Michael F. Simpson

• PhD
• Chair at University of Utah

This paper reports on a new approach to chlorinating molten salts using vaporized NH4Cl. In separate experiments, chlorination of dissolved Li2O and submerged U metal was demonstrated. Ammonium chloride was thermally decomposed in a quartz glass vaporization chamber with an internal temperature ranging from 295 to 384 °C. These decomposition produc...

Electrochemical processing of spent nuclear fuel in molten chloride salts results in radioactive salt waste. Chlorine removal from the salt has been identified as an effective and efficient first step in the management of high-level waste. In this work, a simple salt was dechlorinated with a phosphoric acid phosphate precursor, resulting in a glass...

Open circuit potential (OCP) measurements were made to determine uranium chloride (UClx) species activity in NaCl–MgCl2, which is a candidate salt for use in a molten salt reactor fuel. The operating temperature ranged from 500 to 650 °C, and the measurement system used a U-Zr working electrode (WE) and Ag/AgCl reference electrode (RE). The activit...

We investigated the long-term stability of Ag/AgF, Mn/MnF₂, and Ni/NiF₂ redox couples for use in reference electrodes (REs) for molten fluoride salt systems. A quasi-RE system was used with FLiNaK + 1 wt% AgF, MnF₂, or NiF₂, to evaluate the stability of various metals that may be used for molten fluoride salt RE design. Cyclic voltammetry was used...

Molten CaCl2 is a useful electrolyte for certain pyroprocessing techniques, and the solubility of oxide ions in this salt is a critical characteristic of many of these techniques. CaO is a byproduct of direct oxide reduction (DOR), so the concentration of oxide ions can be used to determine the extent of these reactions. Using cyclic voltammetry (C...

The direct electrolytic reduction (DER) process can be used to reduce UO 2 to U metal in a molten LiCl-Li 2 O electrolyte. In the baseline process, UO 2 is loaded in a steel basket that serves as the cathode, while O 2 gas is generated at a Pt anode. The use of Pt for anodes is non-optimal because of its cost and tendency to corrode via lithium pla...

A direct oxide reduction (DOR) process is used to reduce PuO2 to Pu metal via Ca metal. Molten CaCl2 is commonly used as the electrolyte for this reaction. As the DOR reaction proceeds, CaO accumulates in the salt and must be measured in real-time to know when the reaction has gone to completion. Open circuit potential (OCP) was measured in this st...

Electrochemical measurements of LaCl3 were obtained in eutectic LiCl-KCl at 773 K. Cyclic voltammetry data were acquired using a tungsten electrode LaCl3 amounts in the molten salt in the range of 0.5 to 3.0 wt%. Both cyclic voltammetry and Tafel analyses of the data resulted in calculation of values for the charge transfer coefficient (0.262), dif...

The stability of Ag│Ag ⁺ molten chloride reference electrodes was studied using mullite and magnesia tubes as reference electrode membrane materials in molten MgCl2-KCl-NaCl for continuous immersion up to 31 days. Two electrochemical methods were used to characterize the long-term performance of the reference electrodes at 500°C in an inert atmosph...

Partial chlorination of CeO2, La2O3, and Nd2O3 was achieved using ZrCl4 dissolved in molten LiCl–KCl at 450 °C over a period of 4–5 h. ZrCl4 was pre-volatilized into an argon stream to maintain a constant vapor pressure for equilibration with the salt. This approach was effective at maintaining the ZrCl4 concentration at the solubility limit of 1.3...

Molten salt mixtures containing LiCl-KCl and NaCl-MgCl 2 have been infused with UCl 3 via reaction of U metal and FeCl 2. The process starts with base salt (LiCl-KCl or NaCl-MgCl 2) drying/purification using hydrochlorination via bubbling anhydrous HCl. An auto-titrator running in pH-stat mode was used to determine the point at which there is no ne...

Solubility experiments for magnesium metal in molten MgCl2-KCl-NaCl were run at 500°C, 600°C, and 650°C. This study is relevant for understanding how to control corrosivity of the salt when used for high-temperature heat transfer and storage in systems such as concentrating solar power plants and advanced nuclear reactors. Metal dissolution is a vi...

Molten NaCl–CaCl2–UCl3 was dosed with small amounts of NiCl2 at 600 °C to cause the redox potential to spike upwards. A metallic zirconium rod was used as the working electrode (WE) and was shorted to a stainless steel beaker [counter electrode (CE)] used to contain the salt through a potentiostat operating in zero resistance ammeter mode. The pote...

This paper reports work to further develop a process to chlorinate uranium oxide and cause it to dissolve in a molten salt. The method was first reported by Japanese researchers and involves reaction with ZrCl4 and Zr. The challenge with this approach is achieving a high concentration of ZrCl4 in the molten salt due to its high volatility at the re...

Molten chloride salt fast reactors (MCFRs) will require UCl3 dissolved in molten salt mixtures as fuel for nuclear fission. For infusing the salt with UCl3, bubbling HCl into NaCl-CaCl2 in contact with U metal was investigated. The reaction was run up to 9 h and yielded U concentration up to 0.652 wt.%. Open circuit potential between a W electrode...

Among many mixed ionic electronic conductors (MIECs), lanthanum strontium cobalt iron oxide (LSCF) has been proven as a promising material for use as cathode in SOFCs. The ion and electron conduction in LSCF need to be studied separately. To measure the ionic conductivity of LSCF, YSZ disks were applied to block the electronic current, and multilay...

We report the successful infusion of molten salt mixtures containing LiCl–KCl, NaCl, and NaCl-CaCl2 with UCl3 via reaction of U metal with iron chlorides (FeCl2 and FeCl3). Reaction in LiCl–KCl and NaCl-CaCl2 resulted in a yield of 93% and 96.7% using FeCl2 at 500 and 600 °C, respectively. Reaction to form NaCl-UCl3 at 850 °C had a yield of 70.6%....

A method of ranking the rate of corrosion of metals in molten fluoride or chloride salts is proposed based on a zero-resistance ammeter (ZRA). The metal of interest for the corrosion study is shorted to a relatively noble metal counter electrode. Stainless steel 316 was tested in FLiNaK (LiF-NaF-KF). Haynes 230, stainless steel 316L, and Hastelloy...

The electrochemical interaction of oxide ions with tungsten electrodes in molten calcium chloride (CaCl2) was analyzed by combining electroanalytical techniques with X-ray photoelectron spectroscopy. During a cyclic voltammetry (CV) scan, the oxide ions appear to interact with the tungsten working electrode via a multi-step oxidation and reduction...

Reversible cells can be used as solid oxide fuel cells (SOFC) and solid oxide electrolyzer cells (SOEC). The solid electrolytes used for such cells frequently have pure ionic conduction with insignificant electronic conduction. A previous study (Virkar and Tao, 2015) has shown that cells made of ceria doped yttrium stabilized zirconia (YSZ), a mixe...

Lanthanum strontium cobalt iron oxide (LSCF) is commonly used as a cathode in solid oxide fuel cells (SOFCs), because it is a mixed ionic-electronic conductor with reasonable oxygen ion conductivity and high electronic conductivity. Yttria stabilized zirconia (YSZ) is used as an electrolyte in SOFCs with good oxygen ion conductivity. AC techniques...

This study addressed the problem of measuring the total mass of molten salt in a nuclear system such as a nuclear fuel electrorefiner or a molten salt reactor. In theory, soluble tracers can be added to an unknown amount of salt. Measurement of the tracer concentration after allowing time to homogenize the salt and elemental analysis can be used to...

Garnet type LLZTO ceramic oxide is a promising solid-state electrolyte for use in Li-ion batteries because it has good chemical stability, adequate mechanical strength. However, lithium dendrite growth still needs to be addressed. The opacity of the solid electrolyte hinders in-situ investigation of dendrite growth. Therefore, semi-transparent LLZT...

Molten salt NaCl-CaCl 2 is a viable candidate for the base fuel salt of a molten salt reactor (MSR). Molten salt mixtures NaCl-CaCl 2 -UCl 3 (16 wt%) and NaCl-CaCl 2 -UCl 3 (14.5 wt%)-CeCl 3 (1.0 wt%) were analyzed using electrochemical methods to assess the feasibility of using these methods to measure the concentration of PuCl 3 in MSR fuel in re...

Molten eutectic LiCl-KCl salt is a widely used electrolyte for electrorefining uranium from spent nuclear fuel. Due to the hygroscopic nature of this salt, such operations must be performed under controlled atmospheric conditions, and waste salts require careful storage to avoid deliquescence and corrosion of container materials. This study investi...

A purification process involving simultaneous chlorination and volatilization is reported using aluminum, iron, gallium, and uranium separation from cerium metal as the demonstration system. Anhydrous Cl2 gas was reacted with the metal after it had been converted to small particles of hydride at temperatures ranging from 523 to 973 K. Each of the i...

A study was performed to assess the effect of controlling the reduction mechanism on Li 2 O entrainment in an electrolytic UO 2 reduction process. The reduction mechanism was controlled by isolating the UO 2 particles from the lead, eliminating the direct reduction mechanism. Cathode design made it possible to eliminate the direct UO 2 reduction me...

The properties of a dehalogenated salt waste form (DSWF) were investigated for the immobilization of electrorefiner salt waste from the pyrometallurgical reprocessing of used nuclear fuel. Dehalogenation of salt through reaction with hydrogen-Y zeolite provides a viable means of decreasing solid waste volume prior to immobilization. The presence of...

Electrolytic uranium oxide reduction has the potential to be a key process for recycling spent fuel from commercial light water reactors to advanced nuclear reactors—including molten salt reactors and metal fueled reactors. However, several problems with the process have been identified that need to be addressed to support efficient, cost-effective...

With applications that include thermal energy storage in Gen3 concentrated solar power (CSP), molten salt batteries in grid scale energy storage, fuel and coolant in molten salt reactors (MSR), and oxide reduction/electrorefining of spent nuclear fuel; molten salts are potentially the most important all around medium for advanced energy systems. In...

Molten fluoride salts have attracted interest as fuel and/or coolant for several different molten salt reactor (MSR) designs. Fluoride salts have many advantageous properties for these applications, such as high ionic conductivity, good heat transfer capacity, and high solubility for actinides. However, the impurities of the fluoride salt corrode t...

The U.S. Department of Energy is currently supporting the design and planned construction of the Versatile Test Reactor (VTR) capable of performing experiments in a fast neutron field. This will allow for study of the effect of fast neutron irradiation on nuclear fuels, materials, and functional components. The molten salt irradiation experiment wi...

CaCl2 has applications for electrochemical processing of nuclear materials. Thermal dehydration leads to formation of oxide ions, which are shown to react and cause precipitation of dissolved CeCl3 that was selected as a surrogate for actinide chlorides. Thus, measurement of residual water in CaCl2 is an essential capability. Thermogravimetric anal...

One problematic issue with molten salt reactors (MSRs) is the tendency for the salt’s redox potential to increase with irradiation, causing greater corrosivity. U and Zr were identified as redox buffer candidates in a NaCl-CaCl 2 -UCl 3 salt. Uranium and zirconium are attractive redox buffers since they should not reduce the redox potential below s...

Due to their highly hygroscopic nature, molten chloride salts are known to resist complete thermal dehydration, resulting in incomplete water removal prior to use in high temperature applications. At temperatures required to melt these salts, this can result in hydrolysis, leading to formation of oxides or hydroxides. Depending on application, thes...

The effect of oxide concentration in molten FLiNaK (LiF-NaF-KF) salt on square wave voltammetry (SWV) oxidation peak heights using tungsten working electrodes was determined and compared to the results of acid-base titration of solid salt samples. SWV oxidation current was observed to be more sensitive than that of cyclic voltammetry (CV), possibly...

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...

A novel nuclear waste form was synthesized for the disposal of electrorefiner salt from pyrometallurgical reprocessing of used nuclear fuel. Its synthesis constitutes two steps: waste salt dehalogenation via ion exchange with H–Y zeolite, followed by thermal treatment for phase transition and consolidation. Salt cation-loaded zeolite exhibiting gre...

Hydriding and chlorination of buttons of metallic cerium have been tested as a head-end step to separating metal impurities from actinides. The objective was to achieve complete conversion to chlorides while maintaining high surface area via small particles and open porosity. Hydriding by reaction with 100% H2 gas at 573 K was successful at reducin...

Signature-based safeguards (SBS) is being developed to assist tradition nuclear material accountancy methods in tracking material in pyroprocessing facilities. SBS involves identifying off-normal scenarios that would result in improper movement of material in a pyroprocessing facilities and determining associated sensor response signatures. SBS inv...

The electrochemical response of several alloys (stainless steel 316, Hastelloy C276, Inconel 600, and tantalum) was investigated in molten LiCl–Li2O (1 wt%) at 923 K while bubbling oxygen gas into the molten salt. Tafel and zero resistance ammeter (ZRA) electrochemical methods were used to measure electrochemical effects of oxidation processes at t...

Molten chloride salts have recently gained attention for application to heat transfer and thermal energy storage in concentrating solar power (CSP) plants. In this study, we studied the electrochemical effect of addition of hydroxide ions and/or dissolved metals added to MgCl2–KCl–NaCl salt mixture. Open circuit potentiometry indicated that the sal...

This paper describes experimental and characterization methods for gram-scale uranium oxide electrolytic reduction tests in molten LiCl–Li2O. Post-test analysis of the cathodically reduced UO2 involves acid-base titration and thermogravimetric analysis under flowing air. The titration was used to measure Li2O entrained inside reduced UO2 particles....

Molten salt mixtures containing LiCl-KCl-UCl3-CeCl3 were used as surrogates for molten salt reactor fuel in a study of electrochemical concentration analysis. Current responses to low concentration CeCl3 (0.5 wt% Ce) in molten eutectic LiCl-KCl with high concentration UCl3 (5 and 10 wt% U) were measured using cyclic voltammetry (CV) and square wave...

Investigation of the kinetics of the ion exchange between protonated ultrastable Y-type (USHY) and surrogate electrorefiner (ER) waste salt was performed to optimize the dechlorination process. The kinetics of the ion exchange reaction was investigated by measuring the amount of unreacted Cl. In theory, the kinetics of the ion exchange reaction in...

4H-SiC alpha detectors were fabricated with a 21-μm thick depletion depth and were packaged into a stainless-steel casing with a mineral insulation cable and a standard BNC connector. The packaged detectors had a resolution of 0.624% FWHM at 5.486 MeV prior to salt immersion. The detectors were then immersed in a LiCl–KCl–UCl3 molten salt at 500 °C...

Dechlorination of eutectic LiCl–KCl based electrorefiner (ER) salt is reported via ion-exchange reaction with protonated ultrastable Y-type (USHY) zeolite bound into mechanically fluidized 45–250 μm diameter particles. Evidence of exchange of cations from the salt (Li⁺, K⁺, and fission product cations) into the zeolite lattice replacing H⁺ ions was...

Bismuth aluminoborosilicate glasses were prepared using a standard melt‐quench technique, and the viscosity and thermal properties were measured and analyzed. Electron microprobe analysis was used to examine glass homogeneity. Fragility was calculated based on Mauro–Yue–Ellison–Gupta–Allan model fits to the viscosity curves. The system is character...

Electrochemical methods have been developed to measure metal ion concentrations in spent nuclear fuel electrorefiner by correlating concentration with current response. Analytes UCl3 and GdCl3 were studied in LiCl-KCl eutectic solvent at 773 K. A mixture matrix, which consisted of varying concentrations of UCl3 and GdCl3 from 1 to 10 wt% and from 1...

The alpha emission spectrum of electrodeposited thin film 232Th and depleted uranium sources, including the alpha emitting 232Th decay chain daughters and 238U daughter nuclide 234U, have been measured using a thin film 4H–SiC alpha detector. The energy resolution of this detector was proved suitable for the identification of many of the major char...

Direct electrolytic reduction (DER) of UO2 utilizes a molten LiCl-Li2O electrolyte operating at 650 °C.Productive processes at the cathode are UO2 and Li2O reduction (followed by metallothermic reduction of UO2 by Li metal). Non-productive reduction current has been observed that could be attributed to impurities produced from interaction of the sa...

In a molten salt nuclear reactor system, the redox potential must be controlled for mitigating corrosion of structural materials. The paper presented a critical review on the available knowledge of redox potential control in molten fluoride salt systems. The major phenomena that affect the redox potential and material corrosion are fission, TF prod...

The corrosion of metallic nickel was studied in eutectic LiCl-KCl at 773 K while bubbling argon into the salt with a fixed concentration of H2O. The corrosion process was monitored electrochemically by using continuous open circuit potentiometry (OCP) and intermittent cyclic voltammetry (CV). The OCP of the Ni rod increased with time while H2O was...

A simplified flowsheet for pyroprocessing commercial spent fuel is proposed in which the only salt treatment step is acti-nide drawdown from electrorefiner salt. Actinide drawdown can be performed using a simple galvanic reduction process utilizing the reducing potential of gadolinium metal. Recent results of equilibrium reduction potentials for Gd...

A uranium metal electrodeposition extension to the previously proposed analytical technique for determining actinides in molten salts is presented. The technique combines information from electrodeposition with the output of an alpha particle detector. This technique requires a thin actinide layer to be deposited on a metal substrate. Electrodeposi...

During normal operations, the only deposition on a spent nuclear fuel electrorefiner cathode consists of uranium. A computer model, ERAD, was used to simulate conditions under which the electrorefiner salt was progressively contaminated from spent nuclear fuel processing without operator intervention. The model was adjusted to reflect three scenari...

In this paper, the thermodynamic activity of two lanthanide chlorides (CeCl3 and NdCl3) in molten LiCl-KCl eutectic salt measured using open circuit potentiometry has been reported. Activity coefficient of CeCl3 was found to be strongly dependent on the concentration of CeCl3 in a concentration window of up to 4.66 mol % CeCl3. The activity coeffic...

The activity coefficients of rare earth chlorides in molten salt systems are of great interest for the research, development and commercialization of pyrochemical processing of spent nuclear fuels. In this paper, the activity coefficient of GdCl3 in molten LiCl-KCl eutectic salt has been studied using a two electrode electrochemical cell with a gad...

Densities of molten salt mixtures of eutectic LiCl-KCl with UCl3, CeCl3, or LaCl3 at various concentrations (up to 13 wt%) were measured using a liquid surface displacement probe. Linear relationships between the mixture density and the concentration of the added salt were observed. For LaCl3 and CeCl3, the measured densities were significantly hig...

The drawdown of actinides is an important unit operation to enable the recycling of electrorefiner salt and minimization of waste. A new method for the drawdown of actinide chlorides from LiCl-KCl molten salt has been demonstrated. Using the galvanic interaction between the Gd/Gd(III) and U/U(III) redox reactions, it is shown that concentration in...

An electrochemical study of manganese chloride in molten salt mixtures of eutectic LiCl-KCl was carried out using a variety of electrochemical methods in a high temperature cell including cyclic voltammetry (CV), chronopotentiometry (CP), chronoamperometry (CA), and open circuit potentiometry (OCP). Single step reduction from Mn²⁺ to Mn(0) was obse...

Normal pulse voltammetry has been applied and optimized to develop correlations between diffusional current and concentration for UCl3 and MgCl2 in molten eutectic LiCl-KCl. This method was selected to mitigate working electrode area growth in concentrated molten salt mixtures in which a fairly wide potential range is needed to analyze all actinide...

A new analytical technique for determining actinides in molten salts is proposed, which combines information from electrodeposition with the output of an alpha particle detector. This technique requires a thin actinide layer to be deposited on metal substrate. Electrodeposition of a thin thorium layer from molten LiCl-KCl onto stainless steel and n...

In this paper, we apply an advanced safeguards approach and associated methods for process monitoring to a hypothetical nuclear material processing system. The assessment regarding the state of the processing facility is conducted at a system-centric level formulated in a hybrid framework. This utilizes architecture for integrating both time- and e...

The application of the rotating cylinder electrode (RCE) to molten LiCl-KCl eutectic mixtures for electroanlytical measurements is presented. This enabled the measurement of the limiting current which was observed to follow a linear trend with the rotational rate raised to 0.64–0.65 power on average, which closely agrees with existing RCE mass-tran...

The electrochemical behavior of MgCl2 in molten LiCl-KCl eutectic was investigated to evaluate its suitability as a surrogate for PuCl3 in studies related to the eletrorefining of used nuclear fuel. The reduction of Mg2+ was found to be electrochemically reversible up to 300 mV s−1 at 773 K. The diffusion coefficient for Mg2+ was calculated to be 1...

Rare earth oxides in spent oxide fuel from nuclear plants have poor reducibility in the electrochemical reduction process due to their high oxygen affinity and thermodynamic stability. Here, we demonstrate that the extent of their reduction can be enhanced via co-reduction of NiO in a Li2O–LiCl electrolyte for the electrochemical reduction of a sim...

Alpha spectroscopy can be used to quantify actinide (e.g., U, Pu) concentration in a molten salt electrorefining environment. One could electroplate actinide samples directly onto a semiconductor alpha particle detector to obtain representative isotopic concentrations from a measured alpha particle energy spectrum. In this work, we fabricated a SiC...

The thermodynamic behavior of lanthanides in molten salt systems is of significant scientific interest for the spent fuel reprocessing of Generation IV reactors. In this study, the apparent standard reduction potential (apparent potential) and activity coefficient of LaCl3 were determined in a molten salt solution of eutectic LiCl-KCl as a function...

One of the major sources of error for electroanalytical measurements in high-temperature molten salts is from the measurement of the working electrode (WE) area. A glass sealed working electrode (GSWE) was developed in order to set the exposed area of tungsten and reduce the error associated with WE area. The insulating property of the glass while...

A scheme for analyzing concentration of actinide isotopes in the molten salt electrolyte of nuclear fuel electrorefining systems is currently being developed based on alpha spectroscopy. The objective is to determine elemental and isotopic composition in molten salts in near real-time. Electrodeposition of a thin actinide layer directly onto a semi...

Normal pulse voltammetry was performed on LiCl-KCl-UCl3-MgCl2 molten salt mixtures while varying parameters such as pulse time, relaxation potential, and interval time. Concentrations of UCl3 up to 7 wt% were tested with a fixed 1.5 wt% MgCl2 concentration. The correlation between diffusion limited partial current density to UCl3 concentration exhi...

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 t...

In this paper, we have calculated Raoultian standard reduction potentials (E ⁰ ) for 14 elements of interest for the pyroprocessing of spent nuclear fuel. Also discussed here is the sound experimental methodology and challenges associated with the employment of an electrochemical cell to determine the activity of species in molten salt systems. Fin...

In this paper, we present an empirical evaluation of methods for determining the interfacial area between a working electrode and molten salt—specifically demonstrated using the eutectic LiCl-KCl system. Approaches studied include dipping a cold electrode into the molten salt, using a vertical translator to adjust the height of the electrode, parti...

There is interest by several countries in pursuing pyroprocessing of spent nuclear fuel –including both nuclear weapons states and non-nuclear weapons states. This technology features electrorefiners that can be operated to recover fissile actinides from spent nuclear fuel. This separation capability can be viewed as a proliferation risk factor, wh...

Voltammetry is a promising approach to determining the concentration of actinides and rare earths in molten chloride salts for real time or near real time monitoring of nuclear fuel electrorefining. In this process, actinides and rare earths accumulate in eutectic LiCl-KCl from reaction of UCl 3 with spent nuclear fuel. Monitoring the salt composit...

In the absence of water and oxygen, it is known that molten chloride salts are relatively non-corrosive to a wide range of metals. Corrosion rates are expected to increase when water is present, however. To understand the threshold at which very small concentrations of water can cause corrosion of metals such as nickel and iron, a series of electro...