Method Development for Quantitative Analysis of Actinides in Molten Salts

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This work examines the nucleation and growth behavior of uranium and plutonium from molten LiCl-KCl eutectic on inert electrodes using electrochemical techniques. Current-time transients obtained from chronoamperometric experiments were compared with theoretical models to characterize the type of nucleation (progressive or instantaneous) for deposition of U and Pu, and co-deposition of U-Pu, from molten LiCl-KCl at inert electrodes. It was established that the nucleation mode of actinides present as chlorides in molten chloride salts changes from progressive to instantaneous with an increasing concentration of the trivalent actinide ions in the salt. The effect of the material of the working electrode was investigated, and it was found that changing the material from tungsten to silver improves resolvability of the nucleation peaks and allows more accurate analysis of the experimental measurements. Using the nucleation data, diffusion coefficients were obtained for U 3+ and Pu 3+ , and were found to be in very good agreement with the values obtained from other studies. The density of nuclei produced during instantaneous nucleation, the rate of nucleation for progressive nucleation, and the radius of the deposited nuclei were evaluated and examined at different overpotentials. Pyrochemical reprocessing technology represents a promising alternative to aqueous processes for separating actinides from irradiated fuel by applying electrolytic methods to the molten salt media. [1][2][3][4][5][6][7] Electrorefining is the main step in pyrochemical reprocessing. During this process, actinides are separated from the bulk of the fission product elements by electrotransport onto a solid or liquid cathode. Electrochemical deposition that takes place at the electrode/electrolyte interface occurs by a process of nucleation and growth and is strongly dependent on the overpotential. [8][9][10][11] Nuclei initially form at active sites of the substrate, consistent with some nucleation rate law, and then grow via the incorporation of additional ions from the electrolyte. Determination of the fundamental parameters of the nucleation and growth phenomena and a very good physical understanding of these processes are essential for improved control and efficiency of pyroprocesses.Electrochemical methods are widely used to study nucleation and growth phenomena in aqueous or molten media because the driving force of nucleation can be easily changed by varying the applied potential.12 Chronoamperometry (CA) is the most frequently used technique for quantitative analysis of the deposition process; it entails applying potential step pulses and observing the resulting currenttime transients. Immediately after applying the potential step, there is a decreasing current transient corresponding to the charging of the double layer, followed by a current due to the formation of the new phase, and an increase in the number of nuclei. As the nuclei grow, the coalescence of adjacent diffusion zones gives rise to a current maximum, followed by a decay...

Section: +supporting

confidence: 91%

Section: Methodsmentioning

confidence: 99%

Electrochemical Nucleation and Growth of Uranium and Plutonium from Molten Salts

Tylka¹,

Willit²,

Williamson³

2017

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“…Concentration calculations.-To calculate the concentration of U 3+ in the salt, we used the standard area addition described in detail in our previous work. 1 In this method, the peak currents are plotted as a function of the controlled immersion depth (h) of the working electrode, and from the slope of a generated linear plot ( i p h ) described by Equation 2, the concentration C i is calculated.…”

Section: Resultsmentioning

confidence: 99%

Application of Voltammetry for Quantitative Analysis of Actinides in Molten Salts

Tylka

Willit

Prakash

et al. 2015

Self Cite

Our previous work ("Method Development for Quantitative Analysis of Actinides in Molten Salts" 2015) 1 demonstrated that by following a set of developed procedures and a refined data analysis method, cyclic voltammetry can be used for very precise, real-time quantitative concentration measurements of actinides in molten salts. This work examines the suitability of the established procedures over a wider range of concentrations comparable to those expected in the normal operation of an electrorefiner used in the pyrochemical processing of used nuclear fuel. We found that electrochemical methods can be used for very precise concentration measurements up to approximately 2 wt%. For higher actinide concentrations, the value of the diffusion coefficient decreases and these variations have to be taken into consideration. We also investigated the application of voltammetry for analyzing systems containing multiple elements (U 3+ and Pu 3+ ) and found that a zero current baseline determination of the second or succeeding peak in the voltammogram is not a correct approach, since that peak is affected by the tail from the peak that preceded it. We then used a different method of data analysis that allows for an accurate baseline determination and can be used to analyze systems involving more than one component. Pyrochemical reprocessing technology represents a promising alternative to aqueous processes for separating actinides from the remaining irradiated fuel by employing electrolytic methods in a molten salt media.2-8 The electrorefiner is the centerpiece of the process, in which uranium (U) and transuranic elements are separated from the bulk of the fission product elements by electrotransport onto a solid or liquid cathode. High-precision, real-time concentration measurements of actinides in molten salts are required to monitor the progress of the process and could be used for material control and accountability measurements. The development of these measurements is essential for implementing and operating a commercial fuel reprocessing facility.9-11 Electrochemical techniques are very well-suited for this purpose since they allow rapid real-time measurements, do not require the use of standards, and are compatible with remote handling operations.1,11-13 Unlike other chemical methods, they do not require the collection and preparation of representative samples, so they avoid problems that can result from contaminating the sample and its degradation. Also, the equipment is not affected by the high radiation field present in a fuel processing operation, and the analytical results can be received in a relatively short time (e.g., less than 2 minutes).In our previous work, 1 we investigated the applicability of electrochemical methods for quantitative measurements of actinide concentrations in molten salts. We demonstrated that by following a particular set of procedures, and refining data analysis and experimental methods, very precise and reproducible measurements could be achieved. The concentrations of uranium (U) a...

“…2 The setup consisted of an Inconel crucible containing molten salt into which the electrodes were immersed. The working and quasireference electrodes were 1 mm diameter tungsten rods, and the counter electrode was * Electrochemical Society Member.…”

Section: Methodsmentioning

confidence: 99%

“…Electroanalytical techniques using cyclic voltammograms (CVs) are a promising approach that has been developed to provide these measurements. [1][2][3][4] Earlier voltammetry studies showed that CVs were able to provide very accurate results at low concentrations (0-2 wt%) for molten chloride salts containing U and Pu;…”

mentioning

confidence: 99%

Communication—Quantitative Voltammetric Analysis of High Concentration Actinides in Molten Salts

Hoyt

Willit

Williamson

2017

Previous electroanalytical studies have shown that cyclic voltammetry can provide accurate quantitative measurements of actinide concentrations at low weight loadings in molten salts. However, above 2 wt%, the techniques were found to underpredict the concentrations of the reactant species. This work will demonstrate that much of the discrepancy is caused by uncompensated resistance and cylindrical diffusion. An improved electroanalytical approach has therefore been developed using the results of digital simulations to take these effects into account. This approach allows for accurate electroanalytical predictions across the full range of weight loadings expected to be encountered in operational nuclear fuel processing equipment. Accurate measurement of actinide concentrations is critical for the control and accountancy of nuclear materials within molten salts used in nuclear fuel processing. Electroanalytical techniques using cyclic voltammograms (CVs) are a promising approach that has been developed to provide these measurements. [1][2][3][4] Earlier voltammetry studies showed that CVs were able to provide very accurate results at low concentrations (0-2 wt%) for molten chloride salts containing U and Pu; 2 however, at high concentrations (>2 wt%) the electroanalytical measurements were found to deviate considerably from inductively coupled plasma/atomic emission spectroscopy (ICP/AES) results. The possibility of reduced diffusion coefficients at high concentrations (leading to concomitant reduction in peak currents) was mentioned as one source of the discrepancy.3 However, other effects can lead to similar attenuation. For example, uncompensated resistance, R u , has long been known to alter the current response in CVs, and the effects thereof must be addressed even in high conductivity molten salts. As cylindrical rods are commonly used as electrodes in molten salts, the effects from cylindrical diffusion must too be addressed.Many classic papers exist on the subject of uncompensated resistance, both for soluble-soluble and soluble-insoluble reactions. [5][6][7] Cylindrical diffusion has too been examined in several classic papers. [8][9][10] Specific to molten salts, a number of recent electroanalytical papers have included iR-correction to partially account for the effects of uncompensated resistance, 4,11 but simple ex post facto correction of the applied potential does not account for the alteration to the evolution of the current response and is hence inadequate. Positive feedback iR-compensation is an experimental approach to address the effect of uncompensated resistance, 12 but oscillations and feedback issues that occur above ∼90% R u make the approach suboptimal for electroanalytical measurements.13 Thus, while the existence of both uncompensated resistance and cylindrical diffusion effects is well known, the combined examination of the resulting electroanalytical implications for a soluble-insoluble reaction has not been performed.Therefore, in this work, digital simulations of CVs with combined u...