate the structural integrity of a laboratory-scale PCHE that was designed for molten salt-to-supercritical carbon dioxide heat transfer, with the temperature field obtained from Computational Fluid Dynamics (CFD) simulations. It is shown that the fillets on the semi-circular channel walls are stress concentration regions and that the stress intensity decreases quickly as the distance from the fillets increases. A quick drop in the maximum stress intensity is observed with the increase of the fillet radius. There is no significant increase in the stress intensity for locations around the zigzag sharp corners. With a lower bulk temperature and a higher stress intensity, the region close to the outlet of the PCHE hot channels is more vulnerable to potential failures than the inlet region of the hot channels. In addition, the choice of channel models has a weak impact on the maximum stress intensity around the cold channel fillets.

We assess the sensitivity of LIBS for trace xenon detection in a helium buffer and its suitability for online monitoring of reactor fuel integrity.

A study was performed to assess the effect of controlling the reduction mechanism on Li₂O entrainment in an electrolytic UO₂ reduction process. The reduction mechanism was controlled by isolating the UO₂ particles from the lead, eliminating the direct reduction mechanism. Cathode design made it possible to eliminate the direct UO₂ reduction mechanism, as evidenced by the cathode potential, the result being indirect reduction via electrolytically produced Li metal. Characterization of the reduced product was achieved via acid-base titration to measure Li₂O entrainment and thermogravimetry to measure reduction extent. Significantly increased entrainment was observed when the electrolytic UO₂ reduction portion was eliminated compared to normal operation. In addition to the entrainment the representativeness of dip sampling, compared to cup sampling, was investigated. A statistically significant difference between Li₂O concentration measurements from dip and cup samples was observed, dip samples measured 8 ± 3% less Li₂O than cup samples.

A study was performed to assess the effect of controlling the reduction mechanism on Li₂O entrainment in an electrolytic UO₂ reduction process. The reduction mechanism was controlled by isolating the UO₂ particles from the lead, eliminating the direct reduction mechanism. Cathode design made it possible to eliminate the direct UO₂ reduction mechanism, as evidenced by the cathode potential, the result being indirect reduction via electrolytically produced Li metal. Characterization of the reduced product was achieved via acid-base titration to measure Li₂O entrainment and thermogravimetry to measure reduction extent. Significantly increased entrainment was observed when the electrolytic UO₂ reduction portion was eliminated compared to normal operation. In addition to the entrainment the representativeness of dip sampling, compared to cup sampling, was investigated. A statistically significant difference between Li₂O concentration measurements from dip and cup samples was observed, dip samples measured 8 ± 3% less Li₂O than cup samples.

Electrolytic uranium oxide reduction has the potential to be used for recycling spent fuel to a range of nuclear reactors from commercial light water reactors to advanced nuclear reactors—including molten salt reactors and sodium cooled reactors. However, several process engineering-related problems have been identified that need to be addressed to support efficient, cost-effective commercial implementation. Two keys are cathode product purity and cell current efficiency. High product purity relies upon the effective removal of LiCl-Li₂O from the cathode basket and reduced uranium. Removal of the salt from the basket can be achieved via high speed spinning at 650^oC. But entrained Li₂O will remain and should be minimized during the reduction process. Methods for removing hydroxide impurities from the salt are reported that improve cell-current efficiency. The extent of this reduction can be determined via three distinct techniques (thermogravimetric analysis, elemental oxygen analysis, and bromine dissolution) and is key in helping determine the efficiency of the electrochemical cell.

Methods to prepare and condition a reference electrode for precise potential measurements in molten LiCl-Li₂O are discussed in this paper. The Ni/NiO redox couple is commonly encased in MgO tubes containing porous, low-density MgO plugs that support ionic transport. Open circuit potential measurement between a source of fixed lithium activity and these reference electrodes is a promising approach to achieving real time Li₂O concentration measurement in the salt. But the reference potentials must be extremely stable and reproducible in order for concentration correlations to be accurate for any given reference electrode. It was found in our study that pre-soaking of the reference electrodes is needed to eliminate transient effects. And equalizing the length of the porous MgO plugs is also critical to minimizing variability between different reference electrodes.

Methods to prepare and condition a reference electrode for precise potential measurements in molten LiCl-Li₂O are discussed in this paper. The Ni/NiO redox couple is commonly encased in MgO tubes containing porous, low-density MgO plugs that support ionic transport. Open circuit potential measurement between a source of fixed lithium activity and these reference electrodes is a promising approach to achieving real time Li₂O concentration measurement in the salt. But the reference potentials must be extremely stable and reproducible in order for concentration correlations to be accurate for any given reference electrode. It was found in our study that pre-soaking of the reference electrodes is needed to eliminate transient effects. And equalizing the length of the porous MgO plugs is also critical to minimizing variability between different reference electrodes.