ORNL-TM-4056.txt

. - ) ORNL-TM-4056

  

 

THE EQUILIBRIUM OF DILUTE UF3 SOLUTIONS
CONTAINED IN GRAPHITE

L. M. Toth
L. O. Gilpatrick

MASTER

INSTINTER 27 7003 oincivnaeny 1 UNLITER

 

 

 

Ce
'3‘

OPERATED BY UNION CARBIDE CORPORATION » FOR THE U.S. ATOMIC ENERGY COMMISSION

 
 

 

This report was prepared as an account of work sponsored by the United
States Government. Neither the United States nor the United States Atomic
Energy Commission, nor any of their employees, nor any of their contractors,
subcontractors, or their employees, makes any warranty, express or implied, or
assumes any legal liability or responsibility for the accuracy, completeness or
usefulness of any information, apparatus, product or process disclosed, or
represents that its use would not infringe privately owned rights.

 

 
ORNL-TM- 4056
Contract No. W-7405-eng-26

REACTOR CHEMISTRY DIVISION

THE EQUILIBRIUM OF DILUTE UF3 SOLUTIONS CONTAINED IN GRAPHITE

L. M. Toth and L. 0. Gilpatrick

NOTICE

This report was prepared as an account of work
sponsored by the United States Government, Neither
the United States nor the United States Atomic Energy
Commission, nor any of their employees, nor any of
their contractors, subcontractors, or their employees,
makes any warranty, express or implied, or assumes any
legal liability or responsibility for the accuracy, com-
pleteness or usefulness of any information, apparatus,
product or process disclosed, or represents that its use
would not infringe privately owned rights,

 

 

 

DECEMBER 1972

OAK RIDGE NATIONAL LABORATORY
Oak Ridge, Tennessee 37830
operated by
UNION CARBIDE CORPORATION
for the
U.S. ATOMIC ENERGY COMMISSION

 

DISTRERUTIGN OF THIS DOCUMENT IS URLIMITED

U

 
THE EQUILIBRIUM OF DILUTE UF3 SOLUTIONS CONTAINED IN GRAPHITE
L. M. Toth and L. 0. Gilpatrick

ABSTRACT

The equilibrium of dilute UF3-UF, molten fluoride solutions
in contact with graphite and UC2:

3UF, + UC, < 4UF, + 2C
has been studied as a function of temperature (370-700°C), melt com—
position and_atmospheric contamination. Equilibrium quotients, Q =
(UF3)4/(UF4)3 for the reaction were determined by measuring the UF4
and UF, concentrations spectrophotometrically. The solvents used
were primarily LiF-BeF2 mixtures. Results from this solvent system
were related to the reactor solvents LiF-BeF9~-ZrF,(65.6-29.4-5 mole
%) and LiF-BeFp-ThF,;(72-16-12 mole %). It has been found that the
equilibrium quotient is very sensitive to both temperature and sol-
vent changes increasing as either the temperature increases or the
alkali-metal fluoride content of the solvent decreases.

 

INTRODUCTION

The relative stability of dilute UF3-UF, molten fluoride solutions
contained in graphite is of practical importance to Molten Salt Breeder
Reactors, MSBR, in which these solutions are used as nuclear fuels. Be-
cause the reactors contain a large amount of graphite in the core serving

as a neutron moderator, reaction of UF4 with graphite:

*
4UF3(d) + 2C 1'3UF4(d) + U02 (1-1)

to form UF4 and uranium dicarbide has long been recognized1 as a major

factor limiting the amount of UF3 which can be maintained in solution.

Although typical fuel mixtures consist essentially of 1 mole ¥ U235F4
or less in a solution of LiF and BeFZ, the ease of UF4 reduction to UF3 by
the chromium in the metal containment vessel**

o -
+ -—
ZUFA(d) Cr™ & 2UF3(d) + Cer(d) (1-2)

 

* The subscript "d" indicates that the component is in solution.

**Hastelloy N, a nickel-based alloy containing Cr, Fe and Mo has been
used? to fabricate the metal containment vessel for the Molten Salt
Reactor Experiment, MSRE.
necessitates the consideration of UF; chemistry as well. The effect of the
corrosion reaction of Eq. 1-2 is to leach chromium from the structural

metal and cause it to appear in solution as CrFj.

In order to minimize the corrosion, the equilibrium of Eq. 1-2 is
shifted to the left by reducing a small percentage (approximately 17%) of
the UF, to UF3 through the addition of beryllium metal:3

2UF + Be® T 2UF + BeF (1-3)

4(d) 3(d) 2
Although a small amount of UF3 is beneficial in reversing the corrosion
mechanism, it produces complications due to possible reaction via Eq. 1-1
and the resulting undesirable formation of an insoluble uranium carbide.
Reference to "UFg3 stability" in this paper will therefore mean specifically
the equilibrium concentration of UFj3 relative to UF, as determined by Eq.

1-1.

This equilibrium has never been experimentally measured despite the
fact that it is the major factor in determining UF3 stability for molten
salt reactor systems. Although they used indirect means, Long and
Blankens‘nip4 are the only investigators who have attempted to measure the
equilibrium. Since their work is the basis on which all previous estimates
of UFy stability have been made, it will be reviewed in detail, with the
equilibrium expressions in fractional coefficients as used by the authors.
They studied the reduction of UF, (both pure solid phase UF, and in molten

fluoride solution) with hydrogen:

1

 

T - -
SH, + UF, [ UF, + HF (1-4)
and determined the equilibrium quotients for the above reduction:
X Y
R 3 Fyp R UF,
Q=g 77 =K (1-5)
Xop, pl/2 YuF
4 H2 3

by measuring HF and H, ratios evolved from a reaction vessel containing

UF, and UF,. From the solid-phase UF4 reduction they obtained equilibrium

4 3

constants, KR, for the reduction. These, combined with the equilibrium
R X . . .

quotient, Q , for the dilute solutions and the activity coefficient for

UF3, YUF33 obtained from solubility data, enabled them to calculate the
activity coefficient for UF,, YUF,» in the molten fluoride solution. By
combining the free energy expression for Eq. 1-4 with one for the decompo-

sition of UF3 into UF& and UO:

—>§_ 1.0 -
UF3(d) 7 QUFA(d) +Z{I (1-6)

they obtained an expression for the equilibrium quotient, QD, of Eq. 1-6,

in terms of the equilibrium quotient for Eq. 1-4, QR, and the activity co-
efficients of UF, and HF. (c.f. p 18, Ref. 4, part II). Using free ener-
gies of formation for UCy and UC from Rand and Kubachewski,5 which were
acceptable at the time, they estimated uranium activities in the carbides
and concluded that solutions in which up to 607 of the initial 1 mole % of
UF, is converted to UFq5 are expected to be stable in the presence of graph-
ite. In addition they concluded that temperature and solvent changes

should have little effect on the equilibrium mechanism of Eq. 1-1 since they
found no significant effect from them on the H2 reduction mechanism of Eq.

1-4.

In view of the significance of Eq. 1-1 to Molten Salt Reactor Technol-
ogy, a closer examination of it is clearly warranted. The development of
spectrophotometric techniques for the study of molten fluorides and the
realization of solvent effects on molten fluoride chemistry, have given
impetus to the study. We have aiready identified6 UC,, as the stable uran-
ium carbide phase in equilibrium with UF3-UF, solutions in graphite. The
object of this report is to describe the effects of temperature, solvent,
and atmospheric contamination on the equilibrium. Both the forward and the
back reaction of Eq. 1-1 in the reference solvent system LiF-BeF, have been
followed. The data in the reference solvent system have been related to
practical reactor solvents such as the Molten Salt Reactor Experiment, MSRE,
solvent, LiF-BeF,-ZrF, (65.6-29.4~5 mole %) and the proposed Molten Salt
Breeder Reactor, MSBR, solvent, LiF-BeF;-ThF, (72-16-12 mole %Z)}. Our find-
ings are compared with earlier observations which have not been reviewed

before.
EXPERIMENTAL

Equilibrium quotients for the back-reaction* (Eq. 3-2) were determined
by measuring UF3 and UF, concentrations spectrophotometrically with a Cary
Model 14-H recording spectrometer. The sample system consisted of a control-
led temperature, inert atmosphere furnace shown in Fig. 2-1 which held a
diamond-windowed graphite spectrophotometric cell.7 Molten fluoride salt
solutions and reagent uranium carbides were contained in this cell. Ab-
sorption spectra of the molten salt solution were measured against an air

reference. Net spectra due to UF, and UF4 were determined by subtracting

independently determined solvent slank spectra using standard digital com-
puter techniques. Spectra were measured in the near infra-red and visible
regions from 4000 to 33000 eml.  The absorption spectra of UF5 and UF,

served as the primary means of monitoring these components in solution as a

function of temperature, time, and solvent composition.

Materials - Molten salt solvent compositions were prepared by mixing
calculated amounts of the pure component fluoride salts. Optical quality
crystal fragments from the Harshaw Chemical Co. was the source of LiF.
Beryllium fluoride was prepared by vacuum distillation8 from a large
special purchase supplied by the Brush Beryllium Co. The water-—clear,
glass-like product contained no spectrographically detectable cation im-
purities, but was exceedingly hydroscopic and had to be stored under very
anhydrous conditions. Uranium tetrafluoride was taken from a laboratory
purified spectroscopic standard which contained less than 10 ppm of total
cation impurities. Thorium tetrafluoride was part of a special purchase
from the National Lead Co. which contained no greater than 100 ppm in any

cation impurities.

Storms and coworkers of the Las Alamos Scientific Laboratory supplied
each of the uranium carbides used in this study and supplied the following
analysis:

Uranium dicarbide - UC,  wt % C = 8.83 or 75.74 mole % C
02 = 200 ppm

 

*See Results and Discussion Section for an explanation of why the back-

reaction was measured.

**Although uranium dicarbide is a substoichiometric com.pound,13 it will be
identified as UCy in this paper.
SAMPLE

BEAM

s

REFERENCE

BEAM

s

  
  
 
 
 
 
 
 
 
 
 
 
 
  
 

QO~NOOC WM .

 

2-1

High Temperature Furnace
Molten Fluorides.

ORNL-DWG 72-10653

NEUTRAL SCREEN
LAVITE SUPPORT

_ MINERAL WOOL

- QUARTZ - VACUUM WINDOW

O-RING SEAL
BRASS SHELL
TRANSFER CHAMBER (REMOVABLE)

_ VACUUM SHUTTER { O-RING SEAL ]
. TEFLON SLIDING SEAL

CELL SUPPORT TUBE

. STAINLESS STEEL STiRRER RGO
. VACUUM OR HELIUM

TEFLON GLAND
a-in. TUBE FITTING

.. NEOPRENE O-RING

WATLOW HEATERS (NOT SHOWN)

- COPPER BLOCK

.. INCONEL. FURNACE

. DIAMOND-WINDOWED GRAPHITE CELL
_ Pt-10% Rh STIRRER

TITANIUM SPONGE GETTER

- CELL SUPPORT

System for Absorption Spectra of
3.5251 + 0.00054
5.9962 + 0.00083

Uranium sesquicarbide - U,Cy wt “ZC=6.99 or 59.83 mole % C
0, = 50 ppm

crystal lattice by X-ray a = 8.0889 1_0.0009R

crystal lattice by X-ray a
c

I

These high purity carbides were received as lusterous grayublack granules
which ranged in size from 1/2 to 1 mm3. They were shipped sealed in glass
ampules and stored in a helium filled dry box. Exposure to even the dry
box conditions was kept to the absolute minimum needed for weighings and

cell loadings.

Procedure - Even though the reagent salts were quite free of cation
impurities, they were not free of oxides and H90 to the degree needed. All
compositions were therefore treated while molten at 600°C for oxide removal
by sparging for several hours with reagent HF gas or HF-Hy gas mixtures.9
Residual HF was then stripped from the melt with He prior to cooling. Clean
portions of the recovered salt "button" were then crushed and used to
charge the spectrophotometer cell, by weighing out the fluorides in a hel-
ium drybox which was maintained at a water vapor content < 0.1 ppm and at
an 07 content < 2 ppm. Between 0.5 and 0.6 gm of salt solvent made a con-
venient cell loading to which was added from 5 to as much as 100 mg of the
uranium carbide under study. Poco AXF-5Ql grade graphitelo spectrophoto-
metric cells were used which were purified after fabrication by heating in
an Hyp gas stream to 1000°C and then flushed free of Hy, with He. Subsequent
dry box handling and loading techniques have been discussed earlier.1l A
"dash pot" stirrer made from platinum-10% rhodium (see Fig. 2-1) was used
to hasten the attainment of equilibrium which is otherwise dependent large-
ly on diffusion. It proved to be a great aid in shortening the time needed
to reach equilibrium. We observed a small but temporary loss of transmig-
sion directly after stirring in some cases which was equal to 0.15 absorb-
ance units at 4000 cm™l. We have assumed this recoverable loss to be
caused by the temporary suspension of fine particles which later settle.
Whole grains of the carbide were used after early attempts to increase the
surface area by crushing caused the carbide to collect at the window and

interfere with the optical transparency of the cell. A large excess of the

golid carbide phase was always maintained in the cell. On some occasions,
the experimental sequence was interrupted and additional uranium carbide
was added to demonstrate that an excess was indeed present. No change was
observed in the concentrations of UFg or UF, in the homogeneous solutions

as a result of these additions.

Spectral Measurements - Molar concentrations of dissolved UF3 and UF,

 

were determined simultaneously in solution at a series of temperatures a-
bove the melting point by measuring optical densities at 9174 and 11360 cm-l.
These wave numbers represent the maximum absorbance values for dissolved

UF,4 and UFq4 respectively in the near infra-red region as shown in Figs. 2-2
and 2-3. The strong UF3 absorption in the visible region from 16000 to
33000 cm™ ! was in general too intense to be useful since the solutions
studied had initial UF,; molarities in the range of 0.04 to 0.10. Figures
2-2 and 2-3 show that for spectra of pure UF, and UF4 there is a contribu-
tion from each at the most sensitive absorbance region of the other member.
Stated differently, the absorbance at 9174 em™1 in a mixed solution is
primarily due to UF4, but not entirely so. This condition is solved unique-
ly for the contriburion from each species by the solution of a set of simul-
taneous linear equations equal to number to the number of components in the

system which contribute to the net spectra, in our case 2.

Using Beer's law

-log I/I = A, = (e,) (DL (2-1)
where I = measured optical intensity of the sample
IO = measured optical intensity of the reference solvent
A = total absorbance at a given frequency, V.
(8\))T = molar absorption coefficient at v and temperature T
(M), = molarity of component in solution
2 = cell path length = 0.635 cm

The following set of equations are sufficient to determine the separate

molar concentrations in a mixed solution at a particular temperature.

_ 3 4 _ b 3

Ajp T AL T A T g WM e (M) R (2-2)
3 b 4 3

sz - Av2 + sz - E:\)2(1\{4)1‘£ + E\)2(M3)T2 (2-3)
ABSORPTION COEFFICIENT (liters mole ™! cm™!)

20

18

12

10

ORNL-DWG 72-9634

 

 

l

I

! l |

 

 

| I I l i

 

10

"

12

13

14 16 18 20 22 24 (x 103)

WAVENUMBER {cm™!)

2-2  UF, Spectrum (approximately 1 mole %) in LiF-BeF;-ThFy
(72-16-16 mole %) at 575°C.
ORNL-DWG 72-9635

 

19
O

D
n

1

- n N W L
&) O U o g O

ABSORPTION COEFFICIENT ( liters mole™! cm
S

 

I | | ! | | ;’ 1 1
—~ 900 —
800
700 K
600 -
- 500 -
400
300 :

/
200 —

     

 

 

l ' i | | | i ! i I | l |

 

6 7 8 9 10 4 12 13 14 18 22 26 30 34 (x103)
WAVENUMBER (cm™)

UF3 Spectrum (approximately 0.3 mole %) in LiF-BeFp (66-34 mole %)
10

where: 1

it

9174 cm

11360 cm~ 1

"

1
2
3 = UF5 component
4

fl

UF, component

Solving Eqs. (2-2) and (2-3) simultaneously for (M4)T and (M3)T gives the
desired molarities since AVl and sz are measured and the € values are

known from previous calibrations.

 

 

4 3
oLy = (Evz Ayl T Eu1 sz) k2-4)
4°T 2(83 84 _ €3 €4 )
v2 vl vl v2
4 4
0Ly = (€51 A2 ~ €1 A (2-5)
3T ., 3 4 3 4

(€52%01 = Ev1fu2)
Because spectra were recorded versus a neutral screen in the reference
beam (see Fig. 2-1) it was always necessary to subtract a solvent spectrum,
or blank, which was independently determined for each experimental spectrum,

to get the net absorbance due to species in solution.

Analyzing composite spectra required making calibrations for € with
solvent melts containing a known concentration of pure UF, and UF5. Values
of € are reduced with increasing temperature because of two effects: the
change of molarity caused by thermal expansion and temperature effects on

the absorption spectra themselves.

Changes in molarity due to temperature changes were adjusted by using
S. Cantor's data12 for the molal volume of various fused fluoride salts and
assuming that the molal volumes are additive to within + 3% according to

the following general relations:

NT[xl(\)l)T + xz(vz)T —————— ] = 1000 ml (2-6)

(Ml)T = NT x; (2-7)
where NT = moles per liter of solution at temperature T

x, = mol fraction of component #1

(\)l)T = molar vol of 1 at temperature T in cc/mole

M 1)T = molarity of 1 in moles/l at temperature T
11

Molar absorptions were first measured for pure UF, solutions at
various temperatures using a known concentration and at molarities adjusted

)

and (832) are recorded in Table

for expansion. Measured values of (E4 T

v1°T
2-1.

A corresponding calibration was performed for pure UF3 under identical
conditions. This was most easily achieved by adding an excess of a reduc-
ing agent. Both zirconium and uranium metal were used for this purpose,

they react as shown in Eqs. (2-8) and (2-9).

Zr + 4UF, T 4UF. + ZrF (2-8)

4 3 4

>
U + 3UF4 < 4UF3 (2-9)

The effect on the properties of the solutions caused by the production of
ZrFA in Eq. 2-8 was very small and hence neglected for these dilute solu-
tions. When uranium was used concentrations had to be increased by 1/3
over those calculated for UF4 in the initial solutions as shown in Eq. (2-9).
Pure UF45 solutions in contact with graphite result in the loss of uranium
from solution by the formation of UC2 as shown in Eq. (1-1). Fortunately
this reaction is rather slow under the conditions that we have studied, and
it was possible to correct for this loss by measuring absorbances as a
function of time to determine the rate of loss (dAv/dT), and correcting for
the loss by extrapolating back to zero time. Reducing UF4 with uranium
does not result in a loss of UF3 from solution. The addition of Eq. (1-1)
and (2-9) results in the cyclic conversion of U and C to UC2 with no net

change of UF, concentration in solution as shown in Eq. (2-10).

 

3
4UF3 + 2C ~> 3UF, + UC, (1-1)
3UF4 + U~ 4UF3 (2-9)
U+ 2C -+ UC (2-10)

2
An alternate approach to determining the molar absorption coefficients
(€v) for UF3 in solution has also been used. Since UF4 solutions are more
stable than UF3 solutions under our experimental conditions, the calibra-
tion results for UF4 are more reliable and associated with less error than
are those for UF3. Using this fact the uncertainty associated with the UF3
calibration can be reduced by measuring the absorption spectrum of a mixture
Table 2-1

Molar Absorption Coefficients for Molten Fluoride Solutions of

 

 

 

 

UF4 and UF3
Solution in L2B Solvent:LiF°BeF2 LB SolventILiF'Ber MSBR So:’LventzLiF'BeFZ'ThF4
Mole 4 (66.7-33.9) (48-52) (72-16-12)
Spectra UF3 UF4 UF3 UF4 UF3 UF4
Mol ti
Cge?;ii?zgip ton €11360 €9170 €11360 €9170 €11360 €9170 £11360 E9170 €11360 E9170 €11360 E€9170
Temperature °C
370 44,2 7.6 3.1 18.4
400 43.2 7.8 3.1 17.8
450 46,0 10.0 3.90 18.7 41.7 8.1 3.1 16.9
500 44,2 10.0 3.85 17.9 40.1 8.4 3.1 16.0 58.5 14.5 2.80 19.2
550 41.7 10.0 3.80 17.1 38.5 8.6 3.1 15.1 56.8 14.5 2.70 18.2
600 39.0 10.0 3.75 16.3 36.9 8.9 3.1 14.2 55.0 14.5 2.65 17.2
650 36.2 10.0 3.70 15.4 35.3 9.2 3.1 13.3 53.3 14.5 2.55 16.2
700 33.5 10.0 3.65 14.8 33.7 9.4 3.1 12.4 51.6 14,5 2.50 15.2
750 30.7 10.0 3.60 14.1 49,9 14.5 2.40 14.5
800 48.0 14.5 2.35 13.8

 

 

 

 

 

 

¢t

 
13

of UF5j and UF, where the UFj is generated by partially reducing a dilute
UF, solution of known concentration. (The reductant chosen for partial
reduction was UCp.) The spectrum is then converted to digital form along
with a UF4 reference spectrum. Using iterative computer techniques, varying
amounts of the UF4 spectrum (i.e., k x (UF4 spectrum) where k is the coef-
ficient which is varied in the iteration process) are subtracted until the
resulting spectrum visually matches that of previously measured (uncalibra-
ted) UF3 spectra. When a match is found for a particular value of k, the

concentration of UF, in solution and thus the absorption coefficient can be

3
calculated knowing the total amount of UF4 before reduction. Comparison of
€ values by this method with the total reduction method showed agreement

within a 5% uncertainty.

In Table 2-1, absorption coefficients are listed for the various solu-
tions and temperature ranges that have been studied. Values were taken
from smoothed functions which within the limits of our precision are a

linear function of temperature.

RESULTS AND DISCUSSION
An equilibrium expression such as the one written in Eq. 1-1 implies
that certain criteria are valid: (1) The equilibrium expression should in-
clude all reactants and products which are involved in the reaction and
these components should combine in the stoichiometry indicated by the ex-

pression. (2) The entire process must be reversible.

Before quantitative data for the equilibrium in Eq. 1-1 were measured,
the above criteria were examined in the following manner: The equation
represents a heterogeneous equilibrium between a molten-fluoride solution
of UF3 and UF4 and two solid phases, U02 and graphite. The identification
of the UF3 and UF4 was made by the characteristic absorption spectrum of
each component in the near-infrared and visible regions (4000-33000 cm_l).
The identification of these two solute components is well established be-
cause their absorption spectra have been thoroughly documented.11 In view
of the extensive spectroscopic work which has preceded, there is no spec-

tral evidence for any cations in the solution other than U+3, U+4.
14

The solid phase components, UC, and graphite, exhibit no measurable

2
solubility in molten fluorides. These phases were identified by their

respective X-ray diffraction patterns. A serious anomally arises as a
result of the U02 phase identification since its formation is contrary to
the established phase diagram13 for the U-C system which shows UCZ to be
metastable with respect to U203 and graphite at temperatures less than
1500°C. On the basis of the uranium-carbon phase diagram and the accepted
free energies of formation for the uranium carbides at temperatures less
than 1000°K, U,C, should be the carbide phase which was identified. Never-

273
theless, UC, has been repeatedly shown to form at these temperatures and

2
has been established as the stable carbide phase in the equilibrium of Egq.

1-1. The reader who is interested in the details of this identification
i . 6 :
is referred to an earlier paper. In the present paper we have included a

series of equilibration experiments where excess U2C3 was used to reduce

UF4 solutions via the back reaction of Eq. 1~1. Results are compared with

similar experiments where U02 was used as a reductant.

One of the simplest and yet most convincing observations to offer for
the equilibrium is that the stoichiometry of the soluble uranium fluoride
species follows the four-to~three relationship of Eq. 1-1. When a solution

of approximately 0.1 mole % UF3 in LiF-—BeF2 is allowed to react with graph-

ite it is observed that 4 moles of UF3 form 3 moles of UF4. For example,

when a 0.068 molar solution of UF, was allowed to react via Eq. 1-1 to form

3
UFA’ it was observed that under conditions where reaction was more than 99%

complete, a 0.049 molar UF4 solution resulted. If the process were merely

one of UF3 oxidation, then 4UF3 should form 4UF4. For example:

4UF3(d) + 2MF < 4UF

2(d) * M (3-1)

4y T

where M is a metal such as Ni.

Finally the reversibility of the reaction was demonstrated by the re-
versible temperature dependence of the equilibrium. From a particular tem—
perature at which the system had attained equilibrium and concentrations of

UF3 and UF4 measured, the temperature could be repeatedly raised or lowered

causing the relative concentrations of UF, and UF4 to shift and attain equi-

3
librium concentrations at these new temperatures. When the system was
15

returned to the initial temperature, the original concentrations of UF3 and
UF& were reproduced. Quantitative aspects of the temperature dependence

will be given in the following sections.

The criteria tests established the equilibrium process as written in

Eq. 1-1. We found it more practical to measure the back reaction mechanism:

3UF, + UC, < 4UF, + 2C (3-2)

since, by intentionally adding excess UC2, we could insure that the molten
fluoride solution was always in contact with all the reactive solid phases.
Furthermore, we could interrupt the equilibration and add fresh UC2 to

demonstrate that the original carbide had not been consumed or altered dur-
ing the course of the reaction. This procedure also insured that more ac-—

tive reducing agents, including other uranium carbides, were not present.
An equilibrium quotient for Eq. 3-2 can be written:

4
(UF 5)

Q = (3-3)

(UF,)>
where UF3 and UF4 are expressed in mole fractions of the solution. Q is
simply the reciprocal of the equilibrium quotient, Q', for the forward
reaction of Eq. 1-1. The data in the following paragraphs will be presen-—
ted as Q values in terms of the back reaction and should not be confused

with forward action.

The effect of variables such as temperature, melt composition, carbide
composition and atmospheric contamination on the equilibrium of Eq. 3-2 in
the solvent system LiFwBer have been measured and are treated separately
in the following sections. Since the equilibrium of Eq. 3-2 (also Eq.1-1)
is the central theme of this paper, it will often be cited as simply “the

equilibrium."
Effect of Temperature on the Equilibrium

Previous results4 from the hydrogen reduction of UF4 in molten fluor-
ide solutions indicated that the temperature effect on the equilibrium of
Eq. 1-1 should be small. However, when we measured the equilibrium by
either the forward or the back reaction, we found it to be very sensitive

to temperature. This can be seen qualitatively by examining the molten
16

fluoride absorption spectra of Fig. 3-1 for equilibrium mixtures of dilute
UF3 and UF4 in LiF-BeF2 (66~34 mole %), in LZB’ over excess UC2 at various
temperatures. The spectra are due only to the UF3 and UF4 components of
the solution. Therefore, by comparing these spectra with the spectra of
pure UF4 and pure UF3 (Figs. 2-2 and 2-3 respectively), it can be seen that
at 500°C, most of the uranium in solution is present as UF4 whereas at
700°C, enough UF,
of Fig. 2-3. The composite spectrum at 600°C resembles neither of the two

is present to make the composite spectrum resemble that

pure component spectra but instead an intermediate mixture of the two.

The quantitative aspects of these spectra were calculated by the pro-
cedure described in the experimental section. From absorption spectra such

as those in Fig. 3-1 concentrations of UF,_, and UF4 were determined in mole

fractions and used in Eq. 3-3 to calculatg equilibrium quotients, Q, at
various temperatures. The data are presented in Table 3-1 along with Q
values which are then presented in Fig. 3-2 as 10g10Q versus l/TK (where TK
is the Kelvin temperature). At the top of the figure is shown the centi-

grade scale and at the right side of the figure, the equilibrium ratio,

[UF31

R=_—__..__—____.
{UF3]+[UF4]

(3-4)

where [UFB] and [UF4] are the concentrations in solution. (Note that the
denominator of Eq. 3-4 represents the total uranium fluoride in solution.)

These R values have been the customary manner in which UF -UF4 concentra-

3
tions are expressed within the MSRE program. The two lines drawn through

the data points represent the experimental uncertainty of the data which
arises mainly from the baseline error in the absorption spectra. Equilib-
ria at various temperatures were approached from both the high (open
circles) and low (closed circles) temperature direction. The system was

initially held at ca. 50°C above the temperature desired until the UF, con-

3

centration had ceased to grow (UF4 reacting with UC2 via Eq. 3-2). Then

the temperature was dropped 50° and the UF3 concentration was allowed to
fall by reaction of UF3 with graphite until no further change could be
detected. The equilibrium could be shifted repeatedly in this manner by

varying the temperature of the system. The train of points at any given
17

ORNL-DWG 72-9633
1 l [ I | I

 

ABSORBANCE

 

 

 

 

5000 1O,OOQ1 15,000
WAVENUMBER (cm )

3-1 Spectra of Dilute UF3-UF, Mixtures in LiF-BeFp (66-34 mole %)
Showing Temperature Effect on the Equilibrium: 4UF3 + 2C Z 3UF, + UCj.
18

Table 3-1

Typical Equilibrium data used in Figures 3-2 to 3-5 where

Q and R are defined by
Eqs. 3-3 and 3-4.

 

 

Run Solvent Carbide Temp Measured Absorbance Mole Fraction Q R
Phase  (°C) 11360 9170  UF.(10%)UF, (10%) (x108)
cm™ em 1 3 4
1 LZB uc, 500 0.130 0.515 0.264 7.25 0.128 } 0.035
2 L,B uc, 550 0.217 0.505 0. 804 7.22 11,12 10.10
3 L,B uc, 600 0.485 0.517 2,76 6.68 1951.0f 0.29
4 L,B uc, 650 0.692 0.327 4,57 6.14 19080.0{ 0. 43
5 LZB uc, 700 1.305 0,780 9.83 7.60 212300,0f 0.56
6 L,B U,Cq 500 0.254 0.538 0.99 7.17 26.0{0.12
7 ! " 600 0.409 0.416 2.36 5.29 2070.0; 0,31
8 " " 700 1.083 0.567 8.29 4.74 443000.00.64
9 LB uc, 370 0.147 0.384 0.53 5.52 4.6]0.087
10 " " 400 0.215 0.330 1.04 4.67 115.0(0.18
11 " " 450 0.380 0.328 2,23 4.36 2950.010. 34
12 " " 500 0.482 0.274 3.09 3.21 27600,0{0.49
13 MSBR UC2 550 0.925 1.317 3.97 18.31 401.010.18
14 " " 600 1.257 1.165 6.13 15.16 4050,.0[0.29
15 " " 650 2,575 1.433 14.0 14.32 129000,0{0.49

 

 
19

ORNL-DWG 72-10719R
TEMPERATURE (°C)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2500 9550 573 600 622 650 700
< I I I | /I
/ / 0.6
®
®
-3 /
/ / ® — 0.5
@
/ !/ — 0.4
-4 >
L/
o — 0.3 ¥
-5 _g If
QS $ u?
o 2
g s/ / 0.2
-6 ‘/ %5
I
@
7 :t/ — 0.4
_ /
8 i/
-9 — 0.03
1.30 1.25 1.20 145 .40 1.05 1.00
1000/7 (k)

3-2 Equilibrium quotients, Q = (UF3)4/(UF4)3, versus temperature
for UCy + 3UF4(d) 2z 4UF3(d) + 2C in the solvent LiF-BeFy (66-34 mole 7).
20

temperature represents the approach to equilibrium with only the lowermost
(for open circles) and uppermost (for closed circles) being the best measur-

ed equilibrium value.

The large temperature effect on the equilibrium is exemplified by Fig.
3.2 where the quotient, Q, shifts by 106 in going from 500 to 700°C. 1In
practical terms, this means that the concentration of UF3 relative to the
total uranium fluoride in solution is increased from ca. 5% at 500°C to ca.
60% at 700°C. The same large temperature effect on the equilibrium is
found when U203 (in place of UC2) is equilibrated with UF4 solutions. These
data are presented in Table 3-1 and the resulting Q values are plotted in
Fig. 3-3. Here the Q values are greater at a given temperature than in Fig.
3-2 and therefore support the identification of UC2 as the stable carbide
phase of Eq. 1-1. Furthermore the U,C, equilibration experiments demon-

273

strate that the UF, stability in dilute fluoride solutions as well as the

3
temperature effect on the equilibrium would not be far different from that
presented in Fig. 3-2, even if the identification of UC2 as the stable
carbide phase of Eq. 1-1 were not correct.

The data of Fig. 3-2 can be used to calculate the change in enthalpy
for the equilibrium. By defining the standard state of the solutes UF3 and

UF, as one mole percent in L, B, their activity coefficients are unity and

4 2
then the equilibrium quotients become equilibrium constants, K. The change
in enthalpy, AH, for the reaction in the temperature range of 500-650°C can
be calculated from the slope of the line in Fig. 3-2 using the expression:
d (1nkK)
d(1/T)

where R is the gas constant. The value obtained for AH of Eq. 3-2 is 99.3

AH = -R (3-5)

Kcal/mole which is surprisingly large in view of the enthalpy change calcu-
lated from enthalpies of formation for the pure, undiluted components at
either 298° or 800°K. These values are given in Table 3-2 and yield AH°
-10 Kcal/mole for the undissolved components of Eq. 3-2 at 298°K and ARC =

~12.90 Kcal/mole at 800°K. The process of solvation is not included in the
calculation since no heats of solution for UF3 and UF4 are available. It
should be noted that even the sign of the AH is different: We measure an

endothermic process whereas a slightly exothermic process is expected.
21

ORNL-DWG 72-10717
TEMPERATURE (°C)

 

 

 

 

 

 

1 500 550 573 600 622 650 700
| | l | ] l
-2 /
* 06
/s
. /
— 0.5
s 0 :v
. — 0.4
-4 o ‘f
O O
© 8 5 L
o 2
g g o —0.3
-5 s o 5
| i
x

 

 

 

W/ o
/

1.30 1.25 1.20 1.45 110 1.05 1.00

1000 /7 (o)

3-3 Equilibrium quotients, Q = (UF3)4/(UF4)3, versus temperature
for 1/2UC3 + 3UF4(q) < 4UF3(gq) + 3/2C in the solvent LiF-BeF; (66-34 mole %).

 

 

 

 

 

 

 

 

-8
22

Table 3-2
Enthalpy Data in (Kcal/mole)

Sources of the Data are from Tabulations referenced as Superscripts

 

 

 

 

 

 

UC2 UF4 UF3 C
MY 2013 ~450(5)° ~345(10)° 0
o .o 13 14 15 16
B Eo0g 8.79 14.99 11.8 1.83
Table 3-3

Equilibrium Quotients, Q, and Ratios, R,
for
UF3, UF4 Solutions in Atmospheric Contaminated System

(Taken from Ref. 24)

 

 

 

Solution 675°C 575°C
(Mole %)
LiF-BeF, R Q R Q
6634 025 2.7x107%0 o004 1.8x10713
7 10

48-52 .13 3x10° .03 6.2x10

 
23

Heats of solution could plausibly account for a large amount of the dis-
crepancy since it is observed that the heat of solution for CeF3 in LZB
(600-800°C) is 17 Kcal/mole whereas only 10 to 12 Kcal/mole is predicted.17
From the enthalpy data, without the heats of solution, we can only conclude
that the thermodynamic data is not adequate to predict the change in en-

thalpy for the reaction.

Effect of Solvent on the Equilibrium

 

In the same way that temperature shifted the equilibrium, changes in
the solvent composition did also. The original purpose of this research
was to demonstrate that changes in the fluoride ion concentration (which
have already been shown to affect the c60rdination behavior of dilute UF4
solutionslS) might be related to shifts in redox equilibria as well. The
effect of changing the solvent composition on the equilibrium is exempli-
fied by comparing the equilibrium quotients, Q, for LiF—BeF2 (48-52 mole %),
LB, in Fig. 3-4 with those previously shown in Fig. 3-2 for the LZB compo-
sition. At any given temperature (e.g., 500°C), Q is considerably larger
in the LB composition than in the corresponding LZB melt. Therefore the
equilibrium of Eq. 3-2 is shifted to the right by increasing the concentra-
tion of BeF2 in the solvent, i.e., by making the solvent more F  deficient
through the addition of a component which coordinates strongly with fluo-
ride ions. The ratio, R, of Eq. 3-4 at 500°C has been increased from ca.
0.05 for the L
3-4 at 500°C).

2B solvent to ca. 0.55 for the LB solvent (c.f. Figs. 3-2 and

The magnitude of this change can be compared with that which is pre-
3+

C ;o bt . 1

dicted from Baes' activity coefficients for M~ and M cations. ? Real-
izing that the only difference in equilibrium quotients between the two
melts is the ratio of the respective activity coefficients, v, for UF3 and

UF, raised to the appropriate powers:

4
3-4

370

TEMPERATURE (°C)

400

450

ORNL-DWG 72-40718

500

 

I

T

/

 

v

1.\l\\_

 

 

 

 

 

 

 

 

 

 

 

 

 

 

;/ |
/
/
/ ]

v/
% /
/

1.55 1.50 1.45 1.40 1.35 1.30

1000/7 (ok)

0.6

0.5

0.1

Equilibrium quotients, Q = (UF3)4/(UF4)3, versus temperature
for UCy + 3UF4(d) bl 4UF3(d) 4+ 2C in the solvent LiF-BeFj (48-52 mole 7%).
25

&) @)

QLB

K=4Q
L,B L3 o3
(&UFQ;) (zY ;)

where L2B and LB denote the solvent systems. Since Baes defines all activ-

3-6

ity coefficients as unity in the reference composition, LZB’ then:

&)
o5 \'UF%,
QL2B <:?3§;:f
3
19

The right-hand term can be estimated from Baes' data at 600°C™" where
YUF3::NbeF3 4::YThF = 10 by extrapolating to LiF—ger (48-52

mole %). By this procedure g/ g is estimated to be 4 x 107,
2

 

= 0,7 and YUF

From our data, at 600°C can be determined by extrapolating

QLB/QLQB
the double lines to 600°C and comparing this value, Q

read from Fig. 3~2. We find Q

LB® with the value,

QLzB’ LB/QL2B = 5 x 10" agrees reasonably

with the estimate from Baes' data. Even better agreement could be obtained
if a non-linear extrapolation (which is suggested by the trend in the data
of Fig. 3-4) is made. Furthermore, it should be noted that Baes' activity

coefficients are only approximate for UF, since they are actually based on

3
data for CeF,. The comparison serves to show that the magnitude of the

solvent effegt is in reasonable agreement with previous data and consequen-
tly must be considered when estimating UF3 stabilities in other molten
fluoride solvent systems.

This leads then to the practical question, '"What UF3 stability is ex-
pected in the MSRE and the MSBR solvents?". In these ternary systems the
relative measure of F concentration is more difficult to determine than in
the binary system LiF-BeF, since there are two "acidic™* cations in each
competing for fluoride ions. It is currently regarded that the MSRE sol-
vent is more F deficient than LyB and results of a previous electrochemi-

cal study of UFj3 stability by Manning20 support this contention. Little

 

*By "acidic" we mean, in the Lewis acid concept, the tendency to coordinate
with F .
26

attention has been given to the UF3 stability in MSBR solvents. We have

first attempted to predict it and finally we have measured it directly.

Realizing the solvent effects on UF, stability arise from changes in

3
the available free F , an attempt was made to estimate the F concentration
. . 18

in MSRE and MSBR solvents based on the earlier observation™  that the co-

ordination equilibrium of U4+ ions in LiF-BeF. solvents depended upon F

2
according to:
4"' 3 3- -
UF8 Z UF7 + F 3-8
We have previously suggested21 that the F~ could be measured by determining
the concentration of UF8 " and UF73— and then estimating the F concentra-

tion by Eq. 3-8. This method was found to work for LiF—BeF2 solutions with

Ber concentrations of up to 52 mole 7 and for the MSRE solvent which is

essentially a LiF—BeF2 solvent.

The method then was used to estimate the F concentration in the MSBR

solvent. Because the spectrum of UF4 in this solvent was largely UF8 y &
F concentration greater than that in LZB was suggested from Eq. 3-8. We

concluded that the stability of UF, would be very much less than in L,B,

3

] , 21
and in fact, some earlier UF, stability measurements” tended to support

this conclusion. ’

In contrast to this viewpoint, were activity coefficient data by
C. F. Baes19 and BF3 solubility data by S. Cantor22 which suggested that
the UF3 should be slightly more stable in the MSBR solvent than in LzB.

We examined this discrepancy by experimentally measuring the stability
of UF3 in the MSBR solution over excess UC2 in the graphite spectrophoto-
metric cell. The results are shown as logloQ Vs l/TK in Fig. 3-5 in the
same form as that used for previous figures. These data show that UF3 is
more stable in MSBR than in LZB' From the standpoint of reactor operatioms,
concentration ratios, R, of UF3 (c.f. Eq. 3-4) of up to 0.03 can be main-

tained safely down to the ca. 500°C freezing point of the solution.

The discrepancy in our earlier21 predictions can only be rationalized
by allowing a more complex coordination mechanism for the MSBR solvent than
+
is described in Eq. 3-8. This probably involves U4 which are fluoride

bridged to neighboring Th4+ or Be2+ so that, through bridging, the
log,g @

3-5
for UCy + 3UF,
(72-16-12 mole %

27

ORNL-DWG 72-12321
TEMPERATURE (°C)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

500 550 600 650 700
4 l T l /
-2 7
- 06
®
4 05
-3 / 33'
/s
@
/ - 04
<
_q / 5
o/ 103 o
s ~oL
: 3
/ >
O "
-5 L
8 Jo2 3
x
S/
- 01
7 7 /
-8
130 125 420 1145 110 105 100

‘1000/T (°K)

Equilibrium quotients, Q = (UF3)4/(UF4)3 versus temperature

(d

3 < 4UF3(q) + 2C in the solvent LiF-BeFp-ThFy
28

coordination number (and accordingly by Eq. 3-8, the F concentration) ap-
pears much larger. There is some evidence for this in LiF—BeF2 solvents

o 2
where the Ber concentration is greater than 52 mole Z%. 3

Effect of Atmospheric Contamination on the Equilibrium

 

In earlier attempts to measure the equilibrium quotients for Eq. 1-1,
it was apparent that the equilibrium concentration of UF3 in LiF-BeF2
solvents was unusually low24 compared with the present results. These
results are presented in Table 3-3 and were measured by following the re-
action of UF3 in graphite with no uranium carbides added directly to the
system. These reactions were always accompanied by the formation of UO2
and other unidentified solid phases. However after various improvements
were made which eliminated obvious signs of atmospheric contamination, such
as U0, formation, the stability of UF

2 3
sequently concluded that these earlier measurements involved equilibria of

was greatly enhanced. We have sub-

UF3 and UF4 solutions in graphite and an oxy-carbide phase (as opposed to
a pure carbide phase). It was never possible to identify the oxy-carbide
phase by X-ray analysis despite the fact that the equilibria were very easy

to reproduce from Eq. 3-2.

The effect of atmospheric contamination is clear -- it greatly reduces

the stability of UF, and is therefore a major factor which cannot be ignor-

3
ed when considering UF3 stability in molten fluoride solutions.

Effect of Temperature, Solvent and Contamination Compared

 

All of the effects of the variables have been collected to compare

their relative importance and are shown in Fig. 3-6 as loglOQ Vs T%l in

the same fashion as the previous figures but with a substantial reduction
in scale. The effect of increasing temperature is similar in all cases,

causing an increase in the stability of UF There is no reason for the

lines to be parallel to each other becauseBthey differ principally (except
for the case of the atmospheric contamination) in the activity coefficients
for UF3 and UF4 in the different solutions and these need not change pro-
portionately for all solutions. Neither should it be necessary that the
data be represented by straight lines, implying that AH for the reaction is

constant. They are used here only because the data are insufficient to
29

ORNL~DWG 72-12322
TEMPERATURE (°C)

 

 

 

 

 

 

 

o 370 400 450 500 550 600 650 700
I N I I | I | I
|
_1 . i i
_2 | _
0.6
-3 | / 105 ~
LL¢
o
+
» B 04 *,
2
~
0.3
5l Ty
i
02 &
)
o
g
© 0.1

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

..8 L —
_9 .
—10 —— (A) LiF-BeF, (48-52 mole %) T
(B) LiF-BeFp -Zr F4 (65.6 -29.4-5.0 mole %)
_yy L (C) LiF-BeFy-ThF, (72-16-12 mole %)
(D) LiF-BeF, (66- 34 mole %)
(E) LiF-BeF, (48 -52 mole %)
o OXIDE CONTAMINATED
(F) LiF-BeF, {66 -34 mole %)
OXIDE CONTAMINATED
i3 | | I |
1.6 1.5 1.4 1.3 1.2 1.1 1.0

1000/7 (o)

3-6 A comparison of equilibrium quotients versus temperature for
UCy + 3UF4(q) z 4UF3(d) + 2C in various solvent systems.
30

justify greater detail.

Decreasing the F concentration by the addition of BeF2 is very bene-
ficial in increasing UF3 stability whereas atmospheric contamination causes

the opposite and most disasterous effects on UF, stability.

3
Since the MSRE results do not come from our work, the MSRE line is

broken. The stability of UF, in the MSBR solvent is between that of the

MSRE solvent and L2B. It iSBObViOUS that the region of greatest UF3 sta-
bility is that of high temperature and low F concentration. We therefore
suggest that little UF3 could be maintained in F rich solvents such as
LiF-NaF-KF (46,.5-11.5-42.0 mole %) even if the reported K" reduction by

UF (25)

3 were not to occur. Conversely, the greater stability of pure UF§4)

(i.e., not dissolved in a molten fluoride solvent) is explained by the

absence of solvating F .

Other Considerations

 

If the thermodynamic data are sufficiently accurate then it should be
possible to calculate the free energy change for Eq. 3-2 in the solvent

LiF-BeF, (66-34 mole %) and then the equilibrium constant by:
AG = -RT 1n K (3-9)

The expressions for the free energy of formation are given in Table 3-4

for UCZ’ UF4 and UF3 (where the latter two are for the standard state of

 

Table 3-4

Free Energies of Formation for Pure UC, and the Solutes

2
3 and UF, in LiF-BeF, (66-34 mole %) with standard deviations,

0, in Kcal/mole for relationship: AG = A + B (TK/lDDO)

UF

 

 

Component A B g Source
uc, ~15.82 8.2 - Storms™>
UF, ~338.04 40.26 2 Baes??

26

UF4 -445,92 57.85 2 Baes

 
31

one mole percent each in LZB)‘ These free energy functions, when combined
in the proper stoichiometric proportions yield a change of free energy for
Eq. 3-2 of AG = 1.42-4.31 (TK/lOOO) with a significantly large combined
standard deviation of + 16 Kcal/mole. At 500°C, AG is -1.91 Kcal/mole and
the equilibrium constant from Eq. 3-9 is 3.5. Since these are standard
states for the UF3 and UF4 solutes, then Q is also equal to 3.5 and the
ratio, R, of Eq. 3-4 is 0.89. (c.f. with Q = 1.5 x 10—9 and R = (.03 in
Fig. 3-2). Therefore, from the existing free energies of formation for the
components of the reaction, practically all of a dilute UF3 solution in

contact with graphite should be stable. However, neither our results, nor

those from any other investigators support this high a stability of UF3.

A word of caution should be given at this point. It may seem obvious
to demonstrate UF3 stability via Eq. 1-1 by holding UF3 solutions in graph-
ite and allowing the UF3 to react with graphite. Furthermore, it may be
most convenient to generate a UF3 solution by reducing a dilute UF4 solution
with a strong reductant such as Be, Zr, or U metal within the same graphite
vessel that will be used for the stability measurement. We have observed
that this results in the formation of mixtures of UC, UZCB and UC2 phases
accompanied by the consumption of more reducing metal than is expected for
the complete UF4 reduction. The apparent anomally is caused by the revers-
ibility of Eq. 1-1 since as soon as UF3 is formed in excess of its equilib-
rium concentration within the graphite vessel, it reacts with graphite form-
ing uranium carbide phases and UF4 in solution. The UF4 ig, in turn,
reduced again by the excess reductant, forming more UF3. An example of the

process using Zr metal is:

4UF4 + Zr T 4UF3 + 27:1?4 (3-10)

4UF3 + xC 2 3U:a'4 + UC, (3-11)
so that the net reaction is:

UF4 + Zr + xC 2 ZrF4 + UCX (3-12)

This is one of the major reasons why we found it more practical to study
the equilibrium by the back-reaction mechanism of Eq. 3-2. Although the

UC and U203 phases do finally react leaving ultimately UCZ’ we found that
32

even for our small reaction system of less than 0.5 cc, it took an im-
practical length of time. Larger systems with smaller surface-to-volume

ratios would take even longer.

The question of reaction times brings up the final point to be mention-
ed, that is, the kinetics involved in achieving the equilibrium of Eq. 1-1.
Since UF3 is reacting with graphite to form uranium carbides, the mechanism
is obviously heterogeneous. It is consldered by these authors far too dif-
ficult a mechanism to attempt to clearly describe; but 1f reaction rates
are sought, the initial measurements should demonstrate that the mechanism
is heterogeneous by varying the surface-to-volume ratios of the reacting
system. We predict that the outcome of such a measurement will substan-
tiate the heterogeneous mechanism. Another point of caution should be made.
Since larger surface-to-volume ratios mean slower reaction rates, apparent

high stabilities of UF., may appear whereas they actually involve metastable

3
states of the equilibrium mechanism which include uranium carbide phases

other than UCZ' These other carbides will ultimately be converted to UC2

by the mechanism of Eq. 1-1; but, until the conversion is completed, the

UF3 ratio, R, will remain fixed at a high value.

The ultimate aim of the UF3 stability study has been to describe con-

ditions under which certain UF3 ratios can be maintained in graphite. To

demonstrate the validity of our measurements we mixed dilute UF3 and UF4 in
the LB solvent so that the resulting solution had a UF3 ratio, R = 0.17.
The solution was maintained for a period of a week in the graphite spectro-

photometric cell at 475°C with no loss of UF. or UF4 from solution. (c.f.

3
Fig. 3-4 which shows the maximum R at 475°C to be 0.40-0.45).
33

References

1) W. R. Grimes, '"Chemical Research and Development for Molten Salt
Breeder Reactors', ORNL-TM-1853, 1967.

2) M. W. Rosenthal, P. N. Haubenreich, H. E. McCoy, L. E. McNeese,
At. Energy Rev., 9 [3], 601 (1971).

3) W. R. Grimes, Nuc. Application and Technology, 8, 137 (1970).

4) G. Long and F. F. Blankenship, "The Stability of Uranium Trifluoride",
Part I and II, ORNL-TM-2065, 1969.

5) M. H. Rand and 0. Kubaschewski, The Thermochemical Properties of

 

Uranium Compounds, Interscience Publishers, New York, 1963.

 

6) L. M. Toth and L. 0. Gilpatrick, "Equilibria of Uranium Carbides

in Molten Fluoride Solutions of UF3 and UF4

J. of Inorg. & Nuclear Chem., In press.

Contained in Graphite at 850°K",

7) L. M. Toth, J. P. Young and G. P. Smith, Anal. Chem. 41, 463
(1969).
8) Preparation of BeF; performed by B. F. Hitch of ORNL.
9) James H. Shaffer, "Preparation and Handling of Salt Mixtures for
the Molten Salt Reactor Experiment', ORNL-4616 (Jan. 1971).
10) Poco Graphite Inc. a subsidiary of Union 0il Co. of California,
Decatur,Texas 76234,
11) J. P. Young, Inorg. Chem. 6, 1486 (1967).

12) S. Cantor, Reactor Chem. Div. Annu. Progr. Rept. for Period Ending

 

December 31, 1965, ORNL-3913, p. 27.

 

13) E. K. Storms, Refractory Materials, Vol. 2, "The Refractory

Carbides", Academic Press, New York, 1967.

14) A. S. Dworkin, J. Inorg. Nucl. Chem. 34, 135 (1972).
34

15) C. E. Wicks and F. E. Block,*Thermodynamic Properties of 65
Elements, Their Oxides, Halides, Carbides, and Nitrides% Bureau of Mines
Bulletin 605 (1963).

16) JANAF (Joint Army-Navy-Air Force) Interim Thermochemical Tables,
Thermal Research Laboratory, Dow Chemical Co., Midland, Mich.

17) C. J. Barton, M. A. Bredig, L. O. Gilpatrick, J. A. Fredricksen,
Inorg. Chem. 9, 307 (1970).

18) L. M. Toth, J. Phys. Chem., 75, 631 (1971).

19) C. F. Baes, Jr., MSR Semiannu. Progr. Rept. for Period Ending
Feb. 28, 1970, ORNL-4548, p. 153. |

20) D. L. Manning, private communication, 1970.

21) L. M. Toth, L. O. Gilpatrick, MSR Semiannu. Progr. Rept. for

 

Period Ending Aug. 31, 1971, ORNL-4728, p 77.

22) S. Cantor, MSR Semiannu. Progr. Rept. for Period Ending

 

23) 1. M. Toth, unpublished results.

24) L. M., Toth, L. O. Gilpatrick, MSR Semiannual Progr. Rept. for

 

Perjiod Ending Aug. 31, 1971, ORNL-4728, p. 77.

 

23) F. F. Blankenship, B. H. Clampitt, W. R. Grimes, S. Langer,

"The Stability of UF, in Molten Fluorides," unpublished data.

3
26) C. F. Baes, Jr., "The Chemistry and Thermodynamics of Molten
Salt Reactor Fuels,' presented at AIME Nuclear Fuel Reprocessing Symposium

at Ames Laboratory, Ames, Iowa, August 25, 1969, Published 1969, Nuclear

Metallurgy Symp., Vol. 15 by the USAEC Div. of Tech. Information Extension.
. L. Anderson
F. Baes
Bamberger
Barton
Bettis
Blanco
Blankenship
. Bohlmann
. Boyd
Braunstein
A. Bredig

B. Briggs

R. Bronstein
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Compere
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DeVan
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Engel
Ferguson
. Ferris
Frye
Gabbard
Gallaher
Gilpatrick
Grimes
Guymon
Handley
N. Haubenreich
R. Kasten

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Technical

35

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