Evaluation of Low-Temperature Fluoride Routes to Synthesize Actinide Nitrides and Oxide Solid Solutions

UNLV Theses, Dissertations, Professional Papers, and Capstones 5-2009 Evaluation of low-temperature fluoride outesr to synthesize actinide nitrides and oxide solid solutions Gunanda Waduge Chinthaka Silva University of Nevada, Las Vegas Follow this and additional works at: https://digitalscholarship.unlv.edu/thesesdissertations Part of the Materials Chemistry Commons, Nuclear Commons, Nuclear Engineering Commons, and the Radiochemistry Commons Repository Citation Silva, Gunanda Waduge Chinthaka, "Evaluation of low-temperature fluoride outesr to synthesize actinide nitrides and oxide solid solutions" (2009). UNLV Theses, Dissertations, Professional Papers, and Capstones. 1127. http://dx.doi.org/10.34917/2502495 This Dissertation is protected by copyright and/or related rights. It has been brought to you by Digital Scholarship@UNLV with permission from the rights-holder(s). 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EVALUATION OF LOW-TEMPERATURE FLUORIDE ROUTES TO SYNTHESIZE ACTINIDE NITRIDES AND OXIDE SOLID SOLUTIONS by Gunanda Waduge Chinthaka Silva Bachelor of Science University of Colombo, Sri Lanka 2000 Master of Science in Chemistry University of Nevada, Las Vegas 2005 A dissertation submitted in partial fulfillment of the requirements for the Doctor of Philosophy Degree in Radiochemistry Department of Chemistry College of Sciences Graduate College University of Nevada, Las Vegas May 2009 UMI Number: 3383994 Copyright 2009 by Silva, Gunanda Waduge Chinthaka INFORMATION TO USERS The quality of this reproduction is dependent upon the quality of the copy submitted. Broken or indistinct print, colored or poor quality illustrations and photographs, print bleed-through, substandard margins, and improper alignment can adversely affect reproduction. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. 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Box 1346 Ann Arbor, Ml 48106-1346 Copyright by Gunanda Waduge Chinthaka Silva 2009 All Rights Reserved Dissertation Approval UNW The Graduate College University of Nevada, Las Vegas st April 1 _/20. 09 The Dissertation prepared by Gunanda Waduge Chinthaka Silva Entitled Evaluation of Low-Temperature Fluoride Routes to Synthesize Actinide Nitrides and Oxide Solid Solutions is approved in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Radiochemistry Examination Committee Chair Dean of the Graduate College ExaminatiorrQjmmittee Member Examination Committee Member Graduate College faculty Representative 1017-52 11 ABSTRACT Evaluation of Low-Temperature Fluoride Routes to Synthesize Actinide Nitrides and Oxide Solid Solutions by Gunanda Waduge Chinthaka Silva Dr. Kenneth R. Czerwinski, Examination Committee Chair Professor of Chemistry Head of the Department of Radiochemistry University of Nevada, Las Vegas Actinide mononitrides have been considered as a possible nuclear fuel for the Generation-IV nuclear reactor systems. In the process of evaluating these actinide mononitrides as nuclear fuel, it is important to study different chemical and physical characteristics of these compounds. Synthesis of the materials is thus important. Carbothermic reduction is one of the techniques that have been used to synthesize actinide mononitrides. In this method, a mixture of actinide oxide such as UO2 and excess carbon is heat treated at temperatures greater than 1700 °C under a nitrogen atmosphere. The technique is however not promising in synthesizing the actinide mononitrides due to a number of disadvantages the technique presents. Lack of phase purity due to secondary chemical phases with carbon and oxygen, need of high temperatures such as 2200 °C, and the low density of the final product compared to theoretical density are some of the drawbacks that the researches have been encountered. Most of all this method is tiresome and difficult to handle in ordinary laboratories where the experimental setups and iii conditions are inadequate to synthesize actinide mononitrides up to an acceptable quality for chemical characterizations. Therefore, it is important to explore different routes that can be used to synthesize such actinide nitrides and characterize them properly. A recent development of a low-temperature fluoride route in synthesizing UN2 at 800 °C and UN at 1100 °C, proposed a further investigation of this particular chemical route in synthesizing other nitrides of the actinide series. Thus, the possibility of making nitrides of thorium, neptunium, and mixed uranium-thorium by the above mentioned method was suggested. Mechanisms and kinetics involved in separate reactions were studied. Chemical characterizations of the as-synthesized materials were also completed using different techniques reported in Chapter 2. Optimization of this low-temperature process to minimize the formation of secondary phases such as UO2 was also examined in a typical experimental setup by exploring the uranium system. With the thorium, however, only ThNF could be synthesized up to a temperature of 1100 °C. Addition of lithium amide (LiNlrk) into the reactants in synthesizing ThNx produced TI12N3 with some Th02 impurities. This finding is controversial and will discuss the relevant issues in the corresponding chapter. Characterization of Th2N3/Th02 samples revealed an interchangeable formation of TI1O2 in TI12N3 and vise versa suggesting a possible reason for the high susceptibility of ThNx toward oxygen as general. Evaluation of the neptunium system revealed 6 new compounds with isomorphous crystal structures to that of uranium with similar chemical compositions. XRD powder refinements could be used in solving these crystal structures. NpN was synthesized at 900 °C and further experiments are required to check the lower temperatures for making iv the mononitride. However, further experiments will be necessary to optimize the heating time. Microscopic characterization of NpNx compounds was also conducted with SEM and TEM. Nanostructural studies conducted on these samples displayed high crystallographic order in their structures. Uranium and thorium mixed system was also examined with an eye towards synthesizing uranium-thorium mixed nitrides. Less than 1 wt% thorium solubility was identified in the UN2 with XRD and microscopic studies. Further application of the technique on oxides was explored and a novel route in synthesizing (U, Th)02 solid solutions at temperatures of 1100 °C or less, depending on the chemical composition of the oxide solid solution, was established. Moreover, this novel route itself proposes a new and easy to use low-temperature path to fabricate actinide oxide solid solutions from initial, separate oxide starting phases. v TABLE OF CONTENTS ABSTRACT iii LIST OF TABLES vii LIST OF FIGURES viii ACKNOWLEDGEMENTS x CHAPTER 1 INTRODUCTION 1 1.1 Actinide Nitrides as a Nuclear Fuel 1 1.2 Crystallography of Actinide Nitrides 4 1.3 Carbothermic Reduction 8 1.4 Low-Temperature Fluoride Route 10 CHAPTER 2 METHODOLOGY AND INSTRUMENTATION 18 2.1 Introduction 18 2.2 Reagents 18 2.3 Experimental Procedures 19 2.4 X-ray Powder Diffraction (XRD) 20 2.5 Transmission Electron Microscopy (TEM) 24 2.6 Scanning Electron Microscopy (SEM) 32 2.7 Electron Microprobe Analysis 34 CHAPTER 3 SYNTHESIS AND CHARACTERIZATION OF URANIUM NITRIDES 38 3.1 Introduction 38 3.2 Experimental details 40 3.3 Mechanism and Kinetics of UN2 Decomposition 41 3.4 Microscopic Evaluation of UNX 54 3.5 Discussion 65 3.6 Conclusions 70 CHAPTER 4 EVALUATION OF THORIUM SYSTEM 73 4.1 Introduction 73 4.2 Experimental Details and Characterization Methods 76 4.3 Synthesis and Characterization of (NH^ThFg 78 4.4 Synthesis and Characterization of ThNF 84 4.5 Ammonolysis of ThF4 and the Thermal Behavior of ThNF 91 4.6 Synthesis and Characterization of TI12N3 95 VI 4.7 Discussion 105 4.8 Conclusions Ill CHAPTER 5 SYNTHESIS AND CHARACTERIZATION OF NEPTUNIUM NITRIDES 115 5.1 Introduction 115 5.2 Experimental Details and Characterization Methods 117 5.3 Results and Discussion 119 5.4 Conclusions 155 CHAPTER 6 EVALUATION OF URANIUM-THORIUM MIXED SYSTEM ... 160 6.1 Introduction 160 6.2 Experimental Methods 162 6.3 Characterization methods 164 6.4 Results and Discussion 165 6.5 Conclusions 192 CHAPTER 7 CONCLUSIONS AND RECOMMENDATIONS 195 7.1 Uranium System 195 7.2 Thorium System 196 7.3 Neptunium System 198 7.4 Uranium and Thorium Mixed System 200 VITA 203 vii LIST OF TABLES Table 1.1 Comparison on actinide oxide, carbide, and nitride properties 2 Table 1.2 Crystallography of uranium nitrides 4 Table 1.3 Crystallography of thorium nitrides 6 Table 2.1 Cover gases used for the experiments 18 Table 2.2 Chemicals used in the research work 19 Table 2.3 Chemical composition

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