Dissolution of Uranium Dioxide Microspheres in Carbonate and Hydrogen Peroxide Solutions
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AN ABSTRACT OF THESIS OF Timothy G. Adams for the degree of Master of Science in Radiation Health Physics presented on August 15, 2013. Title: Dissolution of Uranium Dioxide Microspheres in Carbonate and Hydrogen Peroxide Solutions Abstract approved:____________________________________________________ Alena Paulenova Uranium dioxide has been used in industry both as a fuel for power reactors and as a target for the production of radioisotopes. One of the most important radioisotopes produced using these targets is molybdenum-99 (Mo-99, 65.94hr half-life), which is the parent isotope to technetium-99m (Tc-99m, 6.01hr half-life), a radioisotope used in 70% of diagnostic medical isotope procedures performed in the United States of America. [1] [2] Molybdenum-99, produced by either the thermal neutron fission of uranium-235 in nuclear reactors or by neutron activation of molybdenum-98, is purified, packaged, and shipped to hospitals worldwide. The maximal activity of Tc-99m is reached in 22.9hrs, so it can be milked from the parent Mo-99 repeatedly. Mo-99 is one of many fission products generated during the thermal neutron fission of uranium-235. For production of Mo-99, irradiation targets based on metallic uranium, uranium alloys, or uranium dioxide are produced. After neutron irradiation in a reactor, the uranium target must first be dissolved in a suitable medium. This has traditionally been done using boiling nitric acid solutions. In literature and in industry, the use of alkaline solutions, specifically carbonate salt solutions combined with hydrogen peroxide, are being explored as an alternative to the nitric acid based dissolution process. The carbonate-peroxide dissolution scheme has several advantages over traditional nitric acid dissolutions including less damage to equipment during operation and smaller volumes of waste produced during process. This thesis research work explores the initial dissolution rates of uranium dioxide in carbonate medium containing hydrogen peroxide. Effect of three different counter cations- ammonium, sodium, and potassium - on the dissolution behavior of uranium was investigated. The kinetic factors of dissolution, activation energy, frequency factor, and reaction order with respect to both the carbonate salt and hydrogen peroxide were found for each of these systems. Information in this thesis is organized into six chapters and list of cited literature sources. © Copyright by Timothy G. Adams August 15, 2013 All Rights Reserved Dissolution of Uranium Dioxide Microspheres in Carbonate and Hydrogen Peroxide Solutions by Timothy G. Adams A THESIS Submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science Presented August 15, 2013 Commencement June 2014 Master of Science thesis of Timothy Adams presented on August 15, 2013. APPROVED: _____________________________________________________________________ Major Professor, representing Radiation Health Physics _____________________________________________________________________ Head of the Department of Nuclear Engineering and Radiation Health Physics _____________________________________________________________________ Dean of the Graduate School I understand that my thesis will become part of the permanent collection of Oregon State University libraries. My signature below authorizes release of my thesis to any reader upon request. _____________________________________________________________________ Timothy G. Adams, Author ACKNOWLEDGEMENTS I would like to thank Dr. Paulenova and Dr. Reese for their guidance and support throughout this project. I would also like to thank my parents, friends, and co-workers for their advice and support throughout my graduate school career. TABLE OF CONTENTS Section Page 1 Introduction ................................................................................................................. 1 1.1 Molybdenum-99 and Technetium-99m ................................................................ 1 2 Goals ........................................................................................................................... 5 3 Literature Review ....................................................................................................... 6 3.1 CARBEX and Carbonate Dissolution .................................................................. 6 3.2 The Rate Law and Arrhenius Equation ................................................................ 7 3.3 General Equations ................................................................................................ 8 3.4 Effect of Carbonate ............................................................................................ 10 3.5 Effect of Hydrogen Peroxide.............................................................................. 12 3.6 Effect of Temperature ........................................................................................ 14 3.7 Effect of pH ........................................................................................................ 15 3.8 Spectrophotometry ............................................................................................. 16 4 Methods and Materials .............................................................................................. 18 4.1 Uranium Dioxide Microspheres ......................................................................... 18 4.1.1 Properties of Uranium Dioxide microspheres: ........................................... 18 4.1.2 Powered Uranium Dioxide Microspheres ................................................... 20 4.2 Spectrophotometry ............................................................................................. 20 4.3 Imaging with Microscope................................................................................... 23 4.4 pH control ........................................................................................................... 23 4.5 Experimental Setup ............................................................................................ 23 4.6 Experimental Procedures.................................................................................... 24 4.6.1 Effect of carbonate cation, hydrogen peroxide, and temperature ............... 25 4.6.2 Additional Ammonium Carbonate Experiments ........................................ 26 4.6.3 Effect of the UO2 particle size .................................................................... 27 5 Results and Discussion ............................................................................................. 29 5.1 Assumptions and General Observations ............................................................ 29 5.2 Ammonium Carbonate ....................................................................................... 31 5.2.1 Ammonium Carbonate: 0.04g/mL Solid:Liquid Ratio .............................. 32 TABLE OF CONTENTS (CONTINUED) Section .................................................................................................................. Page 5.2.2 Ammonium Carbonate: 0.01g/mL Solid:Liquid System ........................... 36 5.2.3 Time-Extended Dissolution Experiments ................................................... 38 5.3 Sodium Carbonate .............................................................................................. 41 5.4 Potassium Carbonate .......................................................................................... 43 5.5 Summary of the Carbonate Counter Cation -Ion Experiments .......................... 45 5.6 Effect of the Particle Surface Area ..................................................................... 47 5.6.1 Dissolutions of the Un-Fractionated Powdered Microspheres ................... 47 5.6.2 Dissolutions of Size-Fractionated Powdered Microspheres ....................... 49 6 Conclusions ............................................................................................................... 54 6.1 Future Work ....................................................................................................... 55 7 Bibliography ............................................................................................................. 57 APPENDIX ....................................................................................................................... 63 Appendix A – Extinction Coefficients .............................................................................. 64 Appendix B – Experimental Dissolution Rates ................................................................ 68 Appendix C – Uncertainty Analysis ................................................................................. 73 LIST OF FIGURES Figure Page Figure 1: Size distribution of ground uranium dioxide microspheres. ............................. 20 Figure 2: Absorbance spectrum of the 9.86mM uranyl peroxide-carbonate complex (top) and uranyl tricarbonate (bottom). Taken with Ocean Optics QE65000 spectrometer. ..... 22 Figure 3: Typical absorbance spectrum of the 1.14mM uranyl peroxide-carbonate sample scanned with the Cary 6000i spectrophotometer. ........................................................... 22 Figure 4: Uranium dioxide microspheres at 200x magnification. Left: original microspheres without any treatment; Right: microspheres partially dissolved in 0.7M ammonium carbonate and 0.7M hydrogen peroxide at room temperature . .................... 29 Figure 5: Dissolution curve from a typical experiment.