Species Chemistry and Electrochemical Separation in Molten Fluoride Salt

Autor: Wang, Yafei
Rok vydání: 2019
Předmět:
Druh dokumentu: Dissertation
Popis: Fluoride salt-cooled high-temperature reactor (FHR) is a safer and potentially less expensive alternative to light water reactor due to the low pressure of primary system, passive decay heat cooling system, chemically inert coolant salt, and high-temperature power cycle. However, one challenge presented by this reactor is that fission products may leak into the primary system from its TRISO particle fuel during normal operation. Consequently, the circulating fission products within the primary coolant would be a potential radioactive source. On the other hand, the containment material of the molten salt such as nickel-based alloys may be corroded, and its species may stay in the salt. Thus, the installment of the primary coolant clean-up system and the study on the contaminant species' chemistry and separation are necessarily needed. Electrochemical separation technique has been proposed as the online coolant clean-up method for FHR for removing some impurities from the salt such as lanthanides and corrosion products. The present research focuses on the electrochemical separations of fission products and corrosion products in molten FLiNaK salt (46.5LiF-11.5NaF-42KF mol%) which is the surrogate of the primary coolant candidate FLiBe (67LiF-33BeF2, mol%) for FHR. The main objective is to investigate the electrochemical behaviors of fission products and corrosion products in molten FLiNaK salt to achieve its separations, and provide fundamental properties to instruct the conditions needed to be applied for a desired electrochemical separation. La and Ce are two main elements concerned in this study since they are major lanthanide fission products. Electrochemical behavior of LaF3 in molten FLiNaK salt was studied on both W and Mo inert working electrodes. Although the standard reduction potential of La (III) is more cathodic than that of the primary salt melt constituents K (I) and Na (I), the electrochemical separation of La from molten FLiNaK salt was achieved by merely using inert working electrode because of the formed LaF63- when KF or NaF exists as the salt constituents. Fundamental properties of La in molten FLiNaK salt were also studied at various situations by electroanalytical methods including cyclic voltammetry (CV), chronopotentiometry (CP), and potentiodynamic polarization scan (PS). Ce is another fission product to be separated out from molten FLiNaK salt. Both inert (W) and reactive working electrodes (Cu and Ni) were utilized to realize the extraction of Ce. The electrochemical behaviors of Ce observed on inert W electrode are similar to the ones obtained in FLiNaK-LaF3 system. Reactive electrodes Cu and Ni were used to precede the electrochemical deposition potential of Ce by forming intermetallic compounds. It turned out only Ni electrode was feasible for preceding the deposition potential and the intermetallic compound was identified as CeNi5. The dissolution of chromium metal in the form of chromium fluoride into molten FLiNaK salt is the main concern of alloy corrosion in FHR. To understand the alloy corrosion and removal of the corrosion products from the FHR salt coolant, the electrochemical behavior and fundamental properties of Cr in molten FLiNaK salt were investigated in the present study as well. A new analysis method for the Cr two-step electrochemical reaction in the salt was developed. The method can be applied to other two-step reactions as well. Liquid bismuth was proposed to be the extraction media for liquid/liquid multistage separation of fission products in molten salt reactor. It also can be used as the cathode to extract the fission product of which the electrodeposition potential is close to or more negative than that of the main constituents of molten salt. Activity and activity coefficient are essential factors for assessing the extraction behavior and viability of bismuth in separating fission products. Hence, in the present study, the activity and activity coefficient of fission products and alkali metals (Li and K) at different concentrations and temperatures were determined by experiment and simulation methods respectively. To conduct the parametric study for the electrochemical reaction process and predict fundamental properties, an electrochemical model including single-step reversible, irreversible, and quasi-reversible reactions, multiple-reaction, and two-step consecutive charge transfer reaction was developed based on MOOSE. Although the model was not applied to analyze the experimental data in the present study, this model provides an efficient and easy way to understand the effect of various parameters on electrochemical reaction process. The present study supplied a comprehensive study on the electrochemical separation of fission products and corrosion products in molten FLiNaK salt and will contribute greatly to the development of FHR.
Doctor of Philosophy
There is a significant increased demand for the generation of electricity with the fast development of modern society and economy. For well over 100 years, the dominant energy sources for producing electricity in the industrialized world are fossil fuels, notably coal, oil, and natural gas. The generation of electricity from fossil fuels is a major and growing contributor to the emission of greenhouse gases that contribute significantly to global warming. As clean and efficient energy, the nuclear power source has been an attractive alternative to traditional fossil fuels. The fluoride salt cooled high temperature reactor (FHR) is a promising Generation-IV advanced nuclear reactor. FHR is a salt-cooled reactor in which the core contains a solid fuel and liquid salt coolant. It combines attractive attributes from previously developed reactors and has the advantages of, for example, low-pressure operation, high temperature power cycle, and passive decay heat rejection. However, the primary salt coolant can unavoidably acquire fission products from the fuel particles and corrosion products from structural material corrosion. Therefore, it is necessary to have a primary coolant clean-up system installed in the FHR to mitigate the contamination and ensure the continued operation of the reactor. Electrochemical separation technique has been proposed as the online coolant clean-up method for FHR. Electrochemical separation can be typically done in a three-electrode cell system (working, counter, and reference electrodes). Through applying a proper electrical potential or a current, the target metal ions in the molten salt will be deposited on the working electrode. In that way, the contaminants, including fission products and corrosion products, can be taken out with a working electrode from the molten salt coolant. In this study, the fundamental behaviors of separation of La, Ce (represent lanthanide fission products) and Cr (represents corrosion products) in FLINAK were investigated. To achieve their separations, the present dissertation provided a comprehensive study about the electrochemical behaviors of La, Ce, and Cr species in molten FLiNaK salt at various situations, and relevant fundamental properties for guiding the conditions needed to be applied for the desired electrochemical separation. Considering the use of liquid bismuth as the extraction media for liquid/liquid separation and the electrode for electrochemical separation of fission products the fundamental properties of fission products and alkali metals in liquid bismuth are also determined in the present study to evaluate the separation behavior and viability. Finally, an electrochemical model for understanding the electrochemical process in the FHR salt coolant clean-up was developed. Overall, the work performed in this study will contribute greatly to facilitate the FHR development.
Databáze: Networked Digital Library of Theses & Dissertations
Externí odkaz: