High Temperature Electrochemistry III: Nuclear Materials
Sponsored by: TMS Extraction and Processing Division, TMS: Pyrometallurgy Committee, TMS: Hydrometallurgy and Electrometallurgy Committee
Program Organizers: Prabhat Tripathy, Idaho National Laboratory; Guy Fredrickson, Idaho National Lab; Boyd Davis, Kingston Process Metallurgy Inc.
Wednesday 8:30 AM
March 1, 2017
Location: San Diego Convention Ctr
Session Chair: Jerome Downey, Montana Tech of the Univ of Montana; Michael Simpson, University of Utah
Optimized Voltammetry Methods for Measuring Concentration of Multiple Rare Earths and Actinides in Molten LiCl-KCl: Michael Simpson1; Devin Rappleye1; Chao Zhang1; 1University of Utah
Real time measurement of the concentration of actinides and lanthanides in molten LiCl-KCl is needed for process control and safeguards of nuclear fuel electrorefiners. Currently, nuclear facilities operating electrorefiners rely upon sampling and destructive analysis, which results in very slow response times. Several voltammetry methods have been investigated recently by our group to analyze multiple metals simultaneously in the eutectic LiCl-KCl commonly used in electrorefiners. Keys to minimizing measurement error include electrochemical method development, electrode design and preparation, and data analysis. Results of studies to optimize normal pulse voltammetry parameters will be discussed in addition to electrode preparation/conditioning and models to correlate measured current to concentration. This work is primarily performed with LiCl-KCl-UCl3-MgCl2 mixtures, where Mg serves as a surrogate for Pu. In addition to the development of concentration calibration curves, ways to utilize this technology for safeguarding nuclear fuel pyroprocessing systems will be discussed.
Zirconium Management in the Mk-IV Electrorefiner: Guy Fredrickson1; 1Idaho National Laboratory
The Mk-IV electrorefiner has been in service since 1996 to treat approximately 1.4 MT of used sodium-bonded metallic driver fuels from the EBR-II and FFTF sodium-cooled fast reactors. The majority of the treated inventory was U-10Zr binary fuel. The competing interests of i) anodically dissolving a sufficient fraction of the uranium from the anode basket, and ii) cathodically depositing a purified uranium product on the cathode mandrel, has resulted in the accumulation of zirconium metal in the Mk-IV electrorefiner vessel. This presentation will address the historical background of Mk-IV electrorefiner design and operations, the operational choices that resulted in the accumulation of zirconium, the operational consequences of this accumulation, and various approaches to recovering the zirconium from the vessel.
Initial Operation of Kg-Scale Electrolytic Reduction and Salt Distillation Equipment for the Pyroprocessing of Uranium Oxide in a Hot Cell: Steven Herrmann1; 1Idaho National Laboratory
New equipment was designed, fabricated, installed, and operated in the Hot Fuel Examination Facility – an argon atmosphere hot cell at Idaho National Laboratory’s Materials and Fuels Complex – to effect the electrolytic reduction of, and subsequent salt removal from, nuclear oxide fuels at kg-scale. The primary equipment includes an oxide reduction system and an oxide reduction salt distillation apparatus. The oxide reduction system operates with a molten salt pool of LiCl – 1 wt% Li2O at 650 °C. Declad and crushed nuclear oxide fuel is loaded into a permeable steel basket and immersed in the oxide reduction salt pool. The basket is connected to power supplies as a common cathode to two separate inert anodes that are also immersed in the oxide reduction salt pool and positioned adjacent to the cathode basket. The power supplies are controlled to electrolytically reduce nuclear oxide fuel to metal in the cathode basket, liberating its oxygen ions to the molten salt where they are simultaneously oxidized to oxygen gas and discharged from the cell at the anode assemblies. The reduced nuclear fuel in the basket is removed from the oxide reduction system and placed in a salt distillation apparatus, where the salt-encrusted fuel and basket are subjected to elevated temperature and reduced pressure to separate the salt from the fuel basket. The salt is collected and returned to the oxide reduction system. The reduced fuel, devoid of oxide reduction salt, is then amenable to further pyroprocessing, as needed. A series of electrolytic reduction and salt distillation operations were performed with unirradiated uranium oxide (UO2) particulate to qualify the prescribed hot cell equipment prior to commencing operations with irradiated materials. Observations from the initial operation of the oxide reduction and salt distillation systems with unirradiated UO2 will be presented in detail.
10:00 AM Break
Thorium and Uranium Electrodeposition from Molten LiCl-KCl onto Alpha Spectroscopy Semiconductor Detector Surface: Milan Stika1; Joshua Jarrell2; Thomas Blue2; Lei Cao2; Michael Simpson1; 1University of Utah; 2The Ohio State University
Alpha particle spectroscopy can serve as a valuable, isotope-specific, concentration monitoring tool in the harsh environment of molten chloride salts. In order to obtain good quality spectrum, a semiconductor detector needs to be brought as close to the analyzed sample as possible due to limited penetration capability of alpha particles. A thin actinide layer electrodeposited directly onto a semiconductor contact is an optimal solution to this obstacle. Techniques for obtaining a good quality, thin, non-dendritic deposits of thorium and uranium are under investigation. The nature of the periodic concentration monitoring also dictates that the deposit needs to be stripped away from the detector surface after each run. This is achieved by applying anodic voltage after the measurement. Techniques for efficient removal of the actinides from the surface, yet safe for the surface itself, are therefore discussed as well.
Electrochemical Techniques for Nuclear Safeguards in Molten Salt: Vickram Singh1; Dev Chidambaram1; 1University of Nevada, Reno
Molten salt systems are expected to play a significant role in next generation nuclear reactor designs and pyrochemical fuel reprocessing. Thus, nuclear safeguards techniques must continue to accommodate these new systems. Most safeguards techniques currently in use were developed for aqueous rather than for molten salt systems. The electrochemical technique of cyclic voltammetry may offer an approach to material monitoring in molten salt systems. Electrochemical testing was conducted on a eutectic LiCl-KCl system containing lanthanide chlorides serving as actinide surrogates. Since most assumptions associated with cyclic voltammetry and its applications were also developed for aqueous systems, some preliminary studies were conducted to study the reversibility and oxidation tendencies of the above molten-salt system before applying any mathematical relationships to the collected data. Lanthanide surrogate testing was then conducted to evaluate our system and compare results to the literature.
Electrochemistry in Molten 2LiF-BeF2 Salt for the Fluoride Salt-Cooled High Temperature Reactor Applications: William Doniger1; Thomas Chrobak1; Brian Kelleher1; Kieran Dolan1; Guoping Cao1; Mark Anderson1; Kumar Sridharan1; 1University of Wisconsin-Madison
A dynamic beryllium reference electrode has been developed for in-situ study of redox potential in molten LiF-BeF2 (66-34 mol%) (FLiBe) salt, a candidate reactor coolant for the fluoride salt-cooled high temperature reactor (FHR) concept. Measurement and control of redox potential can mitigate corrosion and enable more common structural materials to be used in FHR. This three electrode probe has demonstrated good reproducibility of the salt redox potential with an error of ±4 mV at 500ᵒC. The response of the probe to changes in salt chemistry was investigated by simulating oxidizing corrosion processes. The changes in redox potential were measured by progressively increasing concentrations of fluorides of nickel, iron, and chromium, corrosion products of common structural materials. The reduction potentials of the metal impurity fluorides were measured in solutions containing approximately 250 ppm of a single impurity using cyclic voltammetry. This approach is extended to study electrochemical purification of molten FLiBe.