1. Introduction

Molten salts are ionic liquids composed of cations and anions which form loose structural network [1]. These structures (and therefore the species and their complexations) in the salt systems vary considerably with their composition. Therefore, the ability to monitor the composition of molten salts in nuclear applications, mainly advanced reprocessing technologies of used nuclear fuel— also known as pyroprocessing (e.g., LiCl-KCl found in pyroprocessing technology) and advanced nuclear reactors— most notably molten salt reactors (MSRs) for power generation (e.g., NaCl-CaCl₂ found in fast burner reactor or molten chloride breeder reactor, LiF-BeF₂ found in molten salt reactor experiments or molten salt breeder reactor, and LiF-NaF-KF found in molten fluoride fast breeder reactor), is critical to ensure that operating conditions are within acceptable bounds to prevent inadvertent effects on those systems [2-6]. The succeeding sections serve as a review of three analytical techniques—(i) spectrochemical, (ii) electrochemical, and (iii) spectroelectrochemical—used on molten salts for compositional analysis and behavior characterization with a common focus on uranium ions and their kinetic, electrochemical, thermodynamic, and optical properties. Information from these literature reviews were used to aid in our spectroelectrochemical design. Here, we provide and highlight some key findings, assessments, and learning lessons from our design development focusing on a NaCl-CaCl₂ salt system.

2. Review of the Analytical Methods

2.1 Spectrochemical Techniques on Molten Salts

Spectrochemical analyses can elucidate the complex chemistry of molten salts and have the potential to be implemented as an online monitoring technique. Consider a melt constituted of FLiBe (a name of a fluoride salt mixture: LiF-BeF₂). At low concentrations of BeF₂ in LiF, the melt behaves as an ideal mixture with dissociated cation (Li⁺) and anion (BeF₄²⁻, F⁻) species. As the concentration of BeF₂ increases in the melt, the BeF₄²⁻ anions begin to bond together. These share a common F⁻ ion, first creating Be₂F₇³⁻, then Be₃F₁₀⁷⁻, etc., resulting in a polymer several units of BeF₄²⁻ long. This polymerization phenomenon manifests itself in BeF₂’s transport property, viscosity: the viscosity of BeF₂ increases with increasing concentration and pure BeF₂ has a higher viscosity when compared to other fluorides [1]. These species were observed and analyzed using Vis-NIR absorption spectroscopy [7].

An f-block complexation effect of concentration is perhaps more complicated. For example, lanthanides (Ln = La, Ce, Nd, Sm, Dy, Yb) have been investigated for their structure in varying concentrations of LnF3 in molten KFLnF3 systems. It was determined that at less than 25mol% in solution, the predominant species are LnF₆³⁻ species surrounded by K⁺ cations. However, at higher concentrations, lanthanides begin to share common fluorides and create loose structures of bridged octahedra. These species were observed and analyzed using Raman spectroscopy [8].

These examples are just two of the various studies utilizing spectrochemical techniques to characterize and analyze the molten salts and typical species found in nuclear applications (actinides, lanthanides, etc.). Electron Paramagnetic Resonance (EPR) spectroscopy has been utilized for detection and quantitative analysis of Eu in LiCl-KCl eutectic [9]. Raman spectroscopy was implemented for complexation determination, including octahedral distortions, of rare-earth elements in fluorides, chlorides, and bromides [8, 10]. The use of infrared spectroscopy has allowed for the detection of zirconium species present in fluorides and lanthanide species in chlorides [11]. Raman and infrared spectroscopies have also been used together for the study of magnesium treatment of the corrosivity of chlorides [12]. X-Ray Absorption Fine Structure (EXAFS) and Nuclear Magnetic Resonance (NMR) have both been implemented for determining coordination numbers, the nature of neighbors, and complex formations of actinides and lanthanides in the Aircraft Reactor Experiment and Molten Salt Breeder Reactor type molten salts [13-17].

The majority of spectrochemical studies of molten salts have been conducted in the UV-Vis region of the electromagnetic spectrum [18-36]. Laser induced techniques have been implemented in chlorides, namely (i) fluorescence emission for determining wavelengths and intensities of species in melts [26-27], (ii) fluorescence resonance energy transfer (FRET) for structural change, complexation, and center-to-center distance between Tb3+ and Nd³⁺ species [29], and (iii) time-resolved laser fluorescence spectroscopy (TRLFS) for neodymium species determination and cross-relaxation between those species [28]. The most popular technique used is Electronic Absorption Spectroscopy (EAS) for determining uranium and lanthanide species in fluorides and chlorides and monitoring chemical reactions.

A handful of studies based on EAS techniques have focused solely on the speciation determination and concentration quantification of uranium in the melt by utilizing the collected absorption spectra, absorptivities, and Beer’s law [7, 19-21, 25, 34, 35-37]. With Beer’s law, the absorbance, A, can be calculated by measuring the transmittance (T = Φ ⁄ Φ₀; ratio of transmitted flux through a sample, Φ, to source flux, Φ₀) and relating it to the molar absorptivity of the analytes present, ε₀ [M⁻¹cm⁻¹], pathlength of absorption, b [cm], and concentration of analytes present, C [M], with

Only two studies investigated uranium ions in a FLiNaK (a name of the eutectic fluoride salt mixture LiF-NaF-KF) system [34, 36] while the rest focused on chloride systems [19-21, 25, 35, 37]. The viability of EAS has also been explored in simulated pyroprocessing salt [25]. Carnall and Wybourne [19] have discussed the electronic energy levels of U³⁺ (along with Np³⁺, Pu³⁺, Am³⁺, and Cm³⁺). It should be noted that there is no available literature on the spectrochemical analysis of NaCl-CaCl₂ (will be referred as ClNa- Ca) and that TRLFS has not been conducted on uranium bearing molten salts.

For the TRLFS, the fluorescence process involves the emission of a radiant flux (luminescence, Φ_L [W]) between an excited molecular entity to a lower energy state of the same multiplicity following the absorption of an excitation radiant flux (Φ). The captured spectra have longer wavelengths than the excitation source and are proportional to the absorbed radiant flux, (Φ₀ − Φ), such that

where k is a constant that depends on the species, environment, and efficiency with which an excited state returns to the ground state by emission of a photon. Beer’s law can then be incorporated into the above equation (Φ = Φ 0 10 − ε 0 b C) and simplified with Taylor series expansion to:

When a short-duration pulse excitation source is used, such as a nanosecond laser, the lifetime of luminescence, τ_L, can be determined. The lifetime of luminescence is characteristic of a given molecule and its environment, providing an additional parameter for selectivity and multicomponent studies. The lifetime can be determined by measuring the luminescence intensity with time, I_L(t) [a.u.], from its initial intensity, I L 0 ( a . u . ), and fitting them to the time-decay profile of

The above is for a single component sample which decays by a first-order process. This process allows one to determine speciation within the fluid system [26-28].

2.2 Electrochemical Techniques on Molten Salts

Electrochemical methods have the potential to be implemented as an online monitoring technique in molten salt systems and have elucidated the electrochemical, kinetic, and thermodynamic behaviors of ions in such environments. The behavior of uranium ions in molten salts have been extensively studied using the electrochemical techniques of open circuit potential (OCP), chronoamperometry (CA) and cyclic voltammetry (CV) [38-65]. The majority of these studies have been done in LiCl-KCl melts with and without the addition of impurities, simulating conditions of pyroprocessing salt; a comprehensive review has summarized and tabulated their electrochemical, kinetic, and thermodynamic properties [65]. Zhang noted that these values have mostly been determined with CV under the assumption of near-infinitely dilute solutions, which assumes any intermolecular interactions in a solution are only of analyte-solvent nature and concentration effects are unrealized on analytesolvent properties. With this assumption, the diffusion coefficient of the ions, D [cm²s⁻¹], should remain constant with additions of the analyte into the solution; hence, its variation results from its dependence on temperature, T [K], related by the Arrhenius equation

where D₀ [cm²s⁻¹] is the pre-exponential factor and E_a [Jmol⁻¹] is the activation energy for diffusion to occur and the universal gas constant of R = 8.314 [Jk⁻¹mol⁻¹].

A threshold concentration of U³⁺ ions where this assumption breaks done was observed by Tylka and co-workers [59]. The group investigated a novel (yet strict and reproducible) method for monitoring and detecting actinide concentrations on molten salt mixtures of LiCl-KCl-AnCl₃ (An = U, Pu) for pyroprocessing applications. The method implemented was the electrochemical analysis technique of CV. Due to the soluble-insoluble nature of the U³⁺/U⁰ reduction reaction, the research group utilized a modified version of the Berzins-Delahay equation for analysis of their results. With this, the change in measured current, Δi_p,c [A], resulting from a change of electrode depth, Δh [cm], with an electrode diameter of d [cm] at a potential scan rate of ν [mVs⁻¹] with n electrons transferred in the reaction, and with Faraday’s constant, F = 96,485 [C mol⁻¹], the current density, j [A cm⁻²], can be used to estimate the concentration of U³⁺, C U 3 + [mol cm⁻³], since j ∝ C at a constant temperature.

Interestingly, for the LiCl-KCl-UCl₃ system, the technique was valid only up to approximately 1.73wt% U³⁺ for estimating the ion’s concentration in the melt. Ruling out varied temperature effects, the group determined that for concentrations greater than 1.73wt% U³⁺ in the melt, the diffusion coefficient decreases with increasing concentration. This implies that at higher concentrations of U³⁺, intermolecular interactions of the analyte-analyte type cannot be ignored and that the near-infinite-solution assumption for CV is not valid for predicting uranium species concentrations. Tylka and co-workers [59-60] estimated the diffusion coefficients for the respective concentrations of U³⁺. The diffusion coefficient was calculated using Eq. (6). The group concluded that an alternative method to electrochemical techniques is required for elucidating this phenomenon and for verifying any diffusion coefficients and concentrations approximated to better understand the molten salt systems studied.

Castrillejo’s group reported the electrochemistry of rare earth elements in the ClNaCa system [66-70]. There is also available data on uranium ions in LiCl-NaCl-CaCl₂-BaCl₂ [71], which has been provided in the table below (Table 1). Newton et al. [72] and Zhang et al. [73] were the two recent studies that reported electroanalytical measurements of uranium in ClNaCa salts. Newton et al. [72] did not report any diffusion value but provided valuable synthesis of UCl₃ in the ClNaCa salt systems. Zhang and team [73] attempted to use CV to study uranium with cerium behaviors in ClNaCa salts and experienced concentration effects on their cyclic voltammograms. They further applied a square-wavevoltammetry and experienced the same issue. It should be mentioned that these two recent studies did not provide any diffusion values.

Table 1

Reported Uranium Ions Diffusion Coefficients in FLiNaK and ClNaCa

There are only two available sources on the electrochemical behavior of uranium ions in FLiNaK; these reported values have been included in Table 1 [48, 57]. It should be noted here that these studies have been investigated at a single composition of uranium salt, one of which was contaminated with LaF₃. Thus, it is unclear if the diffusion coefficients reported have a dependence on concentration or impurities.

2.3 Spectroelectrochemical Techniques in Molten Salts

Spectroelectrochemical analyses, a combination of spectrochemical and electrochemical techniques, have been utilized in molten salt applications elucidating the chemical reactions at electrode surfaces and monitoring bulk composition during electrochemical processes. The nature of the combined analysis technique is powerful, allowing for the collection of data of the two individual techniques in a transient state (i.e., monitoring changes in species and complexation over time). However, implementation of spectroelectrochemical analysis techniques is typically limited by how easily the spectroscopic technique can be integrated with the interrogated system. The ease of implementation varies with each spectrochemical technique; thus, the majority of molten salt studies have been limited to EAS coupled with chronoamperometry (often referred to as chronoabsorptometry) [74-89].

Chronoabsorptometry has been extensively used to study lanthanide (Ln = Sm, Eu, Tb, Yb) and actinide (An = U, Np) bearing molten salts, the majority of which have been in different binary salt ACl-ACl (A = Li, Na, K, Rb, Cs) combinations. It is possible to relate the absorption spectrum measured during chronoabsorptometry over time to the concentration or diffusion coefficient of the reducing species with the equation

where ε_R [M⁻¹cm⁻¹] is the molar absorptivity of the reduced species, C_Ox [M] and D_Ox [cm²s⁻¹]) are the concentration and diffusion coefficient of the oxidized species present, respectively, and t [s] is the time allotted for electrolysis. Furthermore, since the ratio of oxidized and reduced species ([Ox] ⁄ [Red]) can be estimated from the absorption spectrum, the formal standard potential of the redox reaction can be determined by the Nernstian equation

where E^0' [V] is the formal reduction, E_app [V] is the applied potential to the cell, R is the universal gas constant, T is the temperature in Kelvin, n is the number of electrons transferred, F is Faraday’s constant, and [Ox] and [Red] are the respective concentrations of the oxidized and reduced species.

Of the lanthanides, the Eu³⁺/Eu²⁺ couple has been studied the most, and out of these studies, Schroll and colleagues have studied the Eu³⁺/Eu²⁺ couple in the most varied chloride systems [86-87]. They obtained the number of electrons transferred, reduction potentials and diffusion coefficient for Eu³⁺ in each eutectic melt. They observed that diffusion coefficients can vary significantly, with decreasing values (in order of appearance) in 3LiCl-NaCl, 3LiCl-2KCl, 3LiCl-2CsCl, and LiCl-RbCl. The group also noted that the diffusion coefficient obtained with chronoabsorptometry agrees well with that obtained using cyclic voltammetry.

Interestingly, Kim and Yun [90] used chronoabsorptometry to measure reduction amounts of the Eu³⁺/Eu²⁺ couple at various temperatures and compared their findings against TRLFS under similar potentiostatic electrolysis conditions (referred to as chronofluorometry). They determined that the quantitative analysis of the reduction process of Eu³⁺ using chronofluorometry of Eu²⁺ was more reliable and precise in comparison to that obtained using chronoabsorptometry. The formal reduction potential of the redox couple was calculated from the two results and averaged. Determination of the diffusion coefficient using either technique was not reported.

Neptunium has been studied in LiCl-KCl for its behavior, interactions with oxide ions, uranium ions, and the nature of its complex ions [81, 82, 84, 91]. Kim et al. [91] noted that it was possible to calculate the formal potential, E^0', of the Np⁴⁺/Np³⁺ reduction at much lower concentrations with chronoabsorptometry (~0.1 mM) compared to CV (~15 mM) with good agreement between the two techniques.

Several studies of uranium have been performed in LiCl-KCl [74-75, 77, 80, 82, 85] and a single study in FLiNaK [76]. In FLiNaK, the oxidation of UF₄ with EAS during sparging and measuring the system using CV before and after as a comparison. Notable from the chloride studies, Cho et al. studied the absorption spectra and metal- ligand bonding properties of [U^IIICl₆]³⁻ and [U^IVCl₆]²⁻ species [75]. From the chronoabsorptograms collected (Fig. 1) they determined that the reduction rate of U⁴⁺ is about four times faster than that of the oxidation of U³⁺, indicating that U³⁺ is chemically more stable than U⁴⁺ in the given conditions. Their results also support the conclusion that U³⁺ and U⁴⁺ act as electron acceptors while the chloride ion assumes the role of an electron, indicating U-Cl covalency. Knowledge of the nature of ligand bonding in molten salt media is essential for understanding optical and electrochemical properties [73]. There is no available literature on simultaneous spectroelectrochemical analysis performed in FLiNaK and ClNaCa salt systems.

Fig. 1

Peak intensity changes at 548 nm during U⁴⁺/U³⁺ redox reactions in a LiCl-KCl eutectic melt at 450℃: (a) reduction, and (b) oxidation [75].

3. Design of Spectroelectrochemical Techniques as a Means for Monitoring and Analyzing Molten Salt Systems

Section 2.1 has shown that spectrochemical analysis techniques, such as EAS and TRLFS, can be applied on fluoride and chloride molten salts for obtaining their optical properties and determining the concentration of those species. In addition, electrochemical analysis techniques (e.g., OCP, CA, CV) can be applied on fluoride and chloride salts for obtaining their kinetic, electrochemical, and thermodynamic properties as well as determining the concentration of those species. When combined in tandem or simultaneously, spectroelectrochemical analysis techniques can be utilized to monitor composition and active species and their important properties can be elucidated further than when the techniques are done separately.

In general, potentiostatic based spectroelectrochemical routine, like chronoabsorptometry, can possibly determine properties of analytes (e.g., diffusion coefficient) which are unattainable by potentiodynamic techniques (e.g., cyclic voltammetry) after a certain threshold concentration is reached. That is, Eq. (7), a combination of Beer’s law and Cottrell equation, can be used to determine the diffusion coefficient. Furthermore, spectroelectrochemical techniques can complement one another. For example, it can be difficult to differentiate between species using the EAS based technique of chronoabsorptometry when the absorption bands of said species are overlapping. Complementing chronoabsorptometry with chronofluorometry by coupling it with Eq. (4) can resolve this since it is based on the more precise, single wave excitation technique of fluorometry. However, there are knowledge gaps in the literature that must be examined to assess the viability of spectroelectrochemical methods as monitoring and analysis techniques in molten salt studies. The conceptual plan is shown in Fig. 2 based on the literature review to aid in design and assess a spectroelectrochemical experiments, which will be discussed in the next section.

Fig. 2

Conceptual plan for an assessment of a developed spectroelectrochemical cell.

3.1 Design of a Spectroelectrochemical System

A spectroelectrochemical cell (SEC) was designed and manufactured for performing spectroelectrochemical analysis techniques on molten salts based on the systems used in previous work [86-87]. Fig. 3 shows the conceptualized drawings with dimensions of the SEC. A general schematic of the interior of the SEC with sample, electrochemistry, and light pathways was briefly reported by McDuffee et al. [92] (see Fig. 4). To house and heat the SEC, a novel furnace was designed and manufactured by ThermCraft. The SEC and novel furnace are shown in Fig. 5. A temperature controller (J-KEM Model 270) was selected for controlling the temperature of the furnace.

Fig. 3

Isometric view (a), front view (b), and dimensions (c) of conceptualized design for a spectroelectrochemical cell.

Main body and spacer composed of stainless steel (dark gray). Electrode guide plates composed of MACOR (tan). Protrusions are SMA adapters (dark gray) for connecting fiber optic patch cords. Not pictured: internal cavity for sample bearing cuvette (Note: Standard 3.5 ml Quartz Cuvette; 10 mm Light Path; Four Polished Sides; Outer Dimension: 12.5 mm × 12.5 mm × 45 mm; Wall Thickness: 1.25 mm (https://www.alphananotechne.com/product-page/chemical-resistant-uv-quartz-cuvettes)).

Fig. 4

A general schematic of the interior of the SEC with sample, electrochemistry, and light pathways for incident beam (thick green arrow), transmitted beam (thin green arrow), and emitted light (thin red arrow) [92].

Fig. 5

SEC placed in its novel furnace with electrodes and thermocouples coming out the top. MACOR cap included as thermal shield to mini-mize heat loss [92].

For the broad-spectrum excitation beam, an Ocean Optics UV-Vis lamp with built in fiber coupling (Ocean Optics DH-2000-S-DUB-TTL; 215–2,500 nm output) was used. A Quantel pulsed laser (Quantel Q-Smart 450 Nd:YAG Pulsed Laser) with a second harmonic generator (Quantel 2ω QSmart) and third harmonic generator (Quantel 3ω Q-Smart) were used for access to 532 nm and 355 nm laser lines, respectively. Custom high temperature fiber optic patch cords (200 μm core) were purchased from FiberGuide to direct excitation light into the glove box, through the SEC in the furnace, and out the glove box. Standard fiber optic patch cords from ThorLabs (200 μm) were coupled to each of the high temperature chords, one of which was coupled to the light source while the other was coupled to a spectrometer (Andor Shamrock 500i). A potentiostat (Biologic VSP 300) was selected for controlling the electrochemical techniques in the study.

3.1.1 Temperature Calibration and Initial Test and Issues

The temperature sensor of the temperature controller needed to be calibrated. The J-KEM thermocouple/digital meter was tested against three independent thermocouples (Omega brand) for room temperature readings. Table 2 shows the temperature discrepancies between the J-KEM thermocouple and the Omega brand thermocouples. The J-KEM thermocouple read higher than the other three thermocouples with a difference of ~5°C between the upper range of Independent TC 1.

Table 2

Temperature reading disparities amongst thermocouples used to measure room temperature. Temperature readings were taken 60 mins after setup

The J-KEM temperature controller was calibrated using boiling water at an atmospheric pressure. Independent TC 1 was used for confirmation of boiling water temperature. A hotplate was used to boil water in an Erlenmeyer flask. Once boiling was achieved, the J-KEM TC and Independent TC 1 were lowered into the boiling water together. A paper towel was wrapped and secured around the thermocouples to prevent any bubble formation on the surface of the thermocouples, which could cause incorrect measurement of temperature. The temperature of the water was taken three times at 0 minutes (insertion), 5 minutes, and 10 minutes. Before the 15-minute mark, the J-KEM temperature controller was calibrated to the boiling water temperature. Table 3 shows the average temperature measurements pre-calibration and the temperature reading post-calibration, with both thermocouples for this calibration process.

Table 3

Average temperature readings pre-calibration and post-calibration of J-KEM temperature controller against boiling water

Dry temperature trials were conducted on the SEC with no solution to determine the maximum temperature difference between the center of the quartz vial (i.e., temperature of air) and the wall of the SS body of the SEC (this is where the temperature controller is placed for reference). It should be mentioned that this study served as (i) a tuning method for the heating assembly for the SEC and (ii) a method to elucidate any issues with the design of the SEC that may result from high temperature conditions prior to use of salts. The experimental setup is shown in Fig. 6.

Fig. 6

Heating assembly. Novel furnace housing SEC is shown (left) with thermocouples from the independent temperature sensor and temperature controller (right).

The tuning temperature range was 200–700°C with an initial reading at room temperature (~21.5°C). The temperatures were recorded once setpoint temperatures were reached (0 minutes), at 15 minutes, and at 30 minutes. These temperatures were then averaged, and the standard deviations were obtained. Table 4 shows the results of this experiment. In general, as the set temperature increases, the difference between the measured temperature in the wall of the SS SEC body and the center of the quartz cell (air) increases as well. The SEC was visually intact with no obvious damage observed during the trials period. Upon successful completion of the tuning process and testing, the SEC and furnace were allowed to naturally cool from 700°C to room temperature.

Table 4

Temperature Trials of the SEC over several temperatures

The SEC was removed from the enclosure for inspection once cooled. All stainless-steel components had oxidized (thermocouples, main body, thumbscrews, spacers). Some of the thumb screws used for securing the MACOR and SS plates and the J-KEM thermocouple had rusted in place. WD-40 was applied to the rusted location and all thumb screws were successfully removed. However, MACOR ceramic plates used for positioning the electrodes cracked while attempting to remove the thermocouple from the main body of the SEC. The quartz cuvette had also shattered during the attempted removal of the thermocouple. Fig. 7 shows the SEC post tuning and temperature testing.

Fig. 7

Damaged MACOR plates, broken quartz vial of SEC with stuck stainless-steel spacers and thermocouple post-tuning (a). A close-up (b).

3.1.2 Modifications to the SEC Design

SEC modifications (re-designs, re-fabrications, and re-machining) were completed. Fig. 8 shows the modified pieces [92]. The thumbscrews used in the previous design were replaced with partially threaded rods that serve as guide pins for the two MACOR plates and the SS spacer. Redesigns were done to ensure an easy removal process of brittle MACOR plates and SS space if the thermocouple or guide pins mate with the SS SEC body after experiencing high temperatures. Additionally, the thermocouple slot in the SS SEC body was increased in diameter to allow for the thermal expansion exhibited by the thermocouple and to further avoid possibility of mating with the SS SEC body.

Fig. 8

Modifications to the SEC design. Top: electrode assembly of SEC/ thermal insulator of furnace. Middle (from left to right): top MACOR plate, SS spacer, bottom MACOR plate. Bottom: SS Body of SEC with partially threaded rods [92].

3.1.3 Assembly and Alignment of Excitation Light Sources

The laser heads and the UV-Vis lamp were assembled onto a laser breadboard for use with the SEC system. Fig. 9 shows the 355 nm and 532 nm laser line sources, as well as the UV-Vis lamp [92].

Fig. 9

View of light sources for spectroelectrochemical techniques. 355 nm laser line, 532 laser line, and UV-Vis lamp [92].

To utilize as much of the free beam coming from the laser heads and sent to the SEC, the laser beam diameters were reduced using optical lenses. The beam diameters were reduced to ~20% of their original diameter (~1 cm). Fig. 10 shows the beam pathlengths of the 355 nm and 532 nm laser, indicating the beginning of the reduction of the beam diameter at lens assembly (A) and collimation at lens assembly (B). The second harmonic generator (2HG) produces the 532 nm laser line. The addition of a 3HG to a 2HG produces the 355 nm laser line. The arrows indicate the direction towards the SEC. Fig. 11 shows the reduction in beam diameter for the 532 nm wavelength before and after the lenses used for reducing the diameter size.

Fig. 10

Beam pathlengths of the 355 nm and 532 nm laser, indicating the beginning of the reduction of the beam diameter at lens assembly (A) and collimation at lens assembly (B).

Fig. 11

Reduction in beam diameter for the 532 nm wavelength before and after the lenses used for reducing the diameter size. Diameter was reduced from ~1 cm to 1 mm.

3.1.4 Assembly of SEC and Heating Assembly in a Controlled Environment

The SEC and heating assembly was moved into the glovebox in preparations for experiments. The glove box was maintained at O₂ and H₂O levels of <5 ppm each. This assembly is shown in Fig. 12.

Fig. 12

SEC assembly in an argon environment glovebox. 1: SEC and novel furnace. 2: temperature controller. 3: fiber optic feedthrough for spectrochemical and spectroelectrochemical analysis techniques.

3.2 Preliminary Test of SEC for Obtaining Transmission and Absorption Background of NaCl-CaCl₂ Salt

The transmission and absorption spectra of NaCl-CaCl₂ were obtained to determine if there are any systematic issues or artefacts present in the spectra due to the SEC prior to experimenting with uranium containing salts. A small batch of equimolar NaCl-CaCl₂ (~10 g) was prepared to obtain the transmission and absorption background spectra of NaCl-CaCl₂. A small sample of this salt mixture was taken and placed in a quartz vial (path length of 10 mm.) which was then placed in the SEC. The heating assembly was set to 600°C and the salt temperature was verified with the external thermocouple TC 1. The transmitted spectrum of the equimolar eutectic from the UV-Vis was captured with the spectrometer from 250–1,050 nm. Note that the transmitted spectra for just the SEC (no cuvette or salt effects) and the SEC with an empty cuvette (no salt effects) were also obtained prior to this in the glovebox at the same temperature and with the same light source for the same wavelength region.

The transmittance and absorbance of the cuvette and the equimolar salt system are plotted in Figs. 13 and 14, respectively. It should be noted that the plots pertaining to the cuvette use the spectrum of empty SEC for the initial intensity in Beer’s law, whereas the plots pertaining to the salt use the spectrum of the empty cuvette for the initial intensity. As expected, the cuvette has high transmittance effects (low absorption effects) across all wavelengths. The salt exhibits lower transmittance effects (higher absorption effects) at the lower wavelengths than at the higher wavelengths. Note that the spikes near the 300, 450–500, and 650 nm wavelengths are likely an artefact inherent in the spectrometer. These are a result of the spectrometer obtaining spectra in 50–100 nm partitions (instead of the entire range all at once) and manifest when spectra are combined at the end. Since the U(III) ions exhibits absorption bands between 400–600 nm, this can likely be mitigated by recording at smaller ranges of wavelengths.

Fig. 13

Transmittance of Cuvette and Salt in SEC.

Fig. 14

Absorption of Cuvette and Salt in SEC.

3.3 Design Issues and Learning Lessons Manifested Post Preliminary Salt Test

After the preliminary spectra were collected, the temperature controller was turned off and the SEC and furnace were allowed to cool to room temperature. An attempt was then made to remove the quartz cuvette containing the examined salt. There were no issues in removing the thermocouple, stainless steel spacer, and MACOR plates. However, the quartz cuvette had mated to the inner cavity of the SEC body, making it extremely difficult to remove. To get better leverage in removing the cuvette, the furnace was disassembled, and the patch cords were removed. During the removal of one of the patch cords, the sheath protecting the inner fiber optic sheared from the rest of the patch cord’s body and sliced through the fiber optic cable. Fig. 15 shows (a) the point of shearing in the sheath, (b) the portion of the patch cord that was sheared off, and (c) the aftermath of the SEC (showing corroded cell). The remaining patch cords were removed without issue.

Fig. 15

Photos of (a) the point of shearing in the sheath, (b) the portion of the patch cord that was sheared off and sliced fiber optic cable, and (c) aftermath of the SEC in opened novel furnace.

Upon removal of the remaining patch cord, an attempt was made to remove the quartz cuvette by disassembling the SEC body. This constitutes of removing four bolts that feed through the bottom section and thread into the top section. During this process, three of the bolts sheared when twisting due to the bolts rusting in place. The final bolt was left in place and the bottom portion of the SEC body was simply rotated about the bolt to expose the bottom face of the quartz. When attempting to remove the cuvette from the bottom, the quartz shattered. Fig. 16 shows the SEC body with rotated parts and the shattered quartz in a mortar atop a portion of the furnace.

Fig. 16

SEC body with rotated parts and the shattered quartz in a mortar atop a portion of the furnace.

To avoid future issues with the SEC, a redesign of the SEC and furnace is anticipated. First, it should incorporate an SEC design that attaches the high temperature fiber optic patch cords without introducing internal mechanical stress to the patch cord body. Additionally, a design feature should be implemented to allow easy removal of the cuvette from the top. Different materials for the SEC body materials should also be considered. A possible solution is to make the SEC out of MACOR to prevent oxidation of the surfaces and mitigate mating of the cuvette to the SEC body.

Additional work needs to be done to couple the free beam of the laser to the fiber optic delivering the signal to the SEC. Damage to the ThorLabs fiber optic cables resulting from calibration were sustained due to the 532 nm laser beam. Fig. 17 shows the difference between an (a) undamaged and (b) damaged fiber optic cable end (melted and deformed). To avoid future damage to fiber optic cables, a better method for coupling the free beam to the fiber optic cable needs to be incorporated. A possible solution to this is purchasing an adapter from a manufacturer that automatically couples the free beam to a fiber optic cable.

Fig. 17

Microscopic view of undamaged (left) and damaged (right) fiber optic cables.

4. Conclusion

We provided an overview of three analytical techniques– spectrochemical, electrochemical, and spectroelectrochemical– for molten salts for compositional information and characterization focusing on uranium ions in molten salts. This review allowed us to develop a preliminary design the spectroelectrochemical experiment for utilization in the molten salt environments. We designed a spectroelectrochemical cell and performed temperature calibrations and dry test runs to check the system’s capability limit. Here, we reported the issues and redesigned the cell after experiencing the failure during the initial test runs. The modification of the SEC allowed us to assemble, align the system with the light sources, and successfully transferred the setup inside the glovebox. A preliminary test was done to obtain transmission and absorption background of NaCl- CaCl₂ salt at 600°C. Results show that the cuvette has high transmittance effects across all wavelengths and the salt exhibits lower transmittance effects at the lower wavelengths than at the higher wavelengths. With the new design, there were no issues in removing the thermocouple, stainless steel spacer, and MACOR plates after the preliminary run. However, the quartz vessel was mated to the inner cavity of the SEC body, making it hard to remove. In addition, further investigation revealed the shearing in the sheath, the portion of the patch cord that was sheared off, deterioration of the SEC, corrosion on the bolts holding the cell assembly, and damage to fiber optic cable due to high energy light source. To avoid these issues in the future design, the designed SEC should attach to a high temperature fiber optic patch cords without introducing internal mechanical stress to the patch cord body. Moreover, the materials for the cell body should be made of MACOR to prevent corrosion of the surface and avoid mating of the cuvette to the SEC body. In the end, a better method for coupling the free beam to the fiber optic cable should be employed and incorporation of an adapter from a manufacturer that couples directly the free beam to a fiber optic cable would be highly recommended.