Project No. 09-780 Thermal Properties of LiCl-KCl Molten Salt for Nuclear Waste Separation Fue l C yc le R&D Dr. Kumar Sridharan University of Wisconsin, Madison In collaboration with: Idaho National Laboratory Michael Simpson, Technical POC James Bresee, Federal POC Thermal Properties of LiCl -KCl Molten Salt for Nuclear Waste Separation Project Investigators: Dr. Kumar Sridharan, Dr. Todd Allen, Dr. Mark Anderson (University of Wisconsin) and Dr. Mike Simpson (Idaho National Laboratory) Post-Doctoral Research Associates: Dr. Luke Olson Graduate Students: Mr. Mehran Mohammadian and Mr. Sean Martin Undergradua te Students: Mr. Jacob Sager and Mr. Aidan Boyle University of Wisconsin -Madison NEUP Final Report Project Contact: Dr. Kumar Sridharan [email protected] Date: November 30th, 2012 Table of Contents 1. Introductory Narrative ............................................................................................................................. 1 2. Motivation ................................................................................................................................................ 2 3. Experimental Setup ................................................................................................................................... 3 3.1 Molten Salt Electrochemistry.............................................................................................................. 3 3.1.1 LiCl-KCl Eutectic Salt ................................................................................................................. 3 3.1.2 Lanthanide Chlorides ................................................................................................................... 4 3.1.3 Glove box and Atmosphere Controls ........................................................................................... 5 3.1.4 Electrochemical Cell .................................................................................................................... 6 3.1.5 Heater and Temperature Control Equipment ............................................................................. 13 3.1.6 Data Acquisition System ............................................................................................................ 14 3.1.7 Laboratory Supplies and Equipment .......................................................................................... 15 3.1.8 Depleted Uranium Electrochemical Studies .............................................................................. 15 3.2 Molten Salt Density .......................................................................................................................... 19 3.3 Differential Scanning Calorimetry .................................................................................................... 22 4. Theory .................................................................................................................................................... 24 4.1 Molten Salt Electrochemistry............................................................................................................ 24 4.1.1 Reduction/Oxidation Chemistry Basics ..................................................................................... 24 4.1.2 Electrochemical Techniques ...................................................................................................... 28 4.1.3 Analytical Techniques................................................................................................................ 35 4.2 Molten Salt Density .......................................................................................................................... 41 4.3 Differential Scanning Calorimetry .................................................................................................... 43 4.3.1 Phase Diagrams .......................................................................................................................... 49 5. Experimental Results ............................................................................................................................. 50 5.1 Electrochemistry of rare-earth chlorides ........................................................................................... 51 5.1.1 Lanthanum (III) Chloride in LiCl-KCl ...................................................................................... 51 5.1.2 Cerium (III) Chloride in LiCl-KCl ............................................................................................. 56 5.1.3 Neodymium (III) Chloride in LiCl-KCl ..................................................................................... 62 5.1.4 Dysprosium (III) Chloride in LiCl-KCl .................................................................................... 68 5.1.5 Depleted Uranium (III) Chloride in LiCl-KCl ........................................................................... 76 5.1.6 Discussion of Electrochemical Results ...................................................................................... 83 5.2 Differential Scanning Calorimetry (DSC) ........................................................................................ 88 5.2.1 Sample Preparation and Initial Testing ...................................................................................... 88 5.2.2 CeCl 3 Concentration Study ........................................................................................................ 91 5.2.3 NdCl 3 Concentration Study ........................................................................................................ 95 5.2.4 LaCl 3 Concentration Study ........................................................................................................ 97 5.2.5 NaCl Concentration Study ......................................................................................................... 99 5.3 Molten Salt Density ........................................................................................................................ 102 6. Programmatic Discussion ..................................................................................................................... 104 7. Summary ............................................................................................................................................... 105 7.1 Electrochemistry ............................................................................................................................. 105 7.2 Differential Scanning Calorimetry and Density .............................................................................. 107 8. References ............................................................................................................................................. 107 1. Introductory Narrative Molten LiCl-KCl eutectic salt is used in pyrochemical processing as a medium for separating of radioactive nuclides in spent nuclear fuel (SNF). The process was developed for processing spent metallic fuel from the sodium-cooled fast reactors like the EBR-II, and its application is being expanded to process spent oxide fuels that have been converted to metal via an oxide reduction process. In the pyrochemical process, the spent fuel is anodically dissolved in the molten LiCl-KCl salt, and pure uranium metal is recovered at a steel cathode. The recent development of the liquid cadmium cathode (LCC) technology at the Idaho and Argonne National Laboratories allows for the separation of uranium, plutonium, and minor actinides (MA) which are collected in a liquid cadmium cathode. The minor actinides which have very long half-lives are recovered with plutonium and can be transmuted in fresh fuels, rather than be sent to repository sites. The process is also applied to recover minor actinides from high level waste generated in PUREX reprocessing facilities. Therefore, pyrochemical processing using molten LiCl-KCl salt is critically important to the United States’ Advanced Fuel Cycle Initiative (AFCI), the Global Nuclear Energy Partnership (GNEP), and non-proliferation. The proposed research addresses both practical and fundamental scientific issues that are of direct relevance to the some of the operational challenges of the molten LiCl-KCl salt pyrochemical process, while providing avenues for improvements in the process. Since the primary challenges stem from the continually changing composition of the molten salt bath during the process, studies will be performed by systematically varying the concentrations of rare earth surrogate elements, lanthanum, cerium, neodymium, and dysprosium that will be added to the molten LiCl-KCl salt. A limited number of focused experiments will be performed by the dissolution of depleted uranium. All experiments will be performed at 500˚C, which is the commonly used temperature for pyrochemical processing. The density of the molten salts will be measured using an instrument specifically designed for this purpose and the melting points will be determined with a differential scanning calorimeter. Knowledge of these properties is essential for salt mass accounting and taking the necessary steps to prevent melt freezing. Cyclic voltammetry studies will be used to determine the redox potentials of the rare earth cations, as well as their diffusion coefficients and activities in the molten LiCl-KCl salt. Anodic stripping voltammetry will be performed to determine the concentration of the rare earth elements and their solubility, and for developing the scientific basis for an on-line diagnostic