Comprehensive nuclear materials 2 06 the u–f system

Comprehensive nuclear materials 2 06 the u–f system Comprehensive nuclear materials 2 06 the u–f system Comprehensive nuclear materials 2 06 the u–f system Comprehensive nuclear materials 2 06 the u–f system Comprehensive nuclear materials 2 06 the u–f system Comprehensive nuclear materials 2 06 the u–f system Comprehensive nuclear materials 2 06 the u–f system

2.06 The U–F System B Morel AREVA Comurhex, Pierrelatte, France S Chatain Commissariat a` l’E´nergie Atomique et aux E´nergies Alternatives, Gif-sur-Yvette, France ß 2012 Elsevier Ltd All rights reserved 2.06.1 2.06.2 2.06.3 2.06.3.1 2.06.3.1.1 2.06.3.1.2 2.06.3.1.3 2.06.3.1.4 2.06.3.2 2.06.3.2.1 2.06.3.2.2 2.06.3.2.3 2.06.3.2.4 2.06.3.3 2.06.3.3.1 2.06.3.3.2 2.06.3.4 2.06.3.4.1 2.06.3.4.2 2.06.4 2.06.4.1 2.06.4.1.1 2.06.4.1.2 2.06.4.2 2.06.4.2.1 2.06.4.2.2 2.06.4.3 2.06.4.3.1 2.06.4.3.2 2.06.4.4 2.06.4.4.1 2.06.4.4.2 2.06.4.5 2.06.5 References Introduction Phase Diagram Condensed Phases UF6: Uranium Hexafluoride Properties Thermodynamic properties Preparation Uses UF4: Uranium Tetrafluoride Properties Thermodynamic properties Preparation Uses UFx (4 < X < 6) – Intermediate Fluorides UF5: Uranium pentafluoride Intermediate fluorides U4F17 (UF4.25) and U2F9 (UF4.5) UF3: Uranium Trifluoride Preparation Properties Gaseous Compounds Gaseous Uranium Hexafluoride Molecular structure and physical properties Thermodynamic properties Gaseous Monomer and Dimer Uranium Pentafluoride Molecular structure Thermodynamic properties Gaseous Uranium Tetrafluoride Molecular structure Thermodynamic properties Gaseous Uranium Trifluoride Vapor pressure Enthalpy of formation Gaseous Uranium Mono- and Difluoride Outlook Symbols C0p HTÀH298 S0 Tfus Standard heat capacity at constant pressure (J KÀ1 molÀ1) Enthalpy increment (kJ molÀ1) Standard entropy (J KÀ1 molÀ1) Temperature of melting (K) DfH0 DfusH0 DfusS0 DsubH0 198 198 199 199 199 200 201 201 202 202 203 204 204 204 204 206 207 207 207 209 209 209 209 210 210 210 211 211 211 213 213 213 213 214 214 Standard enthalpy of formation (kJ molÀ1) Standard enthalpy of fusion (kJ molÀ1) Standard entropy of fusion (kJ molÀ1) Standard enthalpy of sublimation (kJ molÀ1) 197 198 e h l r The U–F System Dielectric constant (F mÀ1) Viscosity (Pa s) Thermal conductivity (W mÀ1  CÀ1) Density (kg mÀ3 or g cmÀ3) may melt congruently at 621 K or undergo decomposition The eutectic compositions between UF4–UF5 and UF5–UF6 are unknown 2.06.2 Phase Diagram 2U4 F17 sị7UF4 sị ỵ UF6 gị U2 F9 sị3=2U4 F17 sị ỵ UF6 gị 3UF5 sịU2 F9 sị ỵ UF6 gị From the equilibrium constant of these reactions K ẳ K0 eDG0 =RT ẳ PUF6 ị , the experimental results can be expressed as log PðUF6 Þ ¼ log K0 À ðDG0 =RT Þ, where K0 and DG0 =R are constants Plotting log PðUF6 Þ versus 1/T gives the stability domain of these compounds 10 000 5000 Triple point 2000 1000 500 200 100 50 20 10 U4 F17 α−UF5 U2F9 b−UF5 −1 1.35 1.55 1.75 2.15 1.95 103/T (ЊK) 100 ЊC −2 298 ЊC −5 200 ЊC Transition point 320 ЊC Among the numerous compounds in the U–F system (UF3, UF4, U4F17, U2F9, UF5, and UF6 as condensed phases, and UF, UF2, UF3, UF4, UF5, U2F10, and UF6 as gaseous species), UF6 is certainly the most known because of the wide use of this gas to enrich the 235U fraction in uranium Indeed UF6 has a vapor pressure of 1500 mbar (1.5 Â 105 Pa) at 337 K that appears as a striking contrast with the refractory UO2, which melts at 3120 K.1,2 This difference is typical of fluoride/ oxide difference, and also VI/IV oxidation state UF6 was first prepared by Ruff in 19113 through reaction of F2 on U metal or carbide The chemistry of UF6 was then more completely investigated in the 1940s due to the development of nuclear technology By the end of 1950, Agron had published a phase diagram including the intermediate fluorides U4F17, U2F9, and UF5 Further research continued at a slower pace in the 1960s on these intermediate fluorides The scientific interest later decreased with the rise of AVLIS laser-based enrichment technology of U metal that did not need UF6 to enrich in 235U In this period, some R&D was also performed on UF6 to define a dry reprocessing route using the fluoride volatility technique, such as the Fluorex process, to extract U from less-volatile fluorides such as fission products On the other end, UF4 had been known for a long time as a green solid used for the preparation of UF6 and uranium metal It was first prepared by the reaction of aqueous HF on U3O8 by Hermann in 1861 More recently UF4 is now considered for molten salt reactor technology Finally, the UF3–UF4 system was then studied more recently from an academic point of view, but UF3 today does not present any industrial application Except for UF4 that only yields a hydrate when exposed to air, all these compounds are unstable when exposed to the humidity of air yielding UO2F2 and/or UF4 UF6 is also very corrosive and can act as a strong fluorinating reagent Hence, the characterization of these intermediate fluorides has always been quite limited For example, the description of the UF5 liquid phase is not well known UF5 Agron has published a phase diagram (Figure 1) for the intermediate fluorides4 based on the three following reactions: Pressure (mmHg) UF6 (g) 2.06.1 Introduction 2.35 2.55 2.75 Figure The equilibrium pressures of the various uranium fluorides in the composition range < F/U < (Agron diagram) From Agron, P., 1948, AECD-1878, Courtesy of Oak Ridge National Laboratory, U.S Department of Energy The U–F System The UF3–UF4 system has been studied by Khripin et al.5 and Slovianskikh et al.6 by differential thermal analysis; UF3 being obtained through the reduction of UF4 with H2 In the two cases, they found a eutectic transition at, respectively, (1152 Ỉ 7) K and 1143 K, which is slightly lower than that at the temperature found by Thoma et al.7 and selected by Knacke et al.8 The eutectic composition is quite different between the two authors with 0.7835 at F (atomic fraction of F) found by Khripin et al.5 (value extrapolated from 199 the liquidus and solidus data) and 0.788 at F by Slovianskikh et al.6 In 1969 Knacke et al published the most complete phase diagram (Figure 2) to date8 with three eutectics at 1165, 621, and 328 K and three congruently melting compounds UF3, UF4, and UF6 at, respectively, 1700, 1309, and 337 K 2.06.3 Condensed Phases 2.06.3.1 UF6: Uranium Hexafluoride 2.06.3.1.1 Properties (1700) (1688) // 1500 1403 // Temperature (K) 1309 1165 1100 UF3 UF4 (703) (663) 621 (608) 700 UF6 UF4.25 UF4.5 UF5 337 (328) 300 // Atomic ratio F/U Figure The U–F system Reproduced from Knacke, V O.; Lossmann, G.; Muăller, F Z Anorg Allg Chem 1969, 370, 91–103 UF6 is solid at room temperature with a significant vapor pressure (P ¼ 105 mbar (1.05 Â 104 Pa) at 298 K) The triple point is 337 K for p ¼ 1.5 bar, as shown on the PUF6 ị ẳ f (T) diagram (Figure 3) The vapor pressure equations are detailed in Section 2.06.4.1.2 The critical temperature was found between 513 and 518 K.9 Many other physical, thermodynamic, and crystallographic properties can be found, respectively, by Llewellyn,9 Settle et al.,10 and Hoard and Stroupe.11 What can be noted about UF6 is the large difference between the density of the liquid and that of the solid at the triple point (4830 kg mÀ3 vs 3630 kg mÀ3) If liquid UF6 solidifies in a process pipe, care must be taken during heating because of the swelling The recommended equations for the density of solid and liquid UF6 are12 rS ẳ 5200 5:77T 273ị rL ẳ 3946 À 4:0628ðT À 273Þ À 1:36102 ðT À 273Þ2 where r is in kilogram per cubic meter 4500 4000 Pressure (mbars) 3500 Liquid 3000 2500 2000 Solid 1500 Gas 1000 500 0 Figure The UF6 phase diagram 20 40 60 80 Temperature (ЊC) 100 120 200 The U–F System The viscosity of liquid UF6 is close to that of water (0.8 cps at 90  C): 0.91, 0.85, 0.80, and 0.75 cps at, respectively, 70, 80, 90, and 100  C.13 Liquid UF6 usually flows by gravity to fill the 48Y containers A 48Y is a container that contains approximately 12.5 tonnes UF6 Liquid UF6 has a dielectric constant e ¼ 2.18 at 65  C typical of a nonpolar solvent The solubility of ionic compounds is low.14 A review of thermal conductivity for UF6 in the solid anroperties of the gaseous uranium pentafluoride as monomer and dimer DfH0 (UF5, g, 298.15 K) (kJ molÀ1) DfH0 (U2F10, g, 298.15 K) (kJ molÀ1) S0 (UF5, g, 298.15 K) (J KÀ1 molÀ1) S0 (U2F10, g, 298.15 K) (J KÀ1 molÀ1) C0p (UF5, g, 298.15) (J KÀ1 molÀ1) C0p (UF5, g, 298.15) (J KÀ1 molÀ1) C0p (UF5, g, T) (J KÀ1 molÀ1) À(1913 Ỉ 15)39 À(3993 Ỉ 30)39 386.4 Ỉ 10.039 577.6 Ỉ 10.087 110.6 Ỉ 5.024 234.7 Ỉ 5.024 116.738 þ 3.13041 Â 10À2T À 1.2538 Â 10À5T2 À 1273000TÀ2 (298–1100)38 functions of UF5(g) are calculated from molecular parameters estimated by Glusko et al.91 Here the molecule is assumed to be a square pyramid with a slight distortion to reduce the symmetry to C2v.39 2.06.4.3 Gaseous Uranium Tetrafluoride 2.06.4.3.1 Molecular structure The molecular structure was studied by electron diffraction by Girichev et al.92 to determine the U–F and F–F distances The analysis of the results indicated that the structure was not tetrahedral but rather D2h or C2v This was confirmed by the analysis of the thermodynamic (vapor pressure) studies.93–95 Konings et al.96 have studied the infrared spectrum of UF4 vapor between 1300 and 1370 K Based on this, and a reanalysis of the previously determined gas electron diffraction data,92 they have demonstrated that the UF4(g) molecule almost certainly has tetrahedral symmetry 2.06.4.3.2 Thermodynamic properties 2.06.4.3.2.1 Vapor pressure There are a lot of studies on uranium tetrafluoride in the gaseous phase Except for Leinaker,88 all studies suggest that UF4 vaporizes congruently to the UF4 monomer The vapor pressure measurements above the solid and liquid UF4 were performed by  transpiration method in the temperature range 1148–1273 K, using argon purified from oxygen and water vapor as carrier gas97;  combination of a quasistatic method and a boiling point technique from 1291 to 1575 K98; 212 The U–F System Table Experimental vapor pressure equations above solid UF4 References Experimental method Equation Transpiration log PPaị ẳ 15:071 Akishin et al.99 Mass spectrometry log Chudinov et al.100 Effusion log Hildenbrand93 Torsion–effusion log Nagarajan et al.101 Transpiration and evaporation log Popov et al 97 Table 10 T range (K) 16140 TðKÞ 70500 PðPaÞ ¼ 15:020 À 4:576TðKÞ 16504:9 PðPaÞ ¼ 30:663 À À 4:876logTKị TKị 15691 PPaị ẳ 14:91 ặ 0:5ị TKị 15994 ặ 176ị PPaị ẳ 15:03 ặ 0:14ị TKị 1148–1223 917–1041 823–1280 980–1130 1169–1307 Experimental vapor pressure equations above liquid UF4 References Experimental method Equation T range (K) Popov et al Transpiration log PPaị ẳ 10:127 12481278 Langer et al.98 Quasistatic and boiling point Transpiration and evaporation 10000 TKị 16840 ặ 44 log PPaị ẳ 7:549 logTKị ỵ 39:086 ặ 0:03ị TKị 12014 ặ 335ị log PPaị ẳ 11:99 ặ 0:24ị TKị 97 Nagarajan et al.101 Tfus = 1309 K DfH0 (UF4, g, 298.15 K) (kJ molÀ1) S (UF4, g, 298.15 K) (J KÀ1 molÀ1) Cp (UF4, g, 298.15) (J KÀ1 molÀ1) Cp (UF4, g, T) (J KÀ1 molÀ1) log p(UF4 Pa) Popov exp Popov fit Akishin et al.97 Hildenbrand and Lau88 Nagarajan fit Nagarajan exp Langer exp Langer fit Chudinov et al.98 Johnsson100 –1 Tfus = 1242 K –2 6.0 6.5 7.0 7.5 8.0 1312–1427 Table 11 Thermodynamic properties of the gaseous uranium tetrafluoride 1302–1575 8.5 9.0 9.5 10.0 10.5 11.0 DsubH0 (UF4, 298.15) (kJ molÀ1) À(1605.2 Ỉ 6.5)39 360.7 Ỉ 5.039 95.1 Ỉ 3.039 103.826 ỵ 9.549 103 T 1.451 106 T2 À 021 320 T À2 (298–3000)39 309.0 Ỉ 5.039 104/T (K) Figure 19 Comparison of the experimental vapor pressure above solid and liquid UF4  mass spectrometry between 917 and 1041 K99;  integral and differential effusion method100 in 823–1280 K temperature range;  torsion effusion method from 980 to 1130 K93;  both transpiration and evaporation temperature methods between 1169 and 1307 K and 1312 and 1427 K, respectively.101 The equations obtained above the solid and liquid are presented, respectively, in Tables and 10 The vapor pressure measurements above the solid UF4 are scattered (Figure 19) The results of (298–3000) is the temperature range for which the Cp(T) function is valid Nagarajan et al.101 and Popov et al.97 are lower than those of Johnson,102 Akishin et al.,99 and Chudinov et al.,100 but the discrepancy is reduced at higher temperature and reaches 10% after the melting point For the pressure above the liquid, the data of Nagarajan et al.101 and Langer et al.98 are very close The values of Popov are excluded because the two points given in the liquid phase are in fact in solid phase Critical analysis of the vapor pressures measurements gives the selected enthalpy of sublimation (Table 11) The U–F System DfH0 (UF3, g, 298.15 K) (kJ molÀ1) S0 (UF3, g, 298.15 K) (J KÀ1 molÀ1) C0p (UF3, g, 298.15) (J KÀ1 molÀ1) C0p (UF3, g, T) (J KÀ1 molÀ1) À(1065 Ỉ 20)1 347.5 Ỉ 101 DsubH0 (UF3, 298.15) (kJ mol1) 76.2 ặ 5.024 81.327 4.3 106T ỵ 2.427 Â 10À6T2 –476300TÀ2 (298–1800)39 447.2 Ỉ 1524 (298–1800) is the temperature range for which the Cp(T) function is valid The entropy and heat capacity at 298.15 K were calculated using molecular parameters for UF4(g) reported by Konings and Hildenbrand103 and electronic levels taken a part from Glushkov et al.91 and Konings and Hildenbrand.103 2.06.4.4 Gaseous Uranium Trifluoride Molecular geometry has not been measured Quantum chemical calculations for the uranium (III) fluoride indicate a pyramidal structure104 but with a bond angle close to the planar 120 The thermodynamic data on solid uranium trifluoride recommended by Grenthe et al.24 are presented in Table 12 Values for the heat capacity and entropy of UF3(g) are calculated from estimated molecular parameters given by Glushko et al.91 2.06.4.4.1 Vapor pressure On heating, solid UF3 does not vaporize congruently but disproportionately into solid UF4 and uranium by the following reaction: 4UF3 ! 3UF4 ỵ U The vapor pressure measurements, which are difficult, can explain the scattered results (Figure 20) Roy et al.105 determined the vapor pressure of UF3(s) by the transpiration technique using hydrogen as the carrier gas in the 1229–1367 K temperature range, and Gorokhov et al.106 deduced it from their mass spectrometry determinations The temperature dependence of the vapor pressure is described, respectively, by the following equations105,106 logpUF3 ẳ13:26ặ0:23ị15666ặ302ị p inPaị T ðKÞ log p(UF3 Pa) Table 12 Thermodynamic properties of the gaseous uranium trifluoride 213 Roy exp Roy fit Gorokhov fit Gorokhov exp –1 –2 –3 0.72 0.74 0.76 0.78 103/T (K) 0.80 0.82 Figure 20 Comparison of the experimental vapor pressure of UF3(s) p inPaị logpUF3 ẳ13:39ặ0:46ị20040ặ0:62ị T ðKÞ The discrepancy is very large, about three orders magnitude 2.06.4.4.2 Enthalpy of formation Enthalpy of formation of gaseous UF3 has been evaluated from experimental studies by Grenthe et al.24 It has been deduced from  the enthalpy of sublimation obtained by thirdlaw analysis of the vapor pressure data measurements,106  mass spectrometric measurements.107,108 2.06.4.5 Gaseous Uranium Mono- and Difluoride There is no experimental data on the molecular structure for both species These molecules appear at high temperature As for the uranium trifluoride, the mono- and difluoride of uranium have been identified in mass spectrometric measurements by different authors Lau et al.108 studied the exchange reactions of the lower uranium fluoride with BaF by mass spectrometry; Zmbov,107 Gorokhov et al.,106 and Hildenbrand et al.90 studied the molecular equilibria between the uranium fluorides among themselves The results for the uranium compounds have been analyzed in detail by Grenthe et al.24 and updated by Guillaumont et al.39 who demonstrated that the results are in reasonable agreement, considering the large number of approximations made in the analysis Almost no experimental 214 The U–F System Table 13 Thermodynamic properties of the gaseous uranium mono- and bifluoride 14 15 DfH0 (UF2, g, 298.15 K) (kJ molÀ1) S0 (UF2, g, 298.15 K) (J KÀ1 molÀ1) C0p (UF2, g, 298.15) (J KÀ1 molÀ1) DfH0 (UF, g, 298.15 K) (kJ molÀ1) S0 (UF, g, 298.15 K) (J KÀ1 molÀ1) C0p (UF, g, 298.15) (J KÀ1 molÀ1) À(540 Æ 25)1 315.7 Æ 1039 56.2 Æ 5.024 À(47 Æ 20)39 251.8 Ỉ 3.039 37.9 Ỉ 3.024 16 17 18 19 20 data on the molecular properties are available and thus the thermal functions are based rather on qualitative estimates, introducing large uncertainties Values for the heat capacity and entropy of UF(g) and UF2(g) are calculated from estimated molecular parameters given by Glushko et al (Table 13) 21 2.06.5 Outlook 25 Although UF6 and UF4 have been well characterized and used over the years, some work remains to be done on the intermediate fluorides and the phase diagram In particular, the mechanisms leading to the formation of these undesirable compounds in industrial reactors are not very well known This would help in improving yields in the UF6 preparation process References 22 23 24 26 27 28 29 30 31 32 33 34 35 10 11 12 13 Konings, R J M.; Morss, L R.; Fuger, J In Thermodynamic Properties of Actinides and Actinide Compounds; Springer: New York, 2006; Chapter 19, pp 2113–2224 Konings, R J M.; Benes, O.; Manara, D.; Sedmindubsky, D.; 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À( 1065 Ỉ 20 )1 347.5 Æ 101 DsubH0 (UF3, 29 8.15) (kJ molÀ1) 76 .2 Æ 5. 024 81. 327 4.3 106T ỵ 2. 427 106T2 –476300T 2 (29 8–1800)39 447 .2 Ỉ 1 524 (29 8–1800) is the temperature range for which the. .. would help in improving yields in the UF6 preparation process References 22 23 24 26 27 28 29 30 31 32 33 34 35 10 11 12 13 Konings, R J M.; Morss, L R.; Fuger, J In Thermodynamic Properties of Actinides... À(540 Ỉ 25 )1 315.7 Æ 1039 56 .2 Æ 5. 024 À(47 Æ 20 )39 25 1.8 Æ 3.039 37.9 Ỉ 3. 024 16 17 18 19 20 data on the molecular properties are available and thus the thermal functions are based rather on