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Simulated Response of Electrochemical Sensors for Monitoring Molten-Salt Fueled Reactors
Abstract
Advanced reactor concepts featuring molten salts as either the primary coolant or the actual fuel are gaining increased interest from DOE and the nuclear power industry. Examples include the Advanced High Temperature Reactor from Oak Ridge National Laboratory, the Waste Annihilation Molten Salt Reactor from MIT, and the Accelerator Driven Sub-Critical Molten Salt reactor from Texas A&M. These advanced nuclear reactor concepts are anticipated to be deployed in the future within and outside the US, potentially including non-nuclear weapon states. Traditional international safeguards approaches rely heavily upon material accountancy, but that may be insufficient for these systems due to the quantities or concentrations of TRU elements in the fuel salt. Continuous and unattended process monitoring should be an effective supplemental safeguards measure in this case to complement material accountancy. This approach, however, requires robust sensors that are sufficiently sensitive to actinide concentrations in the fuel salt. Voltammetric methods which utilize a simple three-electrode probe have widely been studied for this application—including cyclic, square wave, and normal pulse voltammetry. Based on the measured electrode potentials and peak heights, these methods can generally be correlated to concentrations of actinides and other ions in the salts. Some limitations to these methods may stem from the multi-component nature of these fuel salts. Most voltammetry studies published have focused on single actinides in a matrix salt. Even in single component studies, quantitative signal responses were found to be limited to a low range of concentrations. To provide the fundamental basis for development of advanced voltammetry systems that avoid or minimize these issues, a model called Enhanced REFIN with Anodic Dissolution (ERAD) was used to calculate voltage responses in molten LiCl-KCl with a range of UCl3 and ThCl4 concentrations. The model was developed based on first principles of mass transfer and electrochemistry. Based on ERAD simulations, the voltage response due to variations in hydrodynamic conditions and geometric configurations differs depending on the species present. Understanding the effect of these variations on voltage response is critical to developing electrochemical sensors and techniques for monitoring molten salt concentrations in advanced reactors.
... This could involve stalling the scan just after the peak potential or semi-differentiating the CV curve, but these methods require a certain amount of separation between the peak potentials and discard a majority of the data (Bard and Faulkner, 2001; Palys et al., 1991). Alternatively, multivariate analytical techniques, which utilize the complete CV curve rather than a couple data points at the peaks, could provide more reliable and accurate predictions , potentially even when peak almost completely overlap (Rappleye et al., 2014a). Multivariate analytical techniques could also provide additional insights into the conditions of the ER and the measurement device. ...
... Only a brief explanation of PCR and PLS will be provided here which is specific to the analysis of voltammograms. Readers are referred elsewhere for more detailed and general descriptions (Rappleye et al., 2014a; Keithley et al., 2009; Kramer, 1998). However, it is worth noting the PCR and PLS can be applied to any signal that is affected by any set of variables in any process. ...
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.
By around 2025, thorium-based fuel cycles are likely to be deployed internationally. States such as China and India are pursuing research, development, and deployment pathways toward a number of commercial-scale thorium fuel cycles, and they are already building test reactors and the associated fuel cycle infrastructure. In the future, the potential exists for these emerging programs to sell, export, and deploy thorium fuel cycle technology in other states. Without technically adequate international safeguards protocols and measures in place, any future potential clandestine misuse of these fuel cycles could go undetected, compromising the deterrent value of these protocols and measures. The development of safeguards approaches for thorium-based fuel cycles is therefore a matter of some urgency. Yet, the focus of the international safeguards community remains mainly on safeguarding conventional 235U- and 239Pu-based fuel cycles while the safeguards challenges of thorium-uranium fuel cycles remain largely uninvestigated. This raises the following question: Is the International Atomic Energy Agency and international safeguards system ready for thorium fuel cycles? Furthermore, is the safeguards technology of today sufficiently mature to meet the verification challenges posed by thorium-based fuel cycles? In defining these and other related research questions, the objectives of this paper are to identify key safeguards considerations for thorium-based fuel cycles and to call for an early dialogue between the international safeguards and the nuclear fuel cycle communities to prepare for the potential safeguards challenges associated with these fuel cycles. In this paper, it is concluded that directed research and development programs are required to meet the identified safeguards challenges and to take timely action in preparation for the international deployment of thorium fuel cycles.
As a way to characterize the values of key parameters for modeling electrochemical transport of elements in a molten salt system, a meta-analysis of CV reduction peaks for UCl3 in LiCl-KCl eutectic was performed. Published CV data was fit using least squares fitting software coupled with electrochemical simulation software. The analysis yielded estimates of apparent standard potential, the diffusion coefficient, and the exchange current density for each reaction. The results for the UCl3 reduction peak matched well with
existing data.
The applicability of electroanalytical methods – square wave voltammetry (SWV) and normal pulse voltammetry (NPV) – to on-line monitoring of actinide concentrations in molten chlorides was investigated for use in pyrometallurgical processes. For SWV curves for plutonium, the concentration dependence of the peak current was not linear at high concentration. However, the limiting currents for uranium and plutonium determined by NPV were proportional to their concentrations up to about 1.7 wt%. The fission product elements do not seem to affect the accuracy in determination of actinide concentrations because the limiting currents for uranium and plutonium were not influenced by the addition of gadolinium. It was found to be difficult to determine neptunium concentration in the presence of plutonium due to the overlap of the output current waves. From those results, NPV was considered more promising although the optimization of applied potential waveform is necessary for use at higher concentrations.
Molten salt reactors, in the configuration presented here and called Thorium Molten Salt Reactor (TMSR), are particularly well suited to fulfil the criteria defined by the Generation IV forum, and may be operated in simplified and safe conditions in the Th/233U fuel cycle with fluoride salts. The characteristics of TMSRs based on a fast neutron spectrum are detailed in this paper, focusing on their excellent level of deterministic safety. We aimed at designing a critical TMSR able to burn the Plutonium and the Minor Actinides produced in the currently operating reactors, and consequently to convert this Plutonium into 233U. This leads to closing the current fuel cycle thanks to TMSRs started with transuranic elements on a Thorium base, i.e. started in the Th/Pu fuel cycle. We study the transition between the reactors of second and third generations to the Thorium cycle in a European frame.
Principal component analysis has often been dealt with in textbooks as a special case of factor analysis, and this tendency has been continued by many computer packages which treat PCA as one option in a program for factor analysis—see Appendix A2. This view is misguided since PCA and factor analysis, as usually defined, are really quite distinct techniques. The confusion may have arisen, in part, because of Hotelling’s (1933) original paper, in which principal components were introduced in the context of providing a small number of ‘more fundamental’ variables which determine the values of the p original variables. This is very much in the spirit of the factor model introduced in Section 7.1, although Girschick (1936) indicates that there were soon criticisms of Hotelling’s method of PCs, as being inappropriate for factor analysis. Further confusion results from the fact that practitioners of ‘factor analysis’ do not always have the same definition of the technique (see Jackson, 1981). The definition adopted in this chapter is, however, fairly standard.
The thermodynamic and electrochemical properties of actinides and fission products in the molten salt determine the pyroprocessing separation performance. Extensive measurements have been carried out to provide fundamental data for evaluating the separation efficiency and technology feasibility of pyroprocessing although the technology has been very well developed in laboratory. The state of the art of fundamental data for substance or materials involved in pyropocessing will be reviewed in the present article. The available data will be summarized and reanalyzed. New correlations, which extend the available data to a broad range of applications, will be developed based on available data from different measurements. Further research topics on providing fundamental data that is needed for scaling the current laboratory technology to industrial applications are identified.
The electrochemical reduction of actinides (U, Pu, Np and Arn) and lanthanides (La and Nd) chlorides was investigated by cyclic voltammetry and chronopotentiometry at different temperatures in LiCl-KCl eutectic. The diffusion coefficients of these metallic cations were estimated as well as their apparent standard potentials. These values of potentials are compared with existing data measured also by transient electrochemical techniques or e.m.f. measurements. (c) 2005 Elsevier B.V. All rights reserved.
A compilation of constitution diagrams of uranium and thorium alloys is ;
presented. The transformation and melting temperatures of the base metal are ;
discussed, followed by the various alloys in alphabetical order. References are ;
included on each alloy. (J.R.D.)
The phase behavior of the Th-U system was studied by a variety of techniques, including X-ray, electrical, resistivity, thermal, and metallographic measurements. The assessed Th-U phase diagram is shown. The system has monotectic, eutectic, and catatectic reactions. No intermetallic compounds are formed, but both Th and U display allotropic transformations. There is significant solubility of U in (Th), whereas the Th solubility in (U) is extremely low. A wide miscibility gap containing two liquids exists at high temperatures.
Basic approach creating some data classical least-squares inverse least-squares factor spaces principal component regression PCR in action partial least squares PLS in action appendix A - matrices and matrix operations appendix B - errors -some definitions of terminology appendix C - centering and scaling appendix D - F-tests for reduced eigenvalues appendix E - leverage and influence.
A conceptual design has been made of a single-fluid 1000 MW(e) Molten-Salt Breeder Reactor (MSBR) power station based on the capabilities of present technology. The reactor vessel is ~22ft in diameter × 20 ft high and is fabricated of Hastelloy-N with graphite as the moderator and reflector. The fuel is ²³³U carried in a LiF-BeF2-ThF4 mixture which is molten above 930°F. Thorium is converted to ²³³U in excess of fissile burnup so that bred material is a plant product. The estimated fuel yield is 3.3% per year.
The estimated construction cost of the station is comparable to PWR total construction costs. The power production cost, including fuel-cycle and graphite replacement costs, with private utility financing, is estimated to be 0.5 to 1 mill/kWh less than that for present-day light-water reactors, largely due to the low fuel-cycle cost and high plant thermal efficiency.
After engineering development of the fuel purification processes and large-scale components, a practical plant similar to the one described here appears to be feasible.
Electrochemical studies of U(IV) in molten
(46.5‐11.5‐42.0 mole per cent) at 500°C reveal a two‐step reduction process at a platinum electrode. The first step is complicated
by disproportionation of U(III) to regenerate U(IV); the second step involves formation of uranium metal. Standard electrode
potentials with respect to a unit mole fraction Ni(II)/Ni electrode are estimated for the U(IV)/U(III) and U(III)/U couples
as −1.41 and −1.81V, respectively. These values must be considered tentative because of complications in the electrode processes.
The voltammetric oxidation of U(IV) at platinum and pyrolytic graphite electrodes in
occurs at ∼ + 1.3V vs. a Ni(II) (saturated)/Ni reference electrode. The results point to the disproportionation of electrochemically generated U(V).
In voltammetric studies of Th(IV) in molten
at 500°C, a reduction wave is obtained at nickel and tungsten electrodes, with a peak potential of −2.02V (vs. a Ni(II) (saturated)/Ni reference electrode). Analysis indicated that Th(IV) is reversibly reduced to the metal with alloy
formation between the deposited thorium and the nickel. A standard electrode potential for the Th(IV)/Th couple in this melt
is calculated to be −2.13V vs. a unit mole fraction Ni(II)/Ni electrode.
The necessity of obtaining accurate thermodynamic quantities for the actinide elements and their compounds was recognized
at the outset of the Manhattan Project, when a dedicated team of scientists and engineers initiated the program to exploit
nuclear energy for military purposes. Since the end of World War II, both fundamental and applied objectives have motivated
a great deal of further study of actinide thermodynamics. This chapter brings together many research papers and critical reviews
on this subject. It also seeks to assess, to systematize, and to predict important properties of the actinide elements, ions,
and compounds, especially for species in which there is significant interest and for which there is an experimental basis
for the prediction.
A fairly detailed knowledge of the chemistry and thermodynamics of molten salt breeder reactor fuels, typically LiF-BeF2ThF 4-UF4 (72-16-12- < 1 mol %), has been gained from measurements of heterogeneous equilibria involving various gases (e.g. HF, H2O) and solids (usually oxides). This information combined with the thermochemical data available for the pure compounds has given formation free energies for the fual components. Their entropies can be estimated from the known entropies of the corresponding solid oxides and the charge on the cation. Their activity coefficients depend primarily on the mol % of LiF in the salt mixture. The solubility of oxides generally decreases with the square of the cation charge divided by the radius. Thus the actinide dioxides and, especially, Pa2O2 (or an addition compound of it) are of low solubility. Pourbaix diagrams are presented which summarize the conditions of redox potential and oxide concentration wherein the fuel may be contained without significant corrosion or precipitation reactions, or conditions wherein selective oxide precipitation may be carried out for purposes of fuel reprocessing.
This work presents a study of the electrochemical properties of Th chloride ions dissolved in a molten LiCl-KCl eutectic, in a temperature range of 693-823 K. Transient electrochemical techniques such as cyclic voltammetry and chronopotentiommetry have been used in order to investigate the reduction mechanism on a tungsten electrode and the diffusion coefficient of dissolved Th ions. All techniques showed that only one valence state was stable in the melt. The reduction into Th metal was found to occur according to a one-step mechanism, through an irreversible reaction controlled by a nucleation process which requires an overpotential of several 100 mV. At 723 K, the diffusion coefficient is DTh(723K) = 3.7 ± 0.2·10-5 cm2.s-1. EMF measurements indicated that, at 723 K, the standard apparent potential is *0ThCl4 /Th) E (723 K) = - 2.582 V vs. Cl2/Cl-, and the activity coefficient γThCl4(723 K) = 8·10-3 on the mole fraction scale (based on a pure liquid reference state).
1D and 3D Simulation of Electrochemical Behavior of U/UCl3 and Pu/PuCl3 in Molten Salt Systems
- R Cumberland
R. Cumberland, "1D and 3D Simulation of Electrochemical Behavior of U/UCl3 and Pu/PuCl3
in Molten Salt Systems (Master's Thesis)," Korean Advanced Institute of Science and
Technology, Daejeon, South Korea, 2013.
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- Transatomic Power
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Electroanalytical Measurements of Uranium and Plutonium in Molten LiCl-KCl Eutectic
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M. M. Tylka, N. A. Smith, J. L. Willit and M. A. Williamson, "Electroanalytical Measurements
of Uranium and Plutonium in Molten LiCl-KCl Eutectic," in International Pyroprocessing
Research Conference, Fontana, WI, 2012.
Technical White Paper v 1.0.1
- Transatomic Power
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http://transatomicpower.com/white_papers/TAP_White_Paper.pdf. [Accessed 14 April 2014].
Progress in Development of Li,Be,Na/F Molten Salt Actinide Recycler & Transmuter Concept
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- A Merzlyakov
- V Smirnov
- A Surenkov
- I Tretiakov
- R Zakirov
V. Ignatiev, O. Feynberg, I. Gnidoi, A. Merzlyakov, V. Smirnov, A. Surenkov, I. Tretiakov
and R. Zakirov, "Progress in Development of Li,Be,Na/F Molten Salt Actinide Recycler &
Transmuter Concept," in Proceedings of ICAPP, Nice, 2007.