Optimization of oxygen and hydrogen analysis in salts by inert gas fusion.

Molten salts are widely used in both academic research and industrial applications. However, oxygen impurities can react with the salts to form oxides, which can negatively affect their performance. The presence of oxygen promotes oxide formation, significantly altering the salt’s viscosity, accelerating corrosion, and ultimately leading to material degradation and reduced reactor performance. For example, oxygen reacting with LiF-NaF-KF can produce Li2O or K2O, which affects the melting point and viscosity of the salt, reducing its effectiveness for heat transfer.1 Similarly, uranium tetrafluoride can react to form uranium oxide (UO2), altering the neutron economy of the fuel salt.1 These issues underscore the importance of accurately measuring and minimizing oxide concentrations in molten salts. The objective of our research is to design an electrochemical sensor capable of providing in situ measurements of oxide concentrations in molten chloride and fluoride salts using both stationary and rotating cylinder electrodes. The salts investigated in this study include NaCl-KCl and FLiNaK, which are used in nuclear applications and are candidate salts for molten salt reactors. Therefore, an effective method for real-time monitoring of oxide concentration is critical for both scientific and industrial applications.Cyclic voltammetry (CV) and square wave voltammetry (SWV) are implemented to measure incremental oxide concentrations.2-4 SWV is particularly sensitive to low concentrations, while CV helps identify redox reactions in the electrochemical system. For chloride salts, we used a gold working electrode (WE), a graphite counter electrode (CE), and a Ni/NiO reference electrode (RE). For fluoride salts, a Ni/NiF2 RE was used. Incremental additions of Na2O were made to stagnant NaCl-KCl at 730 °C, and SWV was used to measure the resulting oxide concentrations. The results are depicted in Figure 1, which shows the measurements obtained using the stationary electrode setup, in the top graph, and illustrates the linear relationship between peak current and oxide concentration, in the bottom graph..In molten salt reactors, long-term, reliable sensors are essential, especially under dynamic, flowing conditions hence we used the rotating cylinder electrode (RCE) and saturated Ni/NiO RE, which has demonstrated greater stability over the traditional Ag/AgCl RE.2,5 RCE experiments were also conducted to simulate flow conditions and enhance mass transfer by convection at 500 rpm, 750 rpm, and 1000 rpm using the same salts. This research is ongoing, and work with fluoride salts is in progress.Our work demonstrates that electrochemical signals are correlated to changes in oxide content in molten in both stagnant and flowing conditions. Hence, electrochemical sensors can potentially be employed in electro-refiners, molten salt reactors, and other applications to provide near-real-time feedback on oxide impurities in the process. However, further development is needed to increase range and accuracy of oxide correlations and investigate the effect of other species (e.g., uranium, fission products) on the oxide signals.

The intrinsic negative thermal expansion of UF4 belowroom temperature was examined. A redetermination of the structureof UF4 by single-crystal X-ray diffraction at 100, 200,and 300 K accompanied by an evaluation of the atomic displacementparameters (ADPs) of the F atoms was performed. The structure of UF4 was described as the stacking of two subnetworks, respectively,constituted by the U(1) and U(2) atoms. The subnetwork formed by theU(2) atoms consists of infinite layers that run parallel to the (b, c) plan. The layers are composed ofinfinite zigzag chains of corner-sharing U(2)F8 polyhedrarunning along the c-axis. An increase of temperaturefrom 100 to 300 K leads to a decrease of the unit cell volume andthe a and c lattice parameters andan increase of the b lattice parameter. As the temperatureincreases, the intrachain, interchain, and interlayer U–U distancesdecrease. It is proposed that the decrease of the intrachain and interlayerU–U distances causes a contraction of the U(2) subnetwork alongthe a- and c-axis and that a translationof the chains along the c-axis causes an expansionalong the b-axis. Analysis of the ADPs of the F atomsindicates that a guitar string effect in the U–F–U unitsis possibly the origin of the decrease in the U–U distances.A correlation between the U–U distances and the magnitude ofthe ADPs of the F atoms was established.

Uranium tetrafluoride microrods were prepared by chemical transformation from the reaction of UO2 microrods with HF(g).

A laboratory-scale process for producing dilithium beryllium tetrafluoride (FLiBe) with dissolved uranium tetrafluoride

Numerical simulation of uranium tetrafluoride fluorination in a multistage spouted bed using the improved CFD-DEM chemical reaction model

Tetra-uranium fluoride electrowinning by electro-electrodialysis cell (EED)

Chemical transformations of UF4 under controlled temperature and relative humidity

Uranium tetrafluoride production at pilot scale using a mercury electrode cell

Uranium tetrafluoride hydrate (UFH) is formed by immersing anhydrous UF4 under water for 12 h. UFH is therefore clearly a chemical species of environmental concern, as anhydrous UF4 is an intermediate uranium form in the nuclear fuel cycle. We use inelastic neutron scattering (INS) to probe the full vibrational spectra of UFH and its deuterated analogue in an effort to improve the fundamental understanding of its vibrational spectra. Coupled with density functional theory (DFT) calculations, the first for this compound, and full spectral modeling, we generate the complete vibrational spectra of UFH and compare them to prior optical spectroscopic results. In particular, the combination of DFT with INS allows us to identify multiple distinct chemical environments in the water bending and OH stretching regions. Whereas the water molecules directly bound to the U atoms execute OH stretching around 3600 cm<sup>–1</sup>, a second class of H-bonded waters vibrate below 3000 cm<sup>–1</sup>, an indicator of strong H bonding. In addition, a class of librational water modes are observed between 400 and 900 cm–1, which themselves can be separated in energy according to their chemical environments. Furthermore, measurements presented herein directly assist in the assignment of certain spectral features in the infrared spectrum and will inform future investigations of UFH for environmental or forensics purposes.

Vibrational spectroscopy of uranium tetrafluoride hydrates

Uranium tetrafluoride production using the dropping mercury electrode

Molten Salt Reactor (MSR) is one of the next-generation nuclear power systems known as Generation IV. In this paper, the FUJI-U3 MSR type is designed for neutronic analysis that natural uranium is used as a fuel. The standard reactor core model is adopted due to the flux flattening effect in the active core, as reported from previous research. FLiBe eutectic is used both as the coolant salt and moderator in the reactor core. The neutronic calculation is performed using the SRAC 2006 program: PIJ for pin cell calculation and Citation for core calculation. JENDL 4.0 is used as a nuclear data library that provides the cross-section data of the nuclides. The neutronic parameter results, such as the effective multiplication factor, conversion ratio, neutron spectrum, and power density, are obtained from the calculation. The reactor can achieve the criticality condition with a minimum loading Uranium Tetrafluoride (UF4) of 3.6%mol for 2000 effective full power days (EFPD). The results calculation provide some parameter survey for the research and the future development of MSR.

The dissolution–precipitation behavior of zirconium dioxide (ZrO2) in molten lithium fluoride–beryllium fluoride (LiF–BeF2, (2 : 1 mol, FLiBe)) eutectic salt at 873 K was studied. The results of the dissolution experiment showed that the saturated solubility of ZrO2 in the FLiBe melt was 3.84 × 10−3 mol kg−1 with equilibrium time of 6 h, and its corresponding apparent solubility product (K′sp) was 3.40 × 10−5 mol3 kg−3. The interaction between Zr(iv) and O2− was studied by titrating lithium oxide (Li2O) into the FLiBe melt containing zirconium tetrafluoride (ZrF4), and the concentration of residual Zr(iv) in the melt gradually decreased due to precipitate formation. The precipitate corresponded to ZrO2, as confirmed by the stoichiometric ratio and X-ray diffraction analysis. The K′sp was 3.54 × 10−5 mol3 kg−3, which was highly consistent with that from the dissolution experiment. The obtained K′sp of ZrO2 was in the same order of magnitude as that of uranium dioxide (UO2), indicating that a considerable amount of ZrF4 could inhibit the UO2 formation when oxide contamination occurred in the melt containing ZrF4 and uranium tetrafluoride (UF4). Further oxide titration in the LiF–BeF2–ZrF4 (5 mol%)–UF4 (1.2 mol%) system showed that ZrO2 was formed first with O2− addition less than 1 mol kg−1, and the precipitation of UO2 began only after the O2− addition reached 1 mol kg−1 and the precipitation of ZrO2 decreased the ZrF4 concentration to 0.72 mol kg−1 (3 mol%). Lastly, UO2 and ZrO2 coprecipitated with further O2− addition of more than 1 mol kg−1. The preferential formation of ZrO2 effectively avoided the combination of UF4 and O2−. This study provides a solution for the control of UO2 precipitation in molten salt reactors.

The results of applying the methods of scanning electron microscopy and electron-probe microanalysis (SEM–EPMA) in combination with etching of the surface of the microparticles with an ion beam to study the composition and internal structure of single microparticles of uranium tetrafluoride are presented. It is shown that the accuracy of analysis of relief samples can be significantly improved if their relief is preliminarily smoothed by an ion beam. The standard deviation in determining the uranium weight content in microparticles of uranium tetrafluoride after smoothing of their surface with an ion beam decreases from (6.1–8.7) to (1.3–2.7)%.

Effect of oxygen containing compounds in uranium tetrafluoride on its non-adiabatic calciothermic reduction characteristics

Study of the chemical changes of μm-sized particles of uranium tetrafluoride (UF4) in environmental conditions by means of micro-Raman spectrometry

New fluoride target matrix preparation procedure for determination of 236U with accelerator mass spectrometry

Oxidant K (1s) X-ray Emission Spectroscopy (XES) was used to investigate covalency in the Oxidant 2p – Uranium 5f bonds of Uranium Dioxide (UO<sub>2</sub>) and Uranium Tetrafluoride (UF<sub>4</sub>). It will be shown that the width of the 2p Occupied Density of States (ODOS), determined from the XES measurements, correlates with the increased 5f covalency of UO<sub>2</sub> and the increased ionicity of UF<sub>4</sub>. Analysis of the XES results includes comparison to spectral simulations, cluster calculations and peak fitting, demonstrating the potential of the oxidant XES measurements as a probe of 5f covalency.

Study on the fluorination reaction of uranium tetrafluoride by nitrogen trifluoride