A COMBINED CYANEX-923 AND HEH[EHP] PROCESS FOR PARTITIONING USED NUCLEAR FUELS: CHARACTERIZING COMPLEX INTERACTIONS AARON THOMAS JOHNSON

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1 A COMBINED CYANEX-923 AND HEH[EHP] PROCESS FOR PARTITIONING USED NUCLEAR FUELS: CHARACTERIZING COMPLEX INTERACTIONS By AARON THOMAS JOHNSON A dissertation submitted in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY WASHINGTON STATE UNIVERSITY Department of Chemistry MAY 2014

2 To the Faculty of Washington State University: The members of the Committee appointed to examine the dissertation of AARON THOMAS JOHNSON find it satisfactory and recommend that it be accepted. Kenneth L. Nash, Ph.D., Chair Nathalie A. Wall, Ph.D. Scot Wherland, Ph.D. Gregg Lumetta, Ph.D. ii

3 ACKNOWLEDGEMENTS I would first like to acknowledge my advisor, Dr. Kenneth L. Nash, for his support and advice throughout my graduate career. His eagerness to teach, knowledge, and patience has helped guide me through my tenure at WSU. No matter how busy he was, Dr. Nash was always willing and eager to discuss my project. Through the many long conversations about research we have had over the years I have grown significantly as a scientist. I attribute the completion of my Ph. D. to his encouragement and motivation. I am also very appreciative of him for giving me the opportunity to present my research at many different conferences. Through providing me the opportunity to communicate my research, meet and network with scientists in the field, I was able to start the journey of making a name for myself as a scientist. For this, I will be forever grateful. I also wish to thank all my committee members, Dr. Kenneth L Nash, Dr. Nathalie A. Wall, Dr. Scot Wherland, and Dr. Gregg Lumetta, who each had a hand in my scientific development, and who were more than generous with their expertise and valuable time. I would also like to thank the past and present Nash group members for their friendship and scientific input over the past five years. Specifically I would like to thank, Jenifer Braley, Travis Grimes, Colt Heathman, and Julie Muller for the great amount of time they spent training me, answering questions, or the many research discussions with me. I look forward to their continued collaborations and friendships in the years to come. I would also like to thank Joel Alvarez for the research he performed for me as an undergraduate student. Having Joel working under my tutelage was a fantastic experience, and his diligence saved me from having to perform hundreds of titrations by hand. Also, I would like to thank Kristyn Roscioli. This journey would not have been possible without your loving support, understanding, and encouragement. Everything you have done to help iii

4 me, from cooking me meals when I m busy writing, to simply going on walks to get my mind off work have aided me in this journey more than you will ever know. Your smile always picked me up when I was down, you motivated me to be the man I am today. Having your help and encouragement throughout all the challenging times both in graduate school and life are more than any man could ask for. I will be forever grateful for everything you have done for me. I only hope I can be there with you, as you have for me, as we go forth and face the challenges to come in the future. I must offer a very special thank you to my parents, Edward and Pamela Johnson, who together offered me more support and motivation than they could imagine. I will be forever grateful for their unwavering encouragement, support, and love not only in graduate school, but throughout my whole life. They have always inspired me to challenge myself, without that constant encouragement, I would not be where I am today. I would also like to thank my brother, Edward Johnson, who was always supportive of all my endeavors and supplied many laughs throughout the years. Also, I want to thank my sisters, Carrie and Laurie, for their support and their graciousness in letting me stay with them in Seattle, Chelan, or Leavenworth when I needed to escape Pullman for a weekend. Lastly, I d like to thank my dear friends Mittens, Dillon, Arya, Grayson, Denali and Kodi for always being there for me. iv

5 A COMBINED CYANEX-923 AND HEH[EHP] PROCESS FOR PARTITIONING USED NUCLEAR FUELS: CHARACTERIZING COMPLEX INTERACTIONS Abstract by AARON THOMAS JOHNSON, Ph.D. Washington State University MAY 2014 Chair: Kenneth L. Nash The efficient separation of used nuclear fuel into its components remains a significant challenge in the operation of a closed nuclear fuel cycle. Many solvent extraction processes have been either successfully applied or proposed to accomplish this separation. However, most of these processes necessitate several different discrete operations to fully partition fuel, which introduces challenges for industrial implementation. To reduce the cost and complexity of the separations, the concept of combining an acidic and solvating extractant into one process solvent has become the focus of a large amount of research. Initial studies have shown that these processes hold great promise in their ability to partition used nuclear fuel. However, the fundamental chemistry controlling these processes is not well understood. Strong solute-solute complexes (adducts) are known to form between the extractants in the organic phase. Evidence interpreted to indicate the presence of mixed complexes between metals and the extractants also has been reported. These adducts introduce additional equilibria that can complicate industrial application of these processes. To attempt to reduce the strength of the adduct that forms, a new combination of extractants has been proposed, Cyanex-923 (a mixture of C6-C8 trialkyl phosphine oxides) and v

6 HEH[EHP] (2-ethyl(hexyl)phosphonic acid, mono-2-ethyl(hexyl)ester). Each is a monofunctional, industrial organophosphorus extractant. Additionally, the underlying chemistry that controls combined extractant processes is still not well understood. Studies have been complicated by the large and multi-functional extractants that have been used in earlier studies. By using simple solvating (Cyanex-923) and cation exchanging (HEH[EHP]) extractants, the fundamental chemistry that defines the performance of this combination is simpler to elucidate. The combination of these extractants has been investigated by solvent extraction to determine their ability to separate the minor actinides from the fission product lanthanides. In addition, the fundamental chemistry controlling the extraction process was investigated through use of FT-IR, UV-Vis, and time resolved fluorescence spectroscopy, and thermodynamic modeling techniques. The work presented here is the results of this study. It is hoped that these findings can be applied to other systems to better understand the nature of mixed extractant systems. vi

7 TABLE OF CONTENTS ACKNOWLEDGEMENTS iii Page ABSTRACT.v LIST OF TABLES. xii LIST OF FIGURES..xiv DEDICATION.. xix CHAPTER ONE: INTRODUCTION GLOBAL ENERGY OUTLOOK 1 THE CASE FOR NUCLEAR..2 NUCLEAR POWER OVERVIEW.4 FUEL TREATMENT OPTIONS 8 CURRENT REPROCESSING SCHEMES....9 TALSPEAK CHEMISTRY...13 GLOBAL REPROCESSING SCHMES 16 SIMPLIFICATION OF REPROCESSING SCHEMES 18 RESEARCH SCOPE.23 vii

8 REFERENCES...26 CHAPTER TWO PREFACE.31 CHAPTER TWO: A COMBINED CYANEX-923/HEH[EHP]/DODECANE SOLVENT FOR RECOVERY OF TRANSURANIC ELEMENTS FROM USED NUCLEAR FUEL ABSTRACT...32 INTRODUCTION...33 Description of Work..35 EXPERIMENTAL.37 Chemicals and Materials Used...37 Methods..38 RESULTS AND DISCUSSION 40 Eu(III) and Am(III) Extraction From Nitric Acid..40 Ln(III) Series Extraction Profile 45 An(IV, V, VI) Extraction...47 Extraction of Zr(IV) and Mo(VI)...49 Scrubbing...51 Stripping Regime for Eu(III)/Am(III) Separations 53 Trans-Lanthanide Separation Studies 55 viii

9 CONCLUSIONS...57 REFERENCES..59 CHAPTER THREE PREFACE.62 CHAPTER THREE: INTERACTIONS BETWEEN EXTRACTANT MOLECULES: THERMODYNAMICS OF CYANEX-923 ASSOCIATION WITH HNO3, CYANEX-272, HEH[EHP], AND HDEHP IN n-dodecane ABSTRACT...63 INTRODUCTION.64 EXPERIMENTAL.68 Materials 68 NMR..69 FT-IR.69 Titrations 70 Karl-Fischer Analysis 70 RESULTS AND DISCUSSION 71 Characterization of Organic Phase pre-aqueous Contact.71 Comparison between Cyanex-272 and HDEHP 76 Nitric Acid Extraction by Cyanex-923 and its Mixtures with HEH[EHP] 78 Organic Speciation.85 ix

10 Organic Phase Water Content 87 CONCLUSIONS...88 REFERENCES..90 CHAPTER FOUR PREFACE...93 CHAPTER FOUR: STUDY OF ORGANIC-PHASE LANTHANIDE COMPLEXES IN A COMBINED CYANEX-923/HEH[EHP] SEPARATIONS PROCESS USING OPTICAL SPECTROSCOPY ABSTRACT...94 INTRODUCTION.95 EXPERIMENTAL Materials..101 FT-IR Measurements Absorbance Measurements..102 TRFS Measurements 102 RESULTS FT-IR Analysis.103 Absorbance Spectra of Nd(III) and Ho(III) Eu(III) Fluoresence..113 Spectral Modeling 118 x

11 DISCUSSION..120 CONCLUSIONS.130 REFERENCE CHAPTER FIVE PREFACE CHAPTER FIVE: THERMODYNAMIC MODELING OF THE EXTRACTION OF Nd, Eu, Am, AND Ho FROM NITRIC ACID BY COMBINED CYANEX-923 AND HEH[EHP] ABSTRACT.135 INTRODUCTION EXPERIMENTAL Materials..137 Solvent Extraction 138 RESULTS AND DISCUSSION..139 CONCLUSIONS REFERENCES 156 CHAPTER SIX: CONCLUSIONS CONCLUSIONS REFERENCES 163 APPENDIX A.164 xi

12 LIST OF TABLES CHAPTER TWO: A COMBINED CYANEX-923/HEH[EHP]/DODECANE SOLVENT FOR RECOVERY OF TRANSURANIC ELEMENTS FROM USED NUCLEAR FUEL.. 32 Page Table 2.1. The composition of PUREX raffinate of 1 t initially present heavy metal (IHM) LWR fuel with a burn up of 33,000 MWd/t IHM and a cooling time of 3 years. Abbreviations: fission products (FP), corrosion products (CP), actinides (An), neutron poision (NP). Reprinted from (23).45 Table 2.2. Long-lived actinides present in high level waste solutions and their half-lives 47 Table 2.3. ph dependence of Nd(III)/Am(III) separation factors (S.F.) for the HEDTA and TTHA systems. 0.2 M Cyanex M HEH[EHP], 0.1 M Glycine with 20 mm HEDTA or 10 mm TTHA CHAPTER THREE: INTERACTIONS BETWEEN EXTRACTANT MOLECULES: THERMODYNAMICS OF CYANEX-923 ASSOCIATION WITH HNO3, CYANEX-272, HEH[EHP], AND HDEHP IN n-dodecane. 63 Table 3.1. Characteristic IR frequencies of Cyanex-923 and Cyanex-272, HEH[EHP], and HDEHP mixtures in n-dodecane...72 Table 3.2. Equilibrium constants for adduct formation in n-dodecane. Error is indicated in the parentheses to the hundredths place and represents one sigma..76 CHAPTER FOUR: STUDY OF ORGANIC-PHASE LANTHANIDE COMPLEXES IN A COMBINED CYANEX-923/HEH[EHP] SEPARATIONS PROCESS USING OPTICAL SPECTROSCOPY. 94 Table 4.1. Average fluorescence lifetimes for Eu complexes extracted into the organic phase during spectrophotometric titrations, starting as either Eu(C923)3(NO3)3 or Eu(AHA)3. Total [Eu]org = 0.01 M for all experiments Table 4.2. Calculated stability constants by resolving spectrophotometric titrations of Nd, Eu, and Ho. Magnitude of stability constants normalized so the relative strength of the M(AHA)3 complex is equal to CHAPTER FIVE: THERMODYNAMIC MODELING OF THE EXTRACTION OF Nd, Eu, Am, AND Ho FROM NITRIC ACID BY COMBINED CYANEX-923 AND HEH[EHP] xii

13 Table 5.1. logkex values obtained by fitting the results of equation 10 to the experimentally determined distribution ratios Table 5.2. logkex values obtained by fitting the results of equation 10 to the experimentally determined distribution ratios for Am(III) Table 5.3. Equilibrium constants optimized by fitting Equation 10 to the experimentally determined distribution ratios for the extraction of Nd(III) and Ho(III) by 0.2 M Cyanex M HEH[EHP] from varying concentrations of HNO3 in n-dodecane xiii

14 LIST OF FIGURES CHAPTER ONE: INTRODUCTION.. 1 Page Figure 1.1. The relative levels of CO2 emission from the generation of 1 kwh of electricity from various sources of electricity. Reprinted from (7).3 Figure 1.2. Relative radiotoxicity on inhalation as a function of time for the fission products with a burn up of 38 megawatt days/kg. Reprinted from (11). 6 Figure 1.3. The structure of tri-n-butyl phosphate (TBP).. 10 Figure 1.4. The structure of the TRUEX extractant, CMPO...12 Figure 1.5. The radial probability of finding an electron at a distance r from the nucleus for the 4f, 5d, 6s, and 6p valence orbitals of Sm3+ compared to the analogous probabilities of finding an electron in the 5f, 6d, 7s, and 7p orbitals of Pu3+. Reprinted from (35)...14 Figure 1.6. Structures of the main components of the TALSPEAK process. (A) di-(2-ethylhexyl) phosphoric acid (HDEHP), (B) diethylenetriamine-n,n,n,n,n -pentaacetic acid (DTPA), (C) lactic acid.15 Figure 1.7. Structure of N,N -dimethyl-n,n -dioctylhexylethoxymalonamide (DMDOHEMA). 17 Figure 1.8. The structure of CyMe4BTBP. 18 Figure 1.9. Extraction of Am(III) and Eu(III) by 0.1 M CMPO/1 M HDEHP/n-dodecane as a function of initial nitric acid concentration compared to the Am distribution coefficients for the TRUEX process and the Am and Eu distribution coefficients f or the TALSPEAK solvent. (Adapted from (48)) Figure Structures of the extractants used in ALSEP. (A) 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (HEH[EHP]), (B) N,N,N,N -tetraoctyldiglycolamide (TODGA), (C) N,N,N,N -tetra(2- ethylhexyl)diglycolamide (T2EHDGA)...22 CHAPTER TWO: A COMBINED CYANEX-923/HEH[EHP]/DODECANE SOLVENT FOR RECOVERY OF TRANSURANIC ELEMENTS FROM USED NUCLEAR FUEL..32 Figure 2.1. The structures of the extractants used in this study. A) Cyanex-923 where R1 = C6H13 and R2 = C8H17. B) HEH[EHP].37 xiv

15 Figure 2.2. Eu (III) (A) and Am(III) (B) distribution ratios as a function of initial aqueous nitric acid concentration. Organic phase consisted of 0.2 M Cyanex-923, 0.2 M HEH[EHP], or a mixture of 0.2 M Cyanex M HEH[EHP], in n-dodecane.. 41 Figure 2.3. Distribution ratios of Eu(III) (solid lines, solid symbols) and Am(III) (dashed lines, open symbols) as a function of increasing HNO3. Organic phase was 0.2 M Cyanex M HEH[EHP], 0.4/0.4 M, and 0.6/0.6 M in n-dodecane.44 Figure 2.4. Extraction of La, Ce, Pr, Nd and Eu (stable isotopes) from variable nitric acid concentration by 0.1 M Cyanex-923/0.1 M HEH[EHP] in n-dodecane...46 Figure 2.5. The extraction of Th(IV), U(VI), and Np(V) from 0.1 M Cyanex-923/0.1 M HEH[EHP] as a function of initial aqueous nitric acid concentration in n-dodecane..48 Figure 2.6. Distribution ratios of Zr(IV) and Mo(VI) compared to those of Eu(III) by 0.2 M Cyanex M HEH[EHP] in n-dodecane...50 Figure 2.7. Scrubbing of 0.2/0.2 and 0.4/0.4 M Cyanex-923/HEH[EHP] (n-dodecane) mixtures by 0.1 M Glycine at ph 2.2 Shown is the equilibrium ph of the glycine scrubbing solution after 5 contacts with the organic phase (post extraction from 0.5 M HNO3). Also shown is the stripping of Eu(III) and Am(III) by the glycine solution as a function of number of contacts.52 Figure 2.8. The structures of A) HEDTA (ethylenediaminetetraacetic acid) and B) TTHA (triethylenetetramine-n,n,n',n'',n''',n'''-hexaacetic acid)...54 Figure 2.9. Extraction of Eu(III) and Am(III) by 0.2 M Cyanex M HEH[EHP] in n-dodecane as a function of equilibrium ph at A) 0.1 M glycine, 20 mm HEDTA and B) 0.1 M Glycine, 10 mm TTHA. Distribution ratios are presented on the left y axis and solid lines, while the separation factor is presented on the right y axis with dashed lines.. 55 Figure The trans-lanthanide distribution of the lanthanide series (excluding Pm) by 0.2 M Cyanex M HEH[EHP] in n-dodecane from 0.1 M glycine/10 mm TTHA and 0.1 M glycine/20 mm HEDTA. Am(III) distribution under the same conditions is presented for reference.56 CHAPTER THREE: INTERACTIONS BETWEEN EXTRACTANT MOLECULES: THERMODYNAMICS OF CYANEX-923 ASSOCIATION WITH HNO3, CYANEX-272, HEH[EHP], AND HDEHP IN n-dodecane...63 Figure 3.1. Structures of the components used in these studies. (A) Cyanex-923 (B) Bis(2,4,4-trimethylpentyl)phosphinic acid (Cyanex-272) xv

16 (C) 2-Ethyl(hexyl)phosphoric acid mono-2-ethyl(hexyl) ester (HEH[EHP]) (D) Bis-2-ethyl(hexyl) phosphoric acid (HDEHP) Figure 3.2. FT-IR spectra of Cyanex-923 (0.5 M), HEH[EHP] (0.5 M), and their mixtures at 0.5 M HEH[EHP] in n-dodecane with varying concentrations of Cyanex-923. Spectra were collected on a Nicolet is10 FT-IR spectrometer equipped with a diamond crystal and a Smart itr accessory...72 Figure 3.3. Extinction spectra of HEH[EHP]-Cyanex-923 adduct resolved using HypSpec (dashed line) compared with the experimental spectra of (HEH[EHP])2 (dotted line) and Cyanex-923 (solid line) in n-dodecane. Concentrations of HEH[EHP] and Cyanex-923 were 0.5 M..74 Figure 3.4. The probable structure of the adduct formed due to the interaction of Cyanex-923 and HEH[EHP]. The hydrogen bond formed between the phosphoryl group of Cyanex-923 and the P-O-H group of HEH[EHP] is responsible for this interaction..74 Figure 3.5. The 31P chemical shift for Cyanex-923 phosphoryl group as a function of increasing Cyanex-272, HEH[EHP], and HDEHP concentration. Solid, dashed, and dotted lines indicate calculated chemical shifts for Cyanex-272, HEH[EHP], and HDEHP respectively (calculated by HypNMR). [Cyanex-923] = 0.1 M in n-dodecane...76 Figure 3.6. Experimental determination of organic nitric acid concentration. Nitric acid was extracted by solutions of 0.2, 0.4, and 0.6 M Cyanex-923 in n-dodecane. Dashed lines represent calculated values for extracted nitric acid...80 Figure 3.7. Experimental and calculated determination of organic nitric acid concentration for Cyanex-923/HEH[EHP] mixtures. Nitric acid was extracted by solutions of 0.2/0.2, 0.4/0.4, and 0.6/0.6 M Cyanex-923/ HEH[EHP] in n-dodecane. Dashed lines represent calculated values for extracted nitric acid...82 Figure 3.8. FT-IR spectra of 0.4 M Cyanex-923 in n-dodecane after contacting with 0.01, 0.029, 0.066, 0.10, 0.17, 0.23, and 1.0 M HNO3. The solid line is Cyanex-923 before contact with HNO3 and the dashed line after contact with 1.0 M HNO3.84 Figure 3.9. FT-IR Spectra of 0.4 M Cyanex-923/0.4 M HEH[EHP] in n-dodecane after contact with 0.01, 0.09, 0.23, 0.49, 1.4, 2.25 M HNO3. The solid line represents the FT-IR spectra of the Cyanex-923/HEH[EHP] mixture before contact with nitric acid Figure Speciation diagram for the organic phase of a system containing xvi

17 0.4 M Cyanex-923 mixed with 0.4 M HEH[EHP] in n-dodecane after contacting with HNO3. The plot shows the expected concentration of free Cyanex-923, Cyanex- 923 HEH[EHP] adduct, Cyanex-923 HNO3 and free HEH[EHP]2 after contacting with nitric acid Figure Water content of the organic phase as a function of increasing aqueous nitric acid concentration for the systems containing HEH[EHP], Cyanex-923, and their mixtures at 0.2 or 0.4 M in n-dodecane. 87 CHAPTER FOUR: STUDY OF ORGANIC-PHASE LANTHANIDE COMPLEXES IN A COMBINED CYANEX-923/HEH[EHP] SEPARATIONS PROCESS USING OPTICAL SPECTROSCOPY.94 Figure 4.1. The chemical structures of the extractants A) Octyl(phenyl)-N,Ndiisobutylcarbamoylmethylphosphine oxide (CMPO) B) Cyanex-923, where R1 = C6H13 and R2 = C8H17 C) 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (HEH[EHP]) D) Di-(2-ethylhexyl)phosphoric acid (HDEHP) Figure 4.2 FT-IR Spectra of Cyanex-923 and HEH[EHP] loaded with Nd. A) 0.5 M Cyanex-923 in n-dodecane repeatedly contacted with 0.25 M HNO M Nd. B) 0.5 M HEH[EHP] in n-dodecane repeatedly contacted with 0.01 M HNO M Nd Figure 4.3. FT-IR spectrophotometric titrations in n-dodecane of Nd complexes. A) 0.5 M HEH[EHP] loaded with 23 mm Nd with increasing quantities of Cyanex-923 added. B) The same organic phase as previous except C923 was pre-loaded contacted with HNO3 such that it exists 99.7% as C923 HNO Figure 4.4 Extracted Nd complexes of 0.5 M Cyanex M HEH[EHP] in n-dodecane. A) Repeat contacts with 0.01 M HNO M Nd B) Repeat contacts with 0.25 M HNO M Nd Figure 4.5. Absorbance spectra of A) Nd complexes with 0.2 M Cyanex-923 or 0.2 M HEH[EHP] in n-dodecane B) Ho complexes with 0.2 M Cyanex-923 or 0.2 M HEH[EHP] in n-dodecane.109 Figure 4.6 Organic phase Nd complexes. A) Nd(AHA)3 (17 mm) with increasing quantities of Cyanex-923 added. B) Nd(AHA)3 (21 mm) with increasing quantities of C923 HNO3 added. C) Nd(C923)3(NO3)3 (23 mm) complexes with increasing quantities of HEH[EHP] added 111 Figure 4.7. Organic phase Ho complexes in n-dodecane. A) Ho(AHA)3 (24 mm) with increasing quantities of C923 HNO3 added. B) Ho(C923)3(NO3)3 (22 mm) with increasing quantities of HEH[EHP] added Figure 4.8. Extracted complexes from varying initial aqueous acidity from xvii

18 0.2 M Cyanex M HEH[EHP] in n-dodecane. A) Nd complexes and B) Ho complexes. Absorbance intensity normalized to a constant total absorbance..113 Figure 4.9. Fluorescence emission spectra of extracted Eu complexes A) M Eu extracted by 0.1 M Cyanex-923 in n-dodecane. B) M Eu extracted by 0.1 M HEH[EHP] in n-dodecane Figure Fluorescence emission spectra of extracted Eu complexes A) 0.01 M Eu extracted by 0.2 M HEH[EHP] in n-dodecane with variable quantities of C923 HNO3 added up to 0.4 M. B) 0.01 M Eu extracted by 0.2 C923 in n-dodecane with variable quantities of HEH[EHP] up to 0.4 M added..115 Figure 4.11 Molar absorbance spectra resolved by the HypSpec program for organic phase A) Nd B) Eu and C) Ho complexes. The black line represents M(AHA)3, red M(AHA)2(C923)(NO3), blue M(AHA)(C923)2(NO3)2, and green M(C923)3(NO3) CHAPTER FIVE: THERMODYNAMIC MODELING OF THE EXTRACTION OF Nd, Eu, Am, AND Ho FROM NITRIC ACID BY COMBINED CYANEX-923 AND HEH[EHP] Figure 5.1. Example slope analysis results for the extraction of Eu(III) by solvents containing Cyanex-923 combined with HEH[EHP] in n-dodecane. A) Variable quantities of nitrate B) ph dependence C) Variable quantities of Cyanex-923 and D) Variable quantities of HEH[EHP]. All data points were collected in triplicate, the mean values are shown on the plot. Solid lines are the linear fit of the distribution ratios.140 Figure 5.2. Extraction of Eu(III) by 0.2 M Cyanex M HEH[EHP] in n- dodecane from variable initial nitric acid concentration compared to the distribution ratios for this solvent calculated by Equation Figure 5.3. Eu(III) extraction by , , and M Cyanex HEH[EHP] from variable nitric acid in n-dodecane. Symbols represent experimentally determined distribution coefficients, and the dashed line represents distribution ratios calculated by Equation (10) Figure 5.4. Am(III) extraction by , , and M Cyanex HEH[EHP] from variable nitric acid in n-dodecane. Symbols represent experimentally determined distribution coefficients, and the dashed line represents distribution ratios calculated by Equation (10) Figure 5.5. The extraction of Nd(III) and Ho(III) by 0.2 M Cyanex M HEH[EHP] from variable nitric acid concentrations in n-dodecane. The symbols represent experimentally determined distribution ratios, and the dashed line represents the calculated best fit of the experimental values by using Equation xviii

19 Dedication To my parents Edward and Pamela Johnson xix

20 Chapter 1 INTRODUCTION Global Energy Outlook From 1990 to 2011, global electricity demand nearly doubled and future demand for electricity has been projected to grow by approximately 81% from 2011 to This equates to an increase from approximately 19,000 TWh, to 35,000 TWh of electricity consumed. 1 This growing demand for electricity is fueled by unprecedented growth in the global human population. The United Nations predicts that total world population will be equal to 8.7 billion (up from 6.7 billion in 2011) by This population growth coupled with an emerging higher standard of living in developing countries will drive the demand for electricity on a global scale. The United Nations projects that over 70% of increased energy demand will be in India and China. 2 With this looming large demand for electricity, a key question to ask is how this electricity will be generated? As of today, 68% of all electricity produced is based on the combustion of fossil fuels, 13% from nuclear fission, and 19% from hydro and other renewable sources. 3 Such an overwhelming dependence on fossil fuels raises several concerns about future energy policies and supplies. Specifically, a continued reliance on fossil fuels will result in harmful increases of the greenhouse gas CO2 to the atmosphere, which has been predicted to lead to significant changes in global climate. 4 Furthermore, fossil fuels are nonrenewable with a finite supply. Given the concerns arising from the use of fossil fuel, attention has turned to utilizing alternative and sustainable sources of energy. Careful consideration of both the potential long term energy supplies and environmental impacts have resulted in identification of wind and solar power as technologies that have prospective to supply clean, sustainable energy in the future. 5 The main 1

21 advantages to these energy sources are their abundance and also their ability to provide electricity without significant carbon dioxide emission. However, the fundamental challenge in their application for supplying electricity is efficiently harnessing them. Most electricity demand calls for a continuous and reliable supply which has traditionally been provided by base-load generation. Windmills generate electricity only when the wind is blowing; typical wind turbines operate at only about 20-30% utilization and thus supply electricity intermittently. 5 Solar electricity generation utilizes sunlight to act on photovoltaic cells to produce electricity directly or solar thermal installations that use solar heat to produce steam to drive conventional generators 6. Understandably, production of electricity through this method is only possible when the sun is shining, and there are only limited areas worldwide that offer enough sunlight to offset low efficiency of the solar cells. While it is clear that wind and solar energy sources offer considerable potential to meet mainstream electrical needs, integrating them into the supply system is challenging. The wind and sun cannot be controlled in a way to directly provide continuous baseload power. If there were an efficient way to store large amounts of energy from these intermittent producers (e.g., pumping water uphill to help drive a hydroelectric generator), their contributions to supplying global electricity would be greater. However, until a radical improvement in energy storage is developed, an alternative source of sustainable base-load electricity is required. The Case for Nuclear One alternative source of base-load electricity production is through nuclear fission. Nuclear electricity generation is already an established part of global electrical supplies, providing approximately 13% of the world s power. 3 Nuclear power is known to be especially suitable for large-scale, continuous (base-load) power generation. It is therefore ideally matched to the 2

22 increasing global population demands for electricity. The emissions of greenhouse gasses associated with nuclear power generation is in effect zero (see Figure 1.1). The only greenhouse gas emission from nuclear power production is steam. 7 Without nuclear energy, the world would almost certainly have to lean more heavily on fossil fuels to meet demands for base-load energy production. To put this issue in perspective, for every twenty-two tons of uranium used in nuclear power generation, approximately one million tons of CO2 are saved relative to electricity production using coal. In fact, when comparing the electricity that can be harnessed by this process to that of fossil fuels, on a per kilogram basis 235 U contains two to three million times the energy equivalence of coal or oil. 8 Assuming complete combustion or fission, approximately 8 kwh of heat are generated from 1 kg of coal, 12 kwh from 1 kg of oil, and about 24,000,000 kwh from 1 kg of 235 U. 8 As such, a further reliance on fossil fuels for energy production has significant future environmental implications. It is clear that a global expansion of nuclear power is the best option for supplying energy to satisfy the growing global demand. Figure 1.1. The relative levels of CO2 emission from the generation of 1 kwh of electricity from various sources of electricity. Reprinted from (7). 3

23 Nuclear Power Overview Common uranium based nuclear fuels are composed primarily of 235 U and 238 U. When the uranium is irradiated in a reactor with neutrons, two primary processes occur; fissile 235 U preferentially undergoes fission while fertile 238 U captures a neutron. 9 These processes can be summarized by Equations 1 and U + nthermal Fission Products + 2.5n (1) 238 U + nthermal 239 U 239 Np + β Pu + β - (2) For each neutron consumed by 235 U, on average 2.5 new neutrons are released. These new neutrons can be used to fission adjacent 235 U nuclei, leading to the release of even more neutrons which results in formation of a nuclear chain reaction. 9 Equations 1 and 2 are the principle reactions that maintain the nuclear chain reaction inside a reactor core. An important feature of Equation 1 is also the creation of fission product fragments. The kinetic energy of the fission fragments is converted to heat through collisions with other atoms. It is primarily this heat that is ultimately harnessed to turn water into steam to turn a turbine and generate electricity. Unfortunately, the fate of the fuel cannot be simply expressed by equations 1 and 2. Through further neutron capture by the 239 Pu nucleus, many heavier actinide isotopes (often referred to as transuranic elements) are created. 10 An example of this nuclear reaction is described below in Equation Pu + nthermal 240 Pu + nthermal 241 Pu 241 Am + β - (3) Additional notable transuranic elements created in the reactor this way are 237 Np, 241 Am, and 244 Cm. 10 The production of these transuranic elements in the reactor is considered problematic due 4

24 to their long-lived radioactivity (half-lives >7000 y) or high specific radioactivity. Furthermore, through subsequent fission and decay of the many heavy isotopes produced inside the reactor roughly 1/3 of the periodic table is produced as fission products. A major component of the fission products are the lanthanide elements (La-Ho). Several of the lanthanide elements have high neutron capture cross-sections and thus compete with fissile materials for neutrons. As the fission product lanthanides build up, fuel efficiency decreases to the point where the fuel must be replaced in the reactor. 10 The majority of fission products in spent fuel elements are radioactive, however most of them have short half-lives and decay away in a relatively short amount of time. The radioactivity of used fuel, in terms of cooling time, is dominated from years by fission and activation products (of which 90 Sr and 137 Cs are most important). Between million years, the longer-lived transuranic isotopes and their daughter products govern the radioactivity profile of the used fuel. The longest lived isotopes are 243 Am (t1/2 = 7400 years), 239 Pu (t1/2 = 2.4x10 4 years), and 237 Np (t1/2 = 2.14x10 6 years). Figure 1.2 shows the radiotoxicity of all the fission products compared to the original activity found in the uranium ore used to make the fuel. 11 5

25 Figure 1.2. Relative radiotoxicity on inhalation as a function of time for the fission products with a burn up of 38 megawatt days/kg. Reprinted from (11) Properly managing this used fuel is a difficult task and many problems arise concerning management of the used fuel. A key difference between nuclear and fossil fuels is that instead of being burned to produce CO2, nuclear reactor fuel must be isolated from the environment due to its high radiotoxicity. Complicating this task is the composition of the uranium, fission products, and transuranic elements present in the used fuel. They constitute a complex matrix which makes processing of the material both difficult and expensive. The long lifetime of highly radioactive isotopes present in the fuel also adds additional challenges to managing the used fuel. Moreover, without treatment, it is necessary to isolate the fuel from the environment for up to one million years. An ideal waste isolation repository must be in a geologically inert environment and also designed to minimize the potential for human intrusion. Rational prediction of geological conditions that far into the future with today s technology remains a task with great uncertainty. 6

26 Additionally, and perhaps most importantly in the U.S., are the political difficulties. In 1977, President Jimmy Carter signed an executive order that banned nuclear fuel reprocessing due to fears of nuclear weapon proliferation. 12 In 1982, the U.S. passed the Nuclear Waste Policy Act which established a national policy of utilizing an open (single pass without recycle) fuel management philosophy. 13 These decisions resulted in a national policy based on long-term storage of used nuclear fuel in some sort of geologically-stable underground formation. The act (without a reprocessing plan), sought to research and establish suitable geological storage sites. Yucca Mountain, Nevada, (after an amendment of nuclear waste policy act) was targeted in 1987 to be the site of the only national nuclear fuel storage repository. 14 Nearly 30 years and $9 billion were invested into the evaluation and eventual construction of the Yucca Mountain site. However, in 2010 the Obama administration cut its funding and no longer views the site as a viable option for fuel disposal. Perhaps one positive that came from this policy change was the creation of a Blue Ribbon Commission to evaluate the feasibility implementing a full reprocessing scheme in the U.S. with the ultimate goal of closing the nuclear fuel cycle. 15 Today however, there still remains no clear plan defining what to do with used nuclear fuel. Since the first U.S. nuclear power plant came on line in 1958, used fuel has been accumulating in storage pools at 121 different sites domestically (62,000 metric tons as of 2009). 16 Additional nuclear waste streams resulting from weapons production is another significant source of more radiotoxic waste destined for a geological repository. If both waste streams were combined, their total volume would have filled the proposed Yucca Mountain facility to its heat load limit immediately upon its projected opening date, further illustrating the need for a decision on our nuclear waste policies. 14 7

27 Fuel Treatment Options It is clear that some sort of reprocessing scheme should be implemented to manage the used nuclear fuel in the U.S. Currently, there are three major fuel management options which are being considered: 1- direct disposal of the fuel into a geological repository with no treatment; 2- partial recovery of uranium and plutonium for recycle; and 3- a completely closed cycle in which all components of the fuel are recycled or incinerated in a fast breeder reactor. 17 The fundamental problems related to the operation of an open fuel cycle are well-defined. It is plagued by the large volume of high level waste created the combined technical and political issues facing use of multiple geological repositories. Indeed, partial recovery of uranium and plutonium increases efficiency of the fuel by at least 30 % and also reduces the amount of uranium ore that must be mined and processed to support fuel fabrication. Partial recovery of only uranium and plutonium could reduce waste volume by nearly 90%, however the long-lived radiotoxic minor actinides remain present and must be disposed to a final geological repository. Finally, the goal of a completely closed nuclear fuel cycle is to separate the fuel into its major components, notably: bulk uranium and plutonium, minor actinides (Am, Cm, Np), and the lanthanides/fission products. The bulk uranium and plutonium can be recycled as mixed-oxide (MOX) fuels for reuse in power production. The long-lived minor actinides can be further fissioned in a fast breeder reactor, transmuting them into shorter lived radionuclides that are either inert or require much shorter environmental seclusion. 10 Some valuable fission products (such as Pd and Rh) can be recovered from the fuel and recycled, however the lanthanides must be separated from the minor actinides primarily because the lanthanides (as a result of their high neutron capture cross section) act as 8

28 neutron poisons to the transmutation process simultaneously lowering the efficiency of actinide transmutation and complicating reactor control. Current Reprocessing Schemes The process of actually partitioning the fuels is inherently a complex chemical operation due to the wide variety of nuclides present in various oxidation states. Despite this complexity, a variety of solvent extraction processes have been developed which can achieve separation of the fuel components. The method of solvent extraction has a rich 60 year history in research and applications that encompasses many of the main methods for the partitioning of spent fuels. Part of this rich history has been the development of numerous primary examples of hydrometallurgical processes capable of partitioning used fuel into its various components. The Plutonium Uranium REDOX Extraction Process (PUREX), Uranium Extraction Process (UREX), Transuranic Extraction Process (TRUEX), Trivalent Actinide Lanthanide Separations by Phosphorus Extractants and Aqueous Komplexants (TALSPEAK) process, and Diamide Extraction (DIAMEX)/Selective-Actinide Extraction (SANEX) processes are all examples of established processes. The PUREX process was developed in the U.S. (as part of the Manhattan Project) as a means to recover uranium and plutonium from used nuclear fuel for weapons production. 18 Currently, PUREX a commercial reprocessing method used in France at the AREVA-La Hague site as part of the French national reprocessing effort. The PUREX process is used to co-extract U(VI) and Pu(IV) from a feedstock made by dissolving the fuel matrix in concentrated nitric acid. The process works by exploiting the differences in oxidation states of uranium, plutonium, and 9

29 neptunium compared to that of the lanthanides and other fission products. Since the relative size to charge ratio changes as a function of the oxidation state of the metal, one can expect the extractability of a metal according to its oxidation state to follow M(IV) > MO2(VI) > M(III) > MO2(V). 19 Which suggests the (IV) and (VI) are the most extractable oxidation states. Under PUREX aqueous conditions, uranium and plutonium exist primarily in the +VI and +IV oxidation states respectively, while the remaining minor actinides and fission products exist predominately in the +III oxidation state, meaning that uranium and plutonium will be preferentially extracted. The process utilizes tri-n-butyl phosphate (TBP), a neutral monofunctional solvating extractant (Figure 1.3) dissolved in kerosene to accomplish this separation. Figure 1.3. The structure of tri-n-butyl phosphate (TBP) The extraction reactions for uranium and plutonium by TBP are described by Equations 4 and 5. 18,20 UO NO TBP(org) UO2(NO3)2(TBP)2(org) (4) Pu NO TBP(org) Pu(NO3)4(TBP)2(org) (5) After the co-extraction stage of the separation process, plutonium is separated from uranium by exploiting its redox chemistry. The Pu(IV) (still complexed to TBP in the organic phase) is reduced to Pu(III) by using a combination of hydroxylamine and hydrazine or U(IV), whereby it is stripped to a fresh aqueous phase. At this point, the plutonium can be recycled into mixed oxide (MOX) fuel and used again in a reactor for fuel. The remaining uranium in the organic phase is stripped 10

30 by contacting with a solution of dilute nitric acid, after which it can be reused once again. If no further reprocessing is done after PUREX, the remaining minor actinides and fission products must be vitrified and stored as high level waste in a geological repository. As mentioned previously, France has become the global leader for application and use of this reprocessing scheme on power reactor fuels. One can argue that the biggest factor operating against the industrial scale implementation of the PUREX process domestically is lingering mistrust of its origin. PUREX was designed and extensively used to purify plutonium for weapons production. As outlined previously, during the PUREX process a stream of pure plutonium is created which creates the risk of nuclear weapons proliferation. To limit this risk, the U.S. has spent a significant amount of research time and money on investigating reprocessing schemes that either exclude the PUREX process, or alter it so that no pure plutonium stream is made. This led to the development of the UREX process which was first demonstrated at Argonne National Laboratory. The UREX process utilizes acetohydroxamic acid to alter the redox chemistry of plutonium so its primary oxidation state is III which makes it not extractable by TBP. By preventing the extraction of Pu with the U, the creation of a pure plutonium stream in a PUREX style separation is eliminated. 21,22 While the UREX process has been shown to eliminate the one major objection to the PUREX process, it only recycles the reusable uranium from the fuel. If UREX was the only process utilized in the U.S., long term geological repository would still be needed for the remaining components of the fuel. To more fully partition the fuel into its respective components, the U.S. spent substantial amounts of research into developing the UREX+ suite of extraction processes. The UREX+ process(es) integrate up to 5 discrete solvent extraction processes to fully partition 11

31 the fuel. 23,21,24,25 The most relevant of those processes are the TRUEX and TALSPEAK processes for management of trivalent actinides and fission product lanthanides. The TRUEX process in UREX+ was developed at Argonne National Laboratory by Horwitz and coworkers to be a minor actinide extraction process for management of PUREX wastes. 26,27,28, 29 It was adapted to be the main solvent extraction process after the UREX process has removed the reusable uranium (CCD/PEG and NPEX processes prior to TRUEX remove Cs, Sr, Pu, and Np but are outside the scope of this document). The TRUEX process was also developed to be compatible with the UREX raffinate, in that it works efficiently in concentrated (~6 M) nitric acid. It functions by co-extracting the minor actinides (Am and Cm) along with the fission product lanthanides away from all other remaining fission product metals. This extraction is done by the use of octyl(phenyl)-n,n-diisobutylcarbamoylmethylphosphine oxide (CMPO, Figure 1.4) in combination with TBP (acts as a phase modifier to prevent third phase formation) in kerosene. 26 Figure 1.4. The structure of the TRUEX extractant, CMPO. CMPO is a bifunctional solvating extractant that extracts trivalent lanthanides and actinides by the extraction reaction described in equation M NO CMPO(org) M(NO3)3(CMPO)2(org) (6) 12

32 This process, which is fundamentally only the addition of CMPO to the UREX process solvent, was found to extract the trivalent metals efficiently at high nitric acid concentrations. Once the minor actinide and lanthanide metals are extracted away from the remaining fission products, they can be stripped from the organic phase by applying a fresh aqueous phase maintained at a ph of about 3 to The remaining fission products are then sent to interim storage or processed as High Level Waste (HLW). This leaves the only major (and most difficult) separation to accomplish, which is the separation of the minor actinides from the lanthanides. In UREX+, this is accomplished with the TALSPEAK process. TALSPEAK Chemistry The separation of the minor actinides from the lanthanides still remains one of the greatest challenges in f-element separation science. The reason this separation is so challenging lies in the chemical similarity of the minor actinides and lanthanides. The 4f rare earth elements and 5f actinides both act as classic hard acid cations (according to Pearson s Hard-Soft Acid Base Theory) which means that they exhibit stronger bonding interactions with hard base donor atoms. 30 Ionic radii of both the lanthanides and actinides decrease as the atomic number increases and are present in solution primarily in the trivalent oxidation state. Sixty years of research has yielded limited success into developing solutions for this challenging separation. However, some success has been found in exploiting the ability of actinides to participate in covalent bonding, while the lanthanides do not appear to do so in conventional coordination complexes. 31,32,33,34 13

33 Figure 1.5. The radial probability of finding an electron at a distance r from the nucleus for the 4f, 5d, 6s, and 6p valence orbitals of Sm 3+ compared to the analogous probabilities of finding an electron in the 5f, 6d, 7s, and 7p orbitals of Pu 3+. Reprinted from (35). The spatial extension of actinides 5f, 6d, 7s, and 7p orbitals relative to those of the lanthanides 4f, 5d, 6s, and 6p orbitals makes covalent bonding of the actinides possible. 35 Referring to Figure 1.5, bonding of a ligand to the Sm(III) metal center occurs when electrons occupy its 5d, 6s, or 6p orbitals, meaning that the 4f electrons only slightly influence its chemistry. The analogous figure for Pu(III) shows the greater extension of the 5f orbital compared to the 4f of 14

34 Sm(III). The valence 7s and 7p electrons contract closer to the nucleus relative to the lanthanide 6s and 6p, the net result of which is that the 5f electrons of the actinides more readily participate in covalent bonding than the 4f electrons of the lanthanides. This covalent nature of bonding for actinide elements results in them being slightly softer acids than the lanthanide elements, thus they have a higher affinity for ligands containing soft donor atoms compared to the lanthanides. The TALSPEAK process takes advantage of this principle to accomplish the separation of americium from the lanthanides. TALSPEAK was originally developed in the 1960s at Oak Ridge National Laboratory by Weaver and Kappelmann. 36 The process very cleverly takes advantage of the soft donor principle by using a hard donor extractant (di-(2-ethylhexyl) phosphoric acid (HDEHP)) that has very little preference for lanthanides or actinides in combination with an aqueous media that features a soft donor complexing agent (diethylenetriamine-n,n,n,n,n -pentaacetic acid (DTPA)) buffered by lactic acid. 37,38 These reagents are shown in figure 1.6. Figure 1.6. Structures of the main components of the TALSPEAK process. (A) di-(2- ethylhexyl) phosphoric acid (HDEHP), (B) diethylenetriamine-n,n,n,n,n -pentaacetic acid (DTPA), (C) lactic acid. In this system, the extractability of the minor actinides is suppressed by the presence of DTPA. The soft donor nitrogen groups of DTPA appear to bind more strongly to the minor actinides than 15

35 to similar sized lanthanides, as evidenced by the relative values of complex stability constants. 39 DTPA then preferentially binds or holds-back the minor actinides by creating a charged complex in the aqueous phase which is not extractable by HDEHP. The equilibrium expression for DTPA binding to a trivalent metal is represented by the following expression; M 3+ + H3DTPA 2- M(DTPA) H + (7) The formation of this complex results in the separation of minor actinide/lanthanide species with very similar chemistries. TALSPEAK was found to be able to separate the Ln(III) from the An(III), however it has never been implemented at an industrial scale due to complications related to the necessity for rigorous ph control and slow kinetics. Additionally, the role of lactate in the extraction mechanism is still unclear. 37,38 Global Reprocessing Schemes All of the processes discussed above have been developed for implementation in the United States. However, for a global perspective on used fuel recycling strategies other reprocessing schemes should be discussed. Many of the processes which have been developed globally show great promise in their ability to partition used nuclear fuel. European strategies for reprocessing are quite similar to those of the U.S. A key difference is the choice of ligand design. European researchers have chosen to follow the CHON principle, in which all ligands are made only by carbon, hydrogen, oxygen, and nitrogen. This policy ensures that the components of the process are completely incinerable, meaning that at the end of their use within a partitioning process they can be destroyed into gases (predominately CO2, N2, and H2O). Of the proposed European 16

36 processes, the most notable are the (primarily French) Diamide Extraction (DIAMEX) and Selective Actinide Extraction (SANEX) processes. The DIAMEX process, like TRUEX, is designed to be used after the PUREX process. The DIAMEX process uses N,N -dimethyl-n,n -dioctylhexylethoxymalonamide (DMDOHEMA (Figure 1.7)), a diamide based extractant, to co-extract both the lanthanides and minor actinides from the highly acidic PUREX raffinate. 40,41,42 Figure 1.7. Structure of N,N -dimethyl-n,n -dioctylhexylethoxymalonamide (DMDOHEMA) DMDOHEMA, a neutral solvating extractant, extracts metals via the following mechanism. 40 M NO3 - + xl(org) M(NO3)3(L)x(org) (8) After the DIAMEX process, the European partitioning method for the separation of the minor actinides from the lanthanides has been labeled SANEX. 43,44 The SANEX process can be directly compared to the U.S. TALSPEAK process. The main difference in the two processes is that SANEX aims to selectively extract the minor actinides from the DIAMEX product solution, while TALSPEAK aims to selectively retain the minor actinides in the aqueous phase from the TRUEX product solution. To accomplish this selective separation, ligands with several N donor atoms are required. To this end, several bis-1,2,4-triazinyl-bipyridines (BTBPs) have been 17