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The Spectroscopy and Photolytic Decomposition of Some Volatile Uranium Compounds

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The Spectroscopy and Photolytic Decomposition of Some Volatile Uranium Compounds THE SPECTROSCOPY AND PHOTOLYTIC DECOMPOSITION OF SOME VOLATILE URANIUM COMPOUNDS by N. Ghiassee, B.S. , M.Sc., D.I.C. A Thesis submitted for the degree of DOCTOR OF PHILOSOPHY of the University of London Nuclear Technology Laboratories Department of Chemical Engineering & Chemical Technology Imperial College of Science & Technology London, SW7. AUGUST 1979 2 ABSTRACT Methods for the separation of uranium isotopes in volatile compounds of uranium are reviewed, and derivatives of thenoyltrifluoroacetone and of the tetrahydroborates are chosen for further study. The (IV) and (VI) valent complexes of uranium thenoyltrifluoroacetone (TTA) were prepared and sublimation pressures measured. Spectrophotometric studies were also carried out, and are reported for the first time for the (IV) valent complex. Uranium tetrahydroborate, U(BH4)4, was prepared and large well-formed crystals were grown. Infrared and ultraviolet spectra of this compound were examined in the gas phase at room temperature and above ambient temperatures. Analysis of the kinetics of U(BH4)4 thermal decomposition, in the temperature range of 100 to 170°C, and the values of activation energies thus determined, were found and are also reported for the first time. The photolytic decomposition of U(BH4)4 vapour by the action of U.V. light was shown to occur. The quantum yield of one of the decomposition products, diborane gas, (B2H6), was determined. The possible mechanisms for this photochemical reaction are discussed and values of the quantum yield for U(BH4)4 derived. The photochemical stability was investigated employing continuous wave (C.W.) lasers operating in the visible, and infrared spectral regions. No decomposition was observed and the reasons are discussed. Finally, the suitability of these compounds as process material for the separation of uranium isotopes by the laser method is considered. 3 ACKNOWLEDGEMENTS 1 would tike to thank Pxolie44ox G.N. Watton bon ha 4upe,&vi ion, guidance and he2psul 4ugges Lon6 and .ideas dutrLng the coukse o5 this wonfz. My 4.incexest .thanfzd to Dn P.G. C.2ty bon hid genexou4 he2p and advice on ate aapec o o6 this wonfz, and bon second neadLng the th.eo-Lo. Thanfza axe cl4o due to Mn B.A. Bennet and R. Shaddocfz Nuc2ean Technology Labotuzto' Led, Mn 1. Dtuunmond oi Anat ytLca2 Se'w Lce s Labonatony, and Meb.stus. K. Gno44 and C. Smith oti the g.2a6.4 &towi.ng wonfz<ho p Son the co rt t'ucc tLo n o6 the. g&T 64 vacuum tine and othen g.2ao4wane. The Atomic Ene.i g y OnganL4a Lon o Itcan (AE01) pnov.ided 6-Lnanc La.e 4uppo' t. I would Like to ce.aae the oppoti_tuni ty to also thank my panen bon the t monat and S.inancLaL 4uppotr t. Fina tt y, I would titre to thank ! s4 T. Richandbon lion the pnomp.t and accwwte typing o6 the the a. 4 CONTENTS Page Title Page 1 Abstract 2 Acknowledgements 3 Contents 4 Illustrations and Tables 7 Glossary of Symbols and Abbreviations 10 CHAPTER I: INTRODUCTION 12 1.1 The Need for Enrichment 12 1.2 History and Development of 15 Enrichment Processes 1.2.1 Gaseous Diffusion 16 1.2.2 Gas Centrifuge 18 1.2.3 Nozzle Method 19 1.3 Other Separation (Enrichment) 22 Processes Previously Developed 1.3.1 Electromagnetic 22 1.3.2 Electrolysis 23 1.3.3 Chemical Exchange 23 1.3.4 Distillation 24 1.3.5 Thermal Diffusion 24 1.3.6 Ion Exchange 25 1.4 Laser Isotope Separation (LIS) 25 Technique 1.4.1 History of Photoseparation Method 26 1.4.2 Laser Radiation Properties 28 1.4.3 Basic Requirements for a General 29 Laser Isotope Separation Process 1.4.4 Various Methods of Laser Isotope 30 Separation A. Enhancement of Photochemical 31 Reactivity B. Photoionization 32 C. Photodissociation 32 1.4.5 Process Material for Laser Isotope 37 Separation 1.5 Comparison of the Economics of 38 Enrichment Processes. 1.6 The Scope of Present Work 41 CHAPTER II: PREPARATION, ABSORPTION SPECTROSCOPY AND 43 SUBLIMATION OF (IV) and (VI) VALENT URANIUM THENOYLTRIFLUOROACETONE COMPLEXES. 2.1 Introduction 43 2.2 Preparation, Spectrophotometric 46 Studies and Sublimation of Uranyl—TTA Complex (UO2T2,2H2O) 2.2.1 Preparation 46 2.2.2 Absorption Spectrum 47 2.2.3 Sublimation 49 5 Page 2.3 Preparation, Spectrophotometric 51 Studies and Sublimation of Uranium(IV)- TTA Complex (UT4) 2.3.1 Preparation 51 2.3.2 Absorption Spectrum 51 2.3.3 Sublimation 54 CHAPTER III: PREPARATION AND SPECTROSCOPIC STUDIES OF 57 URANIUM TETRAKISTETRAHYDROBORATE, U(BH4)4 3.1 Introduction 57 3.1.1 Development 57 3.1.2 General Properties 57 3.1.3 Structure 59 3.1.4 Review of Spectroscopic Properties 61 3.1.5 Applications 65 3.2 Preparation 65 3.2.1 Experimental and Procedure 65 3.2.2 Other Methods of Preparation 75 3.3 Spectroscopic Studies 76 3.3.1 Infrared Absorption Spectroscopy 76 A. Experimental & Results 76 B. Assignment of Spectrum 79 3.3.2 UV Absorption Spectroscopy 80 Experimental & Results CHAPTER IV: THERMAL DECOMPOSITION OF U(BH4)4 83 4.1 Introduction 83 4.2 Experimental & Procedure 84 4.3 Results 87 4.4 Discussion 93 CHAPTER V: PHOTOLYTIC DECOMPOSITION OF U(BH4)4 VAPOUR 98 5.1 U.V. Photolytic Decomposition 98 5.1.1 Experimental 98 5.1.2 Results 99 5.1.3 Determination of Quantum Yield, 99 5.2 Laser Photolytic Studies in the 113 Visible Region 5.2.1 Experimental 113 5.2.2 Results 113 5.3 I.R. CO2 Laser Photodissociation 114 5.3.1 Introduction 114 5.3.2 Experimental 123 5.3.3 Results & Discussion 124 6 Page CHAPTER VI: GENERAL CONCLUSIONS AND RECOMMENDATIONS FOR 129 FURTHER STUDIES References 131 Appendix A: An Account of the Development of U(BH4)4 137 Appendix B: Representation of Data for Thermal 140 Decomposition of U(BH4) 4. 7 ILLUSTRATIONS AND TABLES ILLUSTRATIONS Page Figure 1.1 Cross-section of the Nozzle System 20 Figure 1.2 The Electromagnetic Process 22 Figure 1.3 Diagram for General Laser Isotope 29 Separation Figure 1.4 Mechanism for Selective Photo- 33 predissociation of Molecules Figure 1.5 The Mechanism by which Selective 35 Two-Step Photodissociation might occur Figure 1.6 Some Volatile Compounds of Uranium 39 Figure 2.1 Absorption Spectrum of UO2.T2 Complex 48 Figure 2.2 Variation of Absorption with 50 Concentration at (I)334nm and (II)378nm for UO2-T2 Complex Figure 2.3 Absorption Spectrum of UT4 Complex 52 Figure 2.4 Variation of Absorption with Concen- 53 tration at 352nm for U-T4 Complex Figure 3.1 Molecular Structure of Zirconium 60 tetrahydroborate, Zr(BH4)4 Figure 3.2 Visible Gas Phase Absorption Spectrum 61 of U(BH4)4. Room Temperature, 72m Path, -0.1mmHg Figure 3.3 Grease-Free, Mercury-Free, GlassHigh- 66 Vacuum Line Figure 3.4 U(BH4)4 Crystals 73 Figure 3.5 U(BH4)4 Crystals Fully Grown 74 Figure 3.6 Nitrogen Glove Box 76 Figure 3.7 Infrared Spectrum of (a) Zr(BH4)4, 78 (b) U(BH 4)4 Figure 3.8 U.V. Absorption Spectrum of U(BH4)4 81 Vapour at ^'35 C Figure 4.1 A Simplified Schematic Diagram of the 86 Heated Demountable Gas Cell Figure 4.2 Infrared Absorption Spectrum of the 88 Solid U(BH4 )3 Figure 4.3 Arrhenius Plot of the Rate Constants 91 for the Thermal Decomposition of U(BH 4)4 in the 100-120°C Temperature Range 8 Page Figure 4.4 Arrhenius Plot of the Rate Constants 92 for the Thermal Decomposition of U(BH4)4 in the 130-170°C Temperature Range Figure 5.1 Appearance of One of B,2H Infrared 100 Absorption Bands at - 16b3cm 1 due to UV Photolytic Decomposition of U(BH4) Vapour 4 Figure 5.2 UV Photodecomposition of H202 103 Figure 5.3 Variation of Concentration of H202 104 at 353 nm, with Time Figure 5.4 Superposition of Infrared Absorption 109 Band of B2H6 at 1600 cm-1 from (I) Photochemical Process, and (II) Thermochemical Process. Figures B-la to B-11a: Patterns of Variation of Absorbance See Appendix of U(BH4)4 at -477 cm_ 1 with Time B at Different Operating Temperatures Figures B-lb to B-11b: Thermal Decompostion of U(BH 4)4 at Different Operating Temperatures: li Tests for the First-Order Kinetics Figures B-lc to B-11c: Thermal Decomposition of U(BH4)4 at Different Operating Temperatures: Tests for the Second-Order Kinetics TABLES Table 1.1 Per Cent Nuclear Electricity Generation in 13 the EEC Countries 1.2 Gaseous Diffusion Plants in the Western World 18 1.3 Announced Enrichment Facilities 21 1.4 Isotope Separation by Laser Induced Chemistry 31 1.5 Isotope Separation by Two-Step Photoionization 32 in Atomic Beams 1.6 Laser Isotope Separation by Selective Photo- 34 predissociation 9 Page 1.7 Isotope Separation by Selective Two-Step 36 Dissociation 1.8 Isotope Separation by Multiple CO2 Photon 37 Absorption 1.9 Comparative Summary of (Economics of) 40 Enrichment Processes 2.1 Absorption Spectrum of Uranyl-TTA Complex 49 2.2 Absorption of Uranyl-TTA and U(IV)-TTA 54 Complexes 2.3 Measurement of Activity in U(IV)-TTA 56 Complex Sublimate 3.1 Major Infrared Active Fundamental Vibrations 64 Commonly Observed for Metal Tetrahydroborates, M(BH ) 4 4 4.1 Second-Order Rate Constants in the 100-120°C 90 Range 4.2 First-Order Rate Constants in the 130-170°C 90 Range 4.3 Data for the Arrhenius Plot in the 100-120°C 90 Range 4.4 Data for the Arrhenius Plot in the 130-170°C 90 Range 4.5 Activation Energies and Frequency Factors 93 for the Thermal Decomposition of U(BH4)4 5.1 Values of Absorbance and Concentration 106 Before and After Irradiation at 353nm (H202.
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