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 ðsÞ⇄7UF4 ðsÞ þ UF6 ðgÞ
7
2 U2 F9 ðsÞ⇄3=2U4 F17 ðsÞ þ UF6 ðgÞ

3UF5 ðsÞ⇄U2 F9 ðsÞ þ UF6 ðgÞ
From the equilibrium constant of these reactions
K ¼ K0 eÀDG0 =RT ¼ PðUF6 Þ , 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
5

U4 F17

α−UF5

U2F9

b−UF5

2

−1
1.35

1.55

1.75

2.15
1.95
103/T (ЊK)


100 ЊC

−2

298 ЊC

−5

200 ЊC

Transition point

1

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 1 The equilibrium pressures of the various uranium
fluorides in the composition range 4 < F/U < 5 (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 Æ 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

0


1

//

3
4
Atomic ratio F/U

5

6

Figure 2 The U–F system. Reproduced from Knacke, V. O.;
Lossmann, G.; Mu¨ller, 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 PðUF6 Þ ¼ 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 ¼ 5200 À 5:77ðT À 273Þ
rL ¼ 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
0
Figure 3 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 Æ 15)39
À(3993 Æ 30)39
386.4 Æ 10.039
577.6 Æ 10.087
110.6 Æ 5.024
234.7 Æ 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 9

Experimental vapor pressure equations above solid UF4

References

Experimental method

Equation

Transpiration

log PðPaÞ ¼ 15:071 À

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 4:876logTðKÞ
TðKÞ

15691
PðPaÞ ¼ ð14:91 Æ 0:5Þ À
TðKÞ
ð15994 Æ 176Þ
PðPaÞ ¼ ð15:03 Æ 0:14Þ À
TðKÞ

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 PðPaÞ ¼ 10:127 À

1248–1278


Langer et al.98

Quasistatic and boiling
point
Transpiration and
evaporation

10000
TðKÞ
16840 Æ 44
log PðPaÞ ¼ À
À 7:549 logTðKÞ þ ð39:086 Æ 0:03Þ
TðKÞ
ð12014 Æ 335Þ
log PðPaÞ ¼ ð11:99 Æ 0:24Þ À
TðKÞ

97

Nagarajan et al.101

Tfus = 1309 K
4

DfH0 (UF4, g, 298.15 K)
(kJ molÀ1)
0
S (UF4, g, 298.15 K)
(J KÀ1 molÀ1)

0
Cp (UF4, g, 298.15)
(J KÀ1 molÀ1)
0
Cp (UF4, g, T)
(J KÀ1 molÀ1)

log p(UF4 Pa)

3

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
0
–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

5

2

1302–1575

8.5

9.0

9.5 10.0 10.5 11.0

DsubH0 (UF4, 298.15)
(kJ molÀ1)


À(1605.2 Æ 6.5)39
360.7 Æ 5.039
95.1 Æ 3.039
103.826 þ 9.549 Â 10À3 T
À 1.451 Â 10À6 T2
À 1 021 320 T À2
(298–3000)39
309.0 Æ 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 9 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

3

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 Æ 20)1

2

347.5 Æ 101


1

DsubH0 (UF3, 298.15)
(kJ molÀ1)

76.2 Æ 5.024
81.327 – 4.3 Â 10À6T
þ 2.427 Â 10À6T2
–476300TÀ2 (298–1800)39
447.2 Æ 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 þ 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
1
logpUF3 ¼ð13:26Æ0:23ÞÀð15666Æ302Þ
ðp inPaÞ
T ðKÞ

log p(UF3 Pa)

Table 12
Thermodynamic properties of the gaseous
uranium trifluoride

213

Roy exp
Roy fit

Gorokhov fit
Gorokhov exp

0
–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).

1
ðp inPaÞ
logpUF3 ¼ð13:39Æ0:46ÞÀð20040Æ0:62Þ
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 Æ 3.039
37.9 Æ 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

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4.
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6.
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8.
9.
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