OCTOBER 1981 THERMOCHEMICAL INVESTIGATIONS ON INTERMETALLIC UMe3 COMPOUNDS (ME = Ru, Rh, Pd) BY G. WUBENGA Thesis Amsterdam University, 9 December l°"l Netherlands Energy Research Foundation ECN does not assume any liability with respect to the use of, or for damages resulting from the use of any information, apparatus, method or process disclosed in this document. Netherlands Energy Research Foundation ECN P.O. Box 1 1755 ZG Petton INH) The Netherlands Telephone (0)2246 • 626? Telex 57211 ECN-102 OCTOBER 1981 THERMOCHEMICAL INVESTIGAl IONS ON INTERMETALLIC UMe3 COMPOUNDS (Me = Ru, Rh, Pd) BY G. WIJBENGA Thesis Amsterdam University, 9 December 1981 - 9 - CONTENTS Page CHAPTER I INTRODUCTION 15 1.1. Fission products in nuclear fuel 15 1.2. Phase diagrams of the U-Me systems 19 1.3. Plutonium enrichment in (U,Pu)Me3 inclusions 22 References 24 CHAPTER II THE CHEMISTRY OF THE UMe3 COMPOUNDS 26 11.1. Preparation of the compounds 26 1.1. Introduction 26 1.2. Preparation of starting materials 27 1.3. Preparation of the UMe compounds 29 1.3.1. UPd3 29 1.3.2. URh3 31 1.3.3. URu3 31 11.2. Chemical reactivity of the UMe. compounds 33 2.1. Introduction 33 2.2. Reactions of UMe3 with non-metallic elements 33 2.3. Reactions of UMe with aqueous acidic solutions 34 2.4. Miscellaneous reactions 34 11.3. The stoichiometry of UPd3 36 3.1. Introduction 36 3.2. Experimental 36 3.3. Results 37 3.4. Discussion 38 References 41 CHAPTER III FLUORINE BOMB CALORIMETRY 43 I7~.l. The calorimetric system 43 - 10 - Paga 1.1. Introduction 43 1.2. The calorimetric equipment 45- 1.3. Experimental procedures and calibration of the calorimeter 43 1.4. The determination of reaction mechanism and analysis of calorimetric samples *I 1.5. Impurity and stoichlometry corrections for UF,, and UF3 56 References 62 111.2. The enthalpies of formation of uranium tetra- fluoride and uranium trifluoride 64 2.1. Introduction 64 2.2. Experimental 64 2.2.1. Principle of the calorimetric reactions 64 2.2.2. Calorimetric system 64 2.2.3. Materials 65 2.2.4. Combustion technique and procedures 66 2.3. Results 67 2.4. Discussion 69 References 30 111.3. The enthalpy of formation of palladium (II) hexafluoropaliadate (IV), Pd(PdF6), by PF3 redu ction calorimetry 63 3.1. Introduction *>3 3.2. Experimental f3 3.2.1. Principle of the calorimetric reactions 33 3.2.2. Calorimetric system 84 3.2.3. Materials 84 3.2.4. Procedures 85 3.3. Results 85 References 88 - 11 - Page III.4. The enthalpy of formation of UPd , by fluorine boab caloriaetry 90 4.1. Introduction 90 4.2. Experimental 90 4.2.1. Principle of the calorimetric reactions 90 4.2.2. Calorimetric system 91 4.2.3. Materials 91 4.2.4. Preliminary experiments 92 4.2.5. Procedures 94 4.3. Results 95 4.4. Discussion 98 References 106 CHAPTER IV EMF MEASUREMENTS 109 IV.1. EMF techniques 109 1.1. Introduction 109 1.2. Description of EMF apparatus and method of measurement 109 1.2.1. Gas purification system 111 1.2.2. Furnace control and data processing 111 1.2.3. Fabrication of electrodes, electrolytes and contacts 112 1.2.4. Diffusion in calcium fluoride 112 1.2.5. Measurements of asymmetry potentials 115 1.2.6. Sources of error in EMF techniques 115 References 117 IV.2. Determination of the Gibbs energies of for&atlon of URh and URu by solid state EMF measurements 118 2.1. Introduction 118 2.2. Experimental 118 2.2.1. Materials 118 - 12 - Page 2-2.2- EHF apparatus 121 2.3. Results 123 2.4. Discussion 126 References 130 IV.3. Glbbs energy of formation of UF} 132 3.1. Introduction 132 3.2. Experimental 132 3.2.1. Materials 132 3.3. Results 133 3.4. Discussion 135 References 139 CHAPTER V ESTIMATION PROCEDURES OF THERMOCHEMICAL DATA OF ACTINIDE-NOBLE METAL COMPOUNDS 140 V.l. Introduction 140 V.2. Description of the predictive models 141 2.1. The Enge1-Brever theory 141 2.1.1. Application of the Engel-Brewer theory to noble metals and actinides 142 2.1.2. Estimates of enthalpies of forma­ tion of actinide-noble metal com­ pounds with the Engel-Brewer theory 148 2.2. The cellular model of Miedema et al. 152 2.3. An electron band theory model 156 V.3. Discussion 159 References 166 CHAPTER VI THE THERMOCHEMICAL PROPERTIES OF THE UMe3 COMPOUNDS (Me - Ru, Rh and Pd) 17" VI.1. Introduction 170 VI.2. High-temperature heat-capacities of UMe compounds 172 - 13 - Pace 2.1. Introduction 172 2.2. Experimental 172 2.2.1. Materials 172 2.2.2. Description of the apparatus 173 2.2.3. Procedure 174 2.3. Results 175 VI.3. The thermodynamic functions of UPd3, URhj and URu3 180 3.1. The tabulation of the thermochemical data 180 VI.4. Plutonium enrichment in (U,Pu)Me3 inclusions 186 References 193 SUMMARY 195 SAMENVATTING 197 LIST OF SYMBOLS 199 - 15 - CHAPTER I INTRODUCTION 1-1. FISSION PRODUCTS IN NUCLEAR FUEL This thesis describes an investigation into the high-teaperature ther­ modynamic properties of the intermetallfc compounds of uranium with the light platinum metals, ruthenium, rhodium and palladium. These inter- metallic compounds are of technological importance because they are formed during fission of nuclear fuel in a reactor. In mixed uranlum-plutonium oxide fuel, (U,Pu)C2, these compounds have been identified as face-centered cubic (U,Pu)Me3 phases (Me = Ru, Rh and Pd) fl]. The importance of (U,Pu)Me, phases In reactor technology is that considerable plutonium enrichment d,5] has been found in these phases compared to the U/Pu ratio in mixed oxide fuels. In addition, the (U,Pu)Me3 compounds have a very high resistance to dissolution in acids, which will cause ur.desired plutonium losses during reprocessing. During neutron-induced fission of uranium and plutonium, according to U(Pu) + n •*• X. + X, + vn + energy the fission products X and X are formed in concentrations which de­ pend on: a. the concentrations of uranium and plutonium in the fuel, b. the neutron energy spectrum (thermal or fast). The symbol v stands for the number of neutrons, which is about 2.5, depending on the neutron energy spectrum. When the yield of the fission products is plotted against mass number a graph is obtained with two peaks (fig. I.I.). For plutonium-239 in a fast reactor the peak on the left is composed mainly of the elements strontium, yttrium, zirconium, molybdenum and technetium, having the mass numbers 88-100, and the light platinum metals (Ru, Rh and Pd) with mass numbers 101-108. The peak on the right consists of the elements xenon, cesium and barium (mass numbers 131-138), and the rare earth metals (La, Ce, Pr, Nd, Pm, Sm) with mass numbers 139-152 [2,3]. The yield of fission products has been given in fig. 1.1. for plutonium- - 16 - 239. For fission of uraniua-235 in a fast neutron flux, and tlso of Pu- 239 and U-235 in a therval neutron flux. the yield of light platinum aetals will be lower coapared with the fission of Pu-239 in fast neu­ trons (fig. 1.2. and fig. 1.3.) [*]. O 5 -I UJ > Z « O « (0 u- 3 2 'to so mono no no MO no w> MASS NUMBER Fig. 1.1. Yield-mass curve for fast-neutron fission of Pu23' f2]. The fission products described above aay react with each other and with the matrix and fora very stable compounds. Kleykaap [5], on the basis of post-irradiation studies, has classified the types of compounds formed into three groups: a. fission product - fuel component phases, b. fission product - fission product phases, and c. fission product - cladding material Inclusions. Among the fission products the light platinum metals form a dominant group. In hypostolchiometrlc mixed-oxide fuel f(U,Pu)02_x)] at high temperatures the noble metals Rh and Pd form stable Intermetalllc - 17 - Fi». !.2- Yield «its curve for fast aad siow-neutron fission of C2,s r*\ Fig. 1.3. Yield-aass curve for slow-neutron fission of Pu»9 UI. - 18 - compounds with the actinides of the type: (U,Pu)(Rh,Pd) [2,6]. This suggests a very negative Gibbs energy of formation of the binary com­ pounds. Palladium is a fission product which alloys In a variety of other fission products, fuel components and cladding materials. Besides the formation of (U,Pu)(Rh,Pd) , precipitation of (Mo,Tc,Ru,Rh,Pd) solid solutions occurs [5]. Actinide containing compounds (U,Pu) (Pd,In,Sn,Te),+x with high Pd contents and high Pu/U ratios have been observed also in the fuel-cladding interface [6]. The (U,Pu)Me phase (Me = Ru, Rh and Pd) has been detected in oxide as well as in carbide fuel [7]. In carbide fuels, the type of compound formed also depends on the stoichiometry. The reactions between hypostoichiometric (UC. ) and hyperstoichiometric (UC.+X) fuel and various fission products have been investigated in detail [8]. In UC, the following noble metal con­ taining phases were identified: U (Ru,Rh)C2, (U,R.E.)a(Ru,Rh)b in which R.E. are rare earth metals, and (U,R.E.)c(Ru,Rh,Pd)d [9]. In hyper­ stoichiometric fuel the phases identified were U (Ru,Rh)C?, and (U.Zr) ?d3_h [10]. - 19 - 1.2. PHASE DIAGRAMS OF THE U-Me SYSTEMS The extreme stability of the UMe, phases is illustrated by the phase diagrams (Me = Ru, Rh and Pd).