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The application of the rotating cylinder electrode (RCE) to molten LiCl-KCl eutectic mixtures for electroanalytical 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-transfer correlations. This is the first publication of electroanalytical RCE measurements in LiCl-KCl eutectic based molten salt mixtures, to our knowledge. These measurements were made in mixtures of molten LiCl-KCl eutectic containing UCl 3 and MgCl 2 . Kinetic parameters were calculated for Mg 2+ in LiCl-KCl eutectic. The exchange current density ( i o ) of Mg 2+ deposition varied with mole fraction ( x ) according to i o (A cm −2 ) = 1.64 x 0.689 . The parameters from RCE measurements were also applied in an electrochemical co-deposition model entitled DREP to detect and predict the deposition rate of U and Mg. DREP succeeded in detecting the co-deposition of U and Mg, even when Mg constituted less than 0.5 wt% of the deposit. [ABSTRACT FROM AUTHOR]

The electrochemical behavior of MgCl 2 in molten LiCl-KCl eutectic was investigated to evaluate its suitability as a surrogate for PuCl 3 in studies related to the eletrorefining of used nuclear fuel. The reduction of Mg 2+ was found to be electrochemically reversible up to 300 mV s −1 at 773 K. The diffusion coefficient for Mg 2+ was calculated to be 1.74 and 2.17 × 10 −5 cm 2 s −1 with and without U 3+ present, respectively, at 773 K using cyclic voltammetry (CV). Upon comparison to literature data, the diffusion coefficient of Mg 2+ differs by only 8.8% (with U 3+ present) from that of Pu 3+ and the difference in peak potentials was only 79 mV. Binary-analyte mixtures of UCl 3 and MgCl 2 in eutectic LiCl-KCl were further investigated using CV, normal pulse voltammetry (NPV), chronoamperometry (CA) and open-circuit potential (OCP) measurements for the purpose of comparing each technique's accuracy in measuring U 3+ and Mg 2+ concentrations. Of all the techniques tested, NPV resulted in the lowest error which was, on average, 11.4% and 9.81% for U 3+ and Mg 2+ , respectively. [ABSTRACT FROM AUTHOR]

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 in contact with molten salt was verified by observing the independence of electrochemical signals with adjustments in the WE position. The integrity of the glass coating was also confirmed by observing the stability of electrochemical signals (±1.9%) over several hours. Cyclic voltammetry (CV) and normal pulse voltammetry (NPV) were applied at 10 different concentrations of UCl 3 . Above 0.17 mol dm −3 UCl 3 , the electrochemical signals from CV and NPV deviate significantly from linearity with concentration, but in opposite directions. A shift from reversible to quasi-reversible behavior and migration are theorized to cause the non-linearity for CV and NPV, respectively. A model is proposed to account for migration in NPV resulting in a significant error reduction in the electrochemical concentration measurement. [ABSTRACT FROM AUTHOR]

Several electrochemical techniques, including cyclic voltammetry (CV), chronopotentiometry-open circuit potential (CP-OCP), chronoamperometry (CA) and normal pulse voltammetry (NPV) were applied to the eutectic LiCl-KCl melt containing LaCl 3 and ThCl 4 at 773K. The dependence of diffusion coefficient of La(III) on concentration of LaCl 3 was investigated by using CV, CP and CA. As the concentration of LaCl 3 increases, the D La(III) value obtained from CV gradually decreases, while the values derived from CA and CP exhibit a dome-shaped variation trend. In the LiCl-KCl-LaCl 3 -ThCl 4 system, the diffusion coefficients of La(III) and Th(IV) are both approximately constant regardless of changes in the composition of LaCl 3 and ThCl 4 . Concentration correlations were developed for LaCl 3 and ThCl 4 using the normalized CV peak heights in addition to the diffusion-limited relationships for CV, CA and NPV. NPV was found to be superior for the concentration prediction over the range studied in this work. [ABSTRACT FROM AUTHOR]

Several countries have shown interest in developing the electrochemical treatment of used nuclear fuel (UNF), commonly termed pyroprocessing. From a proliferation perspective, an advantage of pyroprocessing is its inability to isolate pure plutonium. However, because plutonium is present in the process, it needs to be effectively safeguarded to protect against diversion of plutonium-containing materials that could be post-processed elsewhere. The most complicated unit to safeguard in the process is the electrorefiner where UNF is chemically separated into several material entities. Molten LiCl–KCl serves as the electrolyte into which actinides (including uranium and plutonium), rare earths, and other active metals from the fuel are partitioned after being oxidized to chloride salts. Various voltammetric methods are being developed to measure the concentration of actinides in the molten salt in near-real-time. However, these methods have mostly been applied to molten salt mixtures containing a single actinide. Unfortunately, the presence of multiple actinides will create interferences in the electrochemical responses which could make traditional analysis of voltammetric data inaccurate for determining individual species concentrations. It is proposed to use multivariate techniques to more accurately predict concentrations of multiple actinides from voltammetric data. Two techniques, principal component analysis and partial least squares, are demonstrated on experimental and simulated data for molten salt mixtures containing uranium and plutonium. These techniques consistently yielded more accurate predictions of uranium and plutonium concentrations than simply using peak height. Possible methods of employing multivariate techniques in safeguarding an electrorefiner are also demonstrated. [ABSTRACT FROM AUTHOR]

Signature-based safeguards (SBS) is currently being investigated to assist traditional nuclear material accountancy in tracking special nuclear material (SNM) within a fuel cycle facility. SBS involves the identification and detection of signatures from process monitoring data for off-normal operation scenarios that involve the loss or improper movement of SNM. To determine possible realistic signatures, the electrorefiner (ER) process is modeled using the code Enhanced REFIN with Anodic Deposition (ERAD), and the JCC-31 Neutron Coincidence Counter, a nondestructive assay detector, is simulated using MCNPx-POLIMI. The ERAD model is used to determine the elemental composition of the ER cathode deposit, while the MCNPx model is developed to determine the single and double count rates expected for this deposition using fi8 tallies. For the determination of signatures, changes were made in the ER model for current density and diffusion layer thickness. The signatures in terms of both modeled ER and detector output demonstrate distinct signatures to be expected for off-normal operations. The detector response in particular shows significant changes registered in count rates when plutonium is deposited at the cathode, due to the changes in the simulated ER operating conditions. Note: Some figures in this paper may be in color only in the electronic version. [ABSTRACT FROM AUTHOR]

Highlights: [•] An alternative approach to safeguarding and monitoring pyroprocessing is proposed. [•] Possible signals to be used to monitor an electrorefiner are identified. [•] An inverse model was developed to determine deposition rates at the cathode. [•] The sensitivity of certain parameters in the inverse model are presented. [Copyright &y& Elsevier]

Abstract: A series of three simulations of temporally evolving, planar, nonpremixed ethylene jet flames are performed using the one-dimensional-turbulence (ODT) model. The simulations are performed at a fixed Reynolds number, but varying Damköhler numbers, under conditions that result in significant flame extinction and reignition. Results are compared to corresponding direct numerical simulations (DNSs), which exhibit 40, 70, and nearly 100% peak flame extinction among the three cases. The planar, temporal configuration is ideal for comparison and validation of the ODT model, and identical thermodynamic, transport, and kinetic models were used. The ODT model captures the jet evolution and heat release effects. Line of sight profiles are qualitatively similar between the ODT and DNS. Good agreement is found for scalar dissipation statistics. While ODT captures the flame extinction process, the level of flame reignition is underpredicted, and conditional mean temperature profiles are depressed during reignition compared to the DNS. This is in contrast to previous ODT studies using CO/H2 mixtures (syngas). As a one-dimensional model, ODT is unable to capture multi-dimensional edge flame propagation during reignition, but is able to capture reignition via flame folding. Given the fidelity of the fine-scale transport and reaction processes built into ODT, and good flow modeling observed, more reduced combustion models will be challenged to capture nonpremixed flame reignition. [Copyright &y& Elsevier]

• First report of electrorefining plutonium in CaCl 2 at hundred-gram scale. • Efficiency and yield of the hundred-gram scale were comparable to full-scale. • Iron contamination in the feed and higher depletion of anode were observed. Molten salt extraction and electrorefining of plutonium were successfully conducted with only a 200-g initial charge. Due to this small scale, the electrorefining cell components were redesigned relative to conventional, multi-kg standards. The efficiency and yield of this small-scale process are comparable to those of larger-scale exemplars. Chemical analysis of the plutonium product and the anode heel is presented. The product was 99.8% pure. The anode heel was able to be depleted beyond the expected freezing point without significant impurity carryover. [ABSTRACT FROM AUTHOR]

AbstractThe identities of unknown analytes within four eutectic LiCl-KCl melts were determined using electrochemical methods, simulating the uncertainty of electrochemically probing an electrorefiner salt bath or molten salt nuclear reactor. With a variety of electrochemical methods (e.g. cyclic voltammetry, chronopotentiometry, and square-wave voltammetry), and electroanalytical techniques (e.g. semi-differentiation), every analyte was positively identified, although one false positive occurred because of an unexpected chemical interaction. This study highlights some remaining challenges for the use of electrochemical sensors in nuclear material control and accountability in molten salts: (1) quantification of analytes without the use of calibration curves (e.g. error in property values, such as diffusion coefficient) and (2) additional and interfering electrochemical signals due to interaction and alloying of multiple species. [ABSTRACT FROM AUTHOR]

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 Cl 2 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 impurities should form chlorides with relatively high saturated vapor pressures. The objective of the process was to volatilize these chlorides after first achieving near-complete conversion of the metals to hydride and then chlorides. Complete conversion of hydrides to chlorides could not be achieved using relatively low-grade (99.5%) Cl 2. High decontamination factors for all of the metal impurities required near complete conversion to chloride. It was concluded that oxygen contamination of the feed gas stream likely limits the conversion to chlorides by forming oxides instead, which then limits the volatilization of contaminants. Uranium was particularly challenging to remove in experiments suspected of having high oxygen contamination in the feed gas stream. After switching to ultra-high purity (99.999%) Cl 2 , complete conversion to chlorides within the measurement error was possible for the entire temperature range studied. Decontamination factors for aluminum, iron, gallium, and uranium were found to have average values of 1.9, 9.8, 14, and 5, respectively, at 973 K. While the process was most extensively studied using a once-through gas flow, recycling unreacted Cl 2 gas succeeded at minimizing waste while still maintaining complete conversion and similar decontamination factors. [ABSTRACT FROM AUTHOR]

AbstractChloride volatility processes for purifying actinide and rare earth elements have historically required the use of Cl2 tanks. To minimize the hazards associated with these processes, an apparatus was designed to produce Cl2 via molten salt electrolysis. Within this apparatus, one can generate Cl2, chlorinate metals, and consume excess Cl2. Here, electrode materials were tested for their ability to generate Cl2, the composition of the gaseous electrolysis product was evaluated using a quadrupole mass spectrometer, and a Ce foil sample was successfully chlorinated using the electrochemically generated Cl2. [ABSTRACT FROM AUTHOR]

Bismuth has been investigated as a potential liquid electrode for molten salt electrorefining, but the electrochemical behavior of Bi3+/Bi redox couple has received scant attention in LiCl–KCl and LiCl–KCl–CaCl2 eutectics. Electroanalytical techniques were used to determine properties of Bi3+/Bi. Diffusivities of 6.71 and 8.93 × 10−6 cm2 s−1 and standard apparent potentials of 0.1166 and 0.1187 V versus Ag/AgCl (4.75 mol%) were recommended for BiCl3 in LiCl–KCl and LiCl–KCl–CaCl2 eutectics, respectively, at 680 ± 3.1 K. The ICP-MS analysis of BiCl3 content limited the accuracy of the results. The reliability of the results improved with a more thorough drying procedure for CaCl2. [ABSTRACT FROM AUTHOR]

• Electrochemical behavior of LaCl 3 is investigated in CaCl 2 -LiCl and CaCl 2. • La metal deposition peak is broader than peaks predicted by Berzins-Delahay equation. • Semi-differential analysis distinguished La metal deposition from Ca in CaCl 2. • Current response sensitivity to varying resistance compensation demonstrated. Cyclic voltammetry (CV), square wave voltammetry (SWV) and other electroanalytical methods have been applied to molten chloride salts to study metal ion behavior. These studies have been useful for understanding and optimizing the electrowinning of metals. Notable ions that have been studied are rare earth and actinide metal ions in LiCl-KCl salts. However, the behavior of lanthanum (La3+) ions in some molten chlorides or chloride mixtures (e.g., CaCl 2) has not been studied. CV experiments were conducted to evaluate La3+ behavior in CaCl 2 -LiCl and CaCl 2 and were compared to La3+ behavior in LiCl-KCl and CaCl 2 -NaCl eutectics from other studies. The electrochemical reaction was checked for reversible, quasi-reversible, and irreversible transitions in scan rate by examining the relationship between the scan rate applied and the peak current and potential for La3+ reduction. Properties such as diffusion coefficient (D o) and electrons exchanged (n) were calculated from data. The electrochemical setup was also checked for radial diffusion based on the calculated La3+ diffusion coefficient and electrode radius, which led to the observation that natural convection possibly affected the La3+ reduction peak at low scan rates. For more accurate electroanalytical measurements of La3+ reduction, CV curves with and without LaCl 3 added to CaCl 2 and CaCl 2 -LiCl were compared to establish baselines for the La3+ reduction peak. The impact of resistance (IR) compensation on the La3+ reduction peak was also explored. Techniques and practices that improved electroanalytical measurements include placing the electrolytic cell in a Faraday cage, distancing electrodes from the heating coils, and melting the chlorides twice prior to measurements. [ABSTRACT FROM AUTHOR]

Conducting research experiments on plutonium electrorefining is difficult due to the significant hazards and regulations associated with nuclear materials. Finding a surrogate for plutonium electrorefining studies would enable more fundamental research to be conducted. Potential surrogates were first identified by determining the physical properties required to conduct electrorefining at the same conditions commonly used in plutonium electrorefining, a molten metal and molten CaCl 2 at 1123 K. Ce-CeCl 3, In-InCl 3, and Pb-PbCl 2 were the only potential surrogates identified using these constraints. Sn-SnCl 2 was also tested at these same conditions. More potential surrogates were identified by changing the matrix salt and operating temperature. This expanded the potential surrogate list to also include Zn-ZnCl 2 , Sn-SnCl 2 , and Bi-BiCl 3. Zn-ZnCl 2 was used with the LiCl-CaCl 2 (65:35 mol%) eutectic at 773 K. Sn-SnCl 2 and Bi-BiCl 3 were used with the LiCl-KCl-CaCl 2 (50.5:44.2:5.3 mol%) eutectic at 673–773 K. Ce electrorefining in molten CaCl 2 resulted in a difficult to separate colloid mixture of Ce, Ca and Cl. Electrorefining rates for In in molten CaCl 2 were too slow due to InCl 3 volatilizing out of the molten salt. Only trace amounts of SnCl 2 was retained in the CaCl 2 at 1123 K resulting in impractical electrorefining rates. Zn metal product was successfully collected in the LiCl-CaCl 2 eutectic molten salt, but the metal obtained did not coalesce into one piece. Sn and Bi were successfully electrorefined in the LiCl-KCl-CaCl 2 eutectic molten salt and coalesced into product rings with high yields and coulombic efficiencies. While a surrogate could not be identified using the same conditions as plutonium electrorefining, two possible surrogates, Sn-SnCl 2 and Bi-BiCl 3, were found that could imitate the physical configuration (i.e., molten salt on top of molten metal) of plutonium electrorefining at a reduced temperature using the eutectic LiCl-KCl-CaCl 2 salt at 673–773 K in place of CaCl 2 at 1123 K. [ABSTRACT FROM AUTHOR]

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% H 2 gas at 573 K was successful at reducing cm-scale metal buttons into small particles/fragments. This step was followed by reaction with flowing 99.5% anhydrous Cl 2 gas at temperatures ranging from 423 to 573 K for 60 min. In experiments reported here, up to 92% conversion to cerium chloride was achieved from reacting with the anhydrous chlorine gas. The final chloride product has high porosity coupled with small particle sizes (microns to 1600 μm), which makes it suitable for subsequent purification processes. Higher chlorine concentrations in the gas favor higher conversion to chloride. The optimal temperature to achieve high chlorination is 523 K. Oxygen is a minor impurity in the chlorine gas which appears to prevent complete chlorination from being achieved. [ABSTRACT FROM AUTHOR]

• Diffusion coefficients are calculated and reported for lanthanum and nickel in LiCl. • Soluble-soluble models are inconsistent for metal deposition. • A simplified model for predicting electrons exchanged is proposed and tested. A comparison of square-wave voltammetry models for calculating the number of electrons exchanged in a metal deposition reaction. Experiments were conducted for silver deposition on platinum in an aqueous HNO 3 solution and for nickel and lanthanum deposition on platinum in molten LiCl. Diffusion coefficients for Ni2+ and La3+ in LiCl at 700 °C are calculated. Accuracy of soluble-soluble models versus a soluble-insoluble model is discussed. A simple method for determining the number of electrons exchanged under the constraint of well-compensated ohmic drop is proposed and evaluated. [ABSTRACT FROM AUTHOR]