Project No. 10-905 Heat Transfer Salts for Nuclear Reactor Systems – Chemistry Control, Corrosion Mitigation, and Modeling Reactor Concepts RD&D Dr. Mark Anderson University of Wisconsin, Madison In collaboration with: Pacific Northwest National Laboratory University of California, Berkeley Brian Robinson, Federal POC Michael McKellar, Technical POC Heat Transfer Salts for Nuclear Reactor Systems - chemistry control, corrosion mitigation and modeling CFP-10-100 Dr. Mark Anderson, Dr. Kumar Sridharan, Dr. Dane Morgan , Dr. Per Peterson, Dr. Pattrick Calderoni, Mr. Randall Scheele , Dr. Andrew Casella, Dr. Bruce McNamara, Brian Kelleher , Robert Sellers, Tony (Guiqiu) Zheng, Tommy Cisneros, Oday Albakri, Nick Kuwahara, Jun Yang, Maxwell Strassman, Arjun Kalra, William Ryan, David Pfotenhauer, Brad Motl, Paul Brooks UNIVERSITY OF WISCONSIN-MADISON, UNIVERSITY OF CALIFORNIA - BERKELEY, PACIFIC NORTHWEST NATIONAL LABORATORY i Executive Summary The concept of a molten salt reactor has existed for nearly sixty years. Previously all work was done during a large collaborative effort at Oak Ridge National Laboratory, culminating in a research reactor which operated for 15,000 hours without major error. This technical success has garnished interest in modern, high temperature, reactor schemes. Research using molten fluoride salts for nuclear applications requires a steady supply of high grade molten salts. There is no bulk supplier of research grade fluoride salts in the world, so a facility which could provide all the salt needed for testing at the University of Wisconsin had to be produced. Two salt purification devices were made for this purpose, a large scale purifier, and a small scale purifier, each designed to clean the salts from impurities and reduce their corrosion potential. As of now, the small scale has performed with flibe salt, hydrogen, and hydrogen flu- oride, yielding clean salt. This salt is currently being used in corrosion testing facilities at the Massachusetts Institute of Technology and the University of Wisconsin. Working with the beryllium based salts requires extensive safety measures and health moni- toring to prevent the development of acute or chronic beryllium disease, two pulmonary diseases created by an allergic reaction to beryllium in the lungs. Extensive health monitoring, engineering controls, and environment monitoring had to be set up with the University of Wisconsin depart- ment of Environment, Health and Safety. The hydrogen fluoride required for purification was also an extreme health hazard requiring thoughtful planning and execution. These dangers have made research a slow and tedious process. Simple processes, such as chemical handling and clean-up can take large amounts of ingenuity and time. Other work has complemented the experimental research at Wisconsin to advance high tem- perature reactor goals. Modeling work has been performed in house to re-evaluate thermophysical properties of flibe and flinak. Pacific Northwest National Laboratories has focused on evaluating the fluorinating gas nitrogen trifluoride as a potential salt purification agent. Work there was per- formed on removing hydroxides and oxides from flinak salt under controlled conditions. Lastly, the University of California Berkeley has spent considerable time designing and simulating reactor components with fluoride salts at high temperatures. Despite the hurdles presented by the innate chemical hazards, considerable progress has been made. The stage has been set to perform new research on salt chemical control which could advance the fluoride salt cooled reactor concept towards commercialization. What were previously thought of as chemical undesirable, but nuclear certified, alloys have been shown to be theoretically compatible with fluoride salts at high temperatures. This preliminary report has been prepared to communicate the construction of the basic infrastructure required for flibe, as well as suggest original research to performed at the University of Wisconsin. Simultaneously, the contents of this report can serve as a detailed, but introductory guide to allow anyone to learn the fundamentals of chemistry, engineering, and safety required to work with flibe salt. ii Table of Contents Executive Summary i Table of Contents ii List of Tables vi List of Figures viii 1 Molten Fluoride Salts for Nuclear Power 1 1.1 History and Applications of Molten Salts . 1 1.2 The Molten Salt Reactor Experiment . 2 1.3 Generation IV Systems: The Fluoride Salt Cooled High Temperature Reactor . 5 2 Fluoride Salt Properties and Chemistry 7 2.1 Selection of Salt for Reactor Use . 7 2.2 Assorted Thermophysical Properties of Flibe . 7 2.3 Thermodynamics and Corrosion Mechanisms of Fluoride Salts . 10 2.3.1 Lewis Acid and Base Chemistry, Salt Stability, and Pure Salt Chemistry . 10 2.3.2 Redox Potential . 12 2.3.3 Chemical Equilibrium and Fluorine Potential . 14 2.3.4 Measured Solubilities of Cr, Fe, and Ni in Flibe . 19 2.3.5 Relationship of Fluorine Potential, Redox Potential, and Chemical Equi- librium . 21 2.3.6 Summary of Equilibrium . 21 2.4 Impurity Driven Corrosion . 22 2.4.1 Water Hydrolysis Reactions . 22 2.4.2 Sulfur Corrosion Reactions . 24 2.4.3 Galvanic Corrosion . 26 2.5 Corrosion Control . 26 2.5.1 Controlling Fluorine Potential . 27 2.5.2 The Hydrofluorination Process . 30 2.5.3 Oxide Removal . 32 2.5.4 Removal of Sulfur . 32 2.5.5 Metal and Metal Fluoride Removal . 33 2.6 Past Determination of Salt Composition and Impurities . 35 2.6.1 Measurements of Beryllium . 36 2.6.2 Structural Metal Analysis . 37 2.6.3 Oxide Analysis . 37 iii 2.7 Previous Corrosion Testing . 37 2.8 Hazards . 43 2.8.1 Beryllium . 44 2.8.2 Anhydrous and Aqueous Hydrogen Fluoride . 49 3 Purification Design and Operation 51 3.1 Purpose and Ultimate Use . 51 3.2 Oak Ridge Fluoride Salt Production Facility . 51 3.3 Wisconsin Purifier Materials Selection . 52 3.4 Wisconsin Fluoride Salt Production Facility . 54 3.4.1 Walk-in Fume Hood and Safety Systems . 55 3.4.2 Purification Vessel . 59 3.4.3 Small Purification Vessel . 61 3.4.4 Support Vessel . 62 3.4.5 Storage Vessel . 63 3.4.6 Salt Filtration . 64 3.4.7 Effluent Stream and Caustic Scrubbers . 66 3.4.8 Electronics . 69 3.4.9 Gas Delivery System . 72 3.4.10 Heating and Insulation . 74 3.5 Data Acquisition and Control . 78 3.5.1 Temperature . 79 3.5.2 Pressure . 81 3.5.3 Mass Flow . 81 3.5.4 Power . 82 3.5.5 Salt Weight . 82 3.5.6 Effluent Stream Composition . 83 3.5.7 Ambient Gas Composition . 84 3.6 Calibration and Uncertainties . 84 3.7 Purification Operation . 85 3.7.1 Personal Protection Equipment . 88 3.7.2 Safeguards . 90 3.7.3 Beryllium Monitoring . 90 3.7.4 Clean Up and Disposal . 93 4 Purification Results 94 4.1 Test Run with KF-ZrF4 ................................ 94 4.1.1 Sparge Gases and Salt Composition . 96 4.1.2 Valve Failure . 96 4.1.3 Heater Leads Oxidation . 97 4.1.4 Effluent Stream Contents . 99 4.1.5 Performance of Filtration Unit . 100 iv 4.1.6 Cleaning . 101 7 4.2 Purification of MSRE LiF-BeF2 (66-34 mol%) . 103 4.2.1 MSRE Flibe Canister and Initial Sampling . 105 4.2.2 Tritium Content . 105 4.2.3 Transfer of MSRE Salts . 107 4.2.4 Purification Process . 109 4.2.5 Purified Salt Contents and Appearance . 110 4.3 ICP-OES and ICP-MS Purity Measurements . 112 4.4 Neutron Activation Analysis . 113 5 Nitrogen Trifluoride Analysis 116 5.1 Selection of Salt for Reactor Use . 116 5.2 Heating the Pure Hydroxides and Reaction with NF3 . 116 5.3 Corrosion of the Experimental Setups . 117 5.4 Flow-Through Experiments . 119 5.5 Static Experiments in Stainless Steel . 119 5.6 Static Experiments in Monel . 122 5.7 XRD Data . 123 5.8 Analysis of Impurity Hydroxide or Oxide in Water . 126 5.9 Flinak Evaluations . 130 6 Modeling of Fluoride Salts 133 6.1 Introduction to Modeling . 133 6.2 Details of First-Principles Molecular Dynamics Simulation . 134 6.3 Evaluation of Thermo-Kinetic Properties from FPMD . 136 6.3.1 Effect of XC-Functional and Dispersion . 137 6.3.2 Thermodynamic Properties of Pure Fluoride Salts . 140 Equilibrium Volume, Density and Bulk Modulus . 140 Coefficient of Thermal Expansion . 143 6.3.3 Kinetic Properties of Pure Fluoride Salts . 144 6.4 FPMD Modeling of Solutes in Fluoride Salts . 148 6.4.1 Diffusion of Solute Ions in Fluoride Salts . 149 6.4.2 Structure Surrounding Solutes in Fluoride Salts . 151 Structure Surrounding Dissolved Cr . ..