I
H. A. BERNHARDT, E. J. BARBER, and R. A. GUSTISON' Oak Ridge Gaseous Diffusion Plant, Union Carbide Nuclear Co., Oak Ridge, Tenn.
Chemistry of Processing Metallic Uranium Fuel Elements with Chlorine Trifluoride Uranium hexafluoride of very high purity is obtained
A
FLUORINATION PROCESS has been developed for converting irradiated uranium directly to uranium hexafluoride by reaction with a solution of liquid chlorine trifluoride and hydrogen fluoride ( 9 ) . The uranium hexafluoride is separated from the reaction mixture, plutonium tetrafluoride, and fission product fluorides by distillation (5). The nonvolatile plutonium and fission product fluorides are subsequently dissolved by aqueous treatment (2) for processing. Ithen irradiated uranium has been allowed to decay until the beta-active uranium-237 no longer constitutes a problem in handling the purified uranium hexafluoride (minimum decay time of 70 days), the volatile fission product fluorides produced by the reaction with chlorine trifluoride can readily be separated from uranium hexafluoride by fractional distillation. Except for the maximum valence tellurium, iodine, and molybdenum fluorides (TeF6, IF,, and MoFB) which are more volatile, the fission products remaining after a 70-day decay period give fluorides less volatile than uranium hexafluoride. M'hile plutonium and uranium hexafluorides are so similar in volatility (3) as to be inseparable on a practical basis, the plutonium is con-
verted only to the nonvolatile tetrafluoride; no difficulty in the separation of uranium from plutonium is encountered.
Chlorine monofluoride reacts readily with fluorine at 300" C. to regenerate chlorine trifluoride (73) :
Uranium metal may be directly converted to the hexafluoride with gaseous fluorine ( 7 ) , but the high temperature necessary (400" C.), release of a large quantity of heat ( 2 . 2 X IOs kcal. per kg.) which is difficult to remove in a gas-solid reaction: alternate formation and cracking of a lower fluoride film on the surface, and formation of plutonium hexafluoride make the processunfeasible. Fluorination with gaseous chlorine trifluoride ( 4 ) proceeds at 280" C., but otherwise this gas has no advantages over fluorine. Use of a liquid fluorinating agent would reduce the heat-removal problem, furnish milder fluorinating conditions, and avoid plutonium hexafluoride formation. The reaction proceeds thus:
A H ; s 8 = -33 kcal./mole
u(s)+ 3ClF3(1)
4
UFB(I)
Figure 2. Reaction rates at 30" C. generally decrease wifh increasing uranium hexafluoride concentration
20 SOLUTION
40 COMPOSITION,
fluoride mole ratio of about 0.3, the
At o chlorine trifluoride-hydrogen reverse effect occurs
Present address, Electro Metallurgical Co., Niagara Falls, N. Y.
0
+ 3ClF(g)
= -424.5 kcal./mole
Combining the two reactions gives all the advantages of fluorination in liquid medium but consumes only fluorine. The rate of reaction between uranium metal and pure chlorine trifluoride solution containing dissolved reaction products is primarily a function of the surface area of the uranium and the reaction temperature and is independent of uranium hexafluoride concentration (4, !)). The reaction rate obtained with pure liquid chlorine trifluoride at 30" C., 2.5 X 10-8 mole of uranium per sq. cm. of metal surface per second, is obviously too slow for a feasible process; however, anhydrous hydrogen fluoride accelerates the reaction rate (Figure 1). The maximum initial rate of reaction, at approximately 60 mole 70 chlorine
4501
60
80
I
1
I
l
l
I
I
I
I
I
l
100
MOLE PERCENT CIF,
Figure 1. Initial reaction rates a t 30" C. in absence of d i s solved uranium hexafluoride show greatest acceleration in mole chloride about 40 mole % hydrogen fluoride-60 trifluoride
70
-0
5 10 COMPOSITION,
VOL. 51,
NO.
2
15 20 M O L E PERCENT
25 UFs
FEBRUARY 1959
30
179
MOLE
O F CIF,/HF
RATIO
Figure 3. Maximum reaction rate at 30" C. is obtained at a chlorine triftuoride-hydrogen fluoride mole ratio about 0.3 with reagent solutions saturated with uranium hexafluoride at 25" C.
2.8
2.9
RECIPROCAL
3.0
3.1
3.2
3.3
3.4
3.5
3.6
ABSOLUTE TEMPERATURE, I/T x IO3
Figure 4. Plot qf the logarithm of K as a function of the reciprocal of the absolute temperature indicates greater dependence of reaction rate on temperature above 48" C.
trifluoride or a 1.5 molar ratio of chlorine trifluoride to hydrogen fluoride, is more than an order of magnitude faster than the rate obtained with pure chlorine trifluoride a t the same temperature. The change in reaction rate per unit area of metal with time was correlated with increase in uranium hexafluoride concentration during the reaction. Addition of uranium hexafluoride to solutions that gave maximum rates in the absence of uranium hexafluoride sharply depressed the rate of reaction; with other solutions the depression was less marked. and, at a chlorine trifluoride-hydrogen fluoride molar ratio of about 0.3, the reaction rate increased with the first small increments of uranium hexafluoride (Figure 2). Other experiments at a mole ratio of 0.3 with increased uranium hexafluoride concentrations showed the rate to be essentially independent of uranium hexafluoride concentration. Studies with dissolver solutions nearly saturated with uranium hexafluoride showed maximum rates of dissolution at a 0.3 mole ratio of chlorine trifluoride to hydrogen fluoride (Figure 3). Interpreted processwise, with a chlorine trifluoride-hydrogen fluoride ratio of 0.3, an adequate reaction will be maintained. Using the optimum composition of the dissolver solution Lchich contained varying quantities of uranium hexafluoride, the temperature dependence of the reaction was investigated between 10" and 90' C. (Figure 4). The reaction proceeded smoothly and gave no indication of becoming uncontrollable at elevated temperatures. The rate of attack of the uranium per unit surface area of metal was independent of time and uranium hexafluoride concentration. The change in slope of the curve in Figure 4 at about 48" C. suggests a possible change in the mechanism of attack. The equations for the rate constant below and above 48" C. are Log K
- 759.8 T
=
-4.5041
*=
5.6783 - 4017.;/T
(1)
and
T 1200
Log K
f0
where I; is the moles of uranium dissolved per square centimeter per second. The activation energy of 18.5 kcal. per mole indicates the reaction is markedly more temperature-dependent above than below 48" C., where the activation energy is only 3.6 kcal. per mole. Mechanism of Reaction. Pure chlorine trifluoride undergoes ionization to a slight extent
'8 1000
X
> 800
k
1
&
600
3
0
400 V
2 200
2ClF3(1)
LL
0 W
L m
O
0
Figure 5.
IO
20 30 40 50 6 0 70 COMPOSITION, MOLE PERCENT
80
90
100
HF
Chlorine trifluoride-hydrogen fluoride solutions containing more hydrogen fluoride are more highly ionized than pure chlorine trifluoride or hydrogen fluoride
50 mole
18 0
(2)
% or
INDUSTRIAL AND ENGINEERING CHEMISTRY
+ ClF:(I) + ClF;(I)
However, the anion is unstable, essentially preventing self-ionization in the pure species; the electrical conductivity at 0 " C. is only about 3 X 10-9 (ohmcm.) -1. Addition of hydrogen fluoride increases the conductivity (9) (Figure 5 ) because it allows formation of the more
NUCLEAR PROCESSING
0
02
04
RY TIC
CIFJ
Figure 6. The principal features of interest in condensed phase equilibria for uranium hexafluoride-chlorine triare low solubility of fluoride-hydrogen fluoride ( 1 I uranium hexafluoride in hydrogen fluoride and existence of a shallow miscibility gap having a Minigap temperature (lowest temperature at which miscibility gap exists) of 5 3 ” C. at 49 mole uranium hexafluoride-13 mole % chlorine trifluoride-38 mole hydrogen fluoride and an upper consolute temperature of I O 1 ’ C. at 44 mole % uranium hexafluoride-56 mole hydrogen fluoride
yo
70 70
+ HF,,,
Cll’:,:~.
ClF3,HF(,, CIF+?(I, FHF-c,)
*
+
The intermediate compound, CIF3.HF, was identified in the gas phase by Pemsler and Smith (70). The increased activity
of chlorine trifluoride in hydrogen fluoride solution is believed to result from increased ClF+z concentration. Isolation of Uranium Hexafluoride
from Uranium Hexafluoride-Hydrogen Fluoride-Chlorine Trifluoride Solution. Success of the process depends
08
10
Figure 7. Liquid-vapor equilibria of chlorine trifluoride-hydrogen fluoride system (8) show a minimum boiling azeotrope, 67 mole % chlorine trihydrogen fluoride fluoride-33 mole
70
on the isolation of high purity uranium hexafluoride from the reaction mixture. Knowledge ofthe phase equilibria is vital for understanding the difficulties in the distillation operation and developing techniques to overcome them. Obviously, (Figure 6), the distillation must be carried out above the triple point of uranium hexafluoride (64.02’ C.), and the distillate is more conveniently withdrawn from the vapor than the condensed phase (or phases). The important features of the liquidvapor equilibria of the ternary system have been deduced from the liquid-vapor equilibria of the three binary systems (Figures 7, 8, and 9), exploratory studies of the ternary system by Crews and Davis of this laboratory, and analyses of the distillate from a distillation a t a constant temperature of 6 6 ” C. The tepary system contains no ternary azeotrope, but an “azeotropic trough” connects the binary azeotrope occurring at 67 mole % chlorine trifluoride-33 mole % hydrogen fluoride with the binary azeotrope in the uranium hexafluoride-hydrogen fluoride system. The term “mole” is construed to I
stable bifluoride anion,
06
F R A C T I O N CIF3
MOLE
VOL. 51, NO. 2
.
FEBRUARY 1959
181
W
$
VAPOR LIQUID
I
70
w
L 3 I-
2
w
60
P
5I50
40
30
0
0.2 04 C O M P O S I T I O N , MOLE
0.8
0.6 FRACTION
10
UFs
Figure 9. Positive deviations from Raoult's law occur with a miscibility gap having an upper consolute temperature of
0.2
0 MOLE
0.4
FRACTION
0.6
0.8
1.0
CHLORINE T R I F L U O R I D E
Figure 8. Liquid-vapor equilibria of uranium hexafluoride-chlorine triflworide (7) indicate uranium hexafluoride i s separable from chlorine trifluoride a t 3 atm. or higher without appearance of a solid phase
101" C. (6,72) The four-phase invariant temperature occurs at 61.2' C. At 66" C. the minimum boiling azeotrope contains about 14 mole uranium hexafluoride
70
tion, so that all the uranium could be recovered as the purified hexafluoride, was determined experimentally by distilling at 66" a synthetic solution which contained ( u ) excess uranium hexafluoride and (6) chlorine trifluoride and hydrogen fluoride in a molar ratio of 0.28 to 1. The fractionation was carried out at a reflux ratio of 4 to 1 in a column determined to have 23 theoretical plates a t total reflux. The separation between the chlorine trifluoride-hydrogen fluoride azeotrope distilling at 107 p.s.i.a. and the uranium hexafluoride-hydrogen fluoride azeotrope distilling at 85 p.s.i.a. is not particularly sharp: but the separation of the latter azeotrope from pure uranium hexafluoride is very sharp. The combined distillate from the azeotropic fractions contained 19.5 mole 70 chlorine trifluoride, 69.7 mole 70 hydrogen fluoride, and 10.8 mole 7 0 uranium hexafluoride. Vse of a dissolver solution of the above composition assures a maximum dissolution rate and the uranium hexafluoride produced can be recovered by distillation.
c.
mean one simple formula weight, although hydrogen fluoride is known to be associated in systems of this kind (6). The composition of uranium hexafluoride-hydrogen fluoride azeotrope changes from 12 to 18 mole yo uranium hexafluoride as the distillation temperature is varied from 62" to 90' C. The maximum pressure a t a given temperature is obtained with the composition corresponding to the binary azeotrope in the chlorine trifluoride-hydrogen fluoride system and may be calculated from the data of McGill, Wendolkowski, and Barber ( 8 ) . Separation of the chlorine trifluoride-hydrogen fluoride azeotrope from ternary mixtures having compositions lying in the vicinity of the "azeotropic trough" requires use of an efficient distillation column. Distillation of a mixture in which the chlorine trifluoride-hydrogen fluoride molar ratio is less than about 2 to 1 will result in lossof some uranium hexafluoride as uranium hexafluoride-hydrogen fluoride azeotrope before pure uranium hexafluoride can be recovered. As the objectives include recovery of all the uranium as the purified hexafluoride, three possible procedures have been examined.
1 82
1. The amount of hydrogen fluoride could be limited, so that solutions are always richer in chlorine trifluoride than the chlorine trifluoride-hydrogen fluoride azeotrope. The relatively slow reaction rate which decreases with increasing uranium hexafluoride concentration (Figures 2 and 3) makes the process noncompetitive economically. 2. The dissolution could be run with the optimum chlorine trifluoride-hydrogen fluoride molar ratio of about 0.3 to 1 and sufficient chlorine trifluoride added to bring the ratio to 2 to 1. An additional operation must be performed to break the chlorine trifluoride-hydrogen fluoride azeotrope before these materials could be used for the next batch. 3. The generally preferable alternative is to start with a dissolver solution which contains enough uranium hexafluoride (of the same assay as the uranium to be dissolved) to satisfy the azeotropic requirements of a system containing chlorine trifluoride and hydrogen fluoride a t a molar ratio of about 0.3. If batches of uranium metal of widely different assays were being dissolved, use of alternatives 1 or 2 probably would be necessary. The uranium hexafluoride concentration required in the initial dissolver solu-
INDUSTRIAL AND ENGINEERING CHEMISTRY
Plutonium
Fluoride
Dissolution.
Another requirement for successful application of the chlorine trifluoride treatment to processing irradiated uranium metal is a method of dissolving the residues that contain the plutonium and nonvolatile fission product fluorides.
NUCLEAR PROCESSING The volatile components of the system, including fission product fluorides and uranium hexafluoride, are removed by simple distillation, leaving plutonium tetrafluoride and nonvolatile fission product fluorides behind as a difficultly soluble residue. Several alternative processes, including direct oxidation, metathesis with a basic slurry followed by dissolution of the slurry in acid, complexing of the cations with a stronger complexing agent than fluoride ion, and complexing of the fluoride ion with metals such as aluminum, iron, and zirconium, have been considered. The last two techniques were investigated (2), because they showed promise of being successful without unduly complicating operations. A complexing agent, L'ersene in 1% solution at pH 7 to 9, dissolved more than 99% of the plutonium and fission product fluorides in a few days at room temperature. A 5% solution of aluminum nitrate acidified to pH 0.5 \vith nitric acid when agitated at room temperature dissolved more than 99% of the plutonium and fission product residue in 1 day. At 100°C. dissolution with the latter solution is rapid, and the residue left in a reactor probably can be dissolved in 1 to 3 hours. The efficiency of the two dissolving solutions, both readily adapted to solvent extraction, was estimated by studying the solubility of plutonium tetrafluoride. The lY0 L'ersene solution. at pH 9, dissolved 4.3 grams of plutonium per liter of solution, and the 5% aluminum nitrate, acidified to pH 0.5, dissolved 23.2 grams under comparable conditions. As the Versene is unstable toward radiation and is less efficient than aluminum nitrate, a 5 7 G aluminum nitrate solution acidified to pH 0.5 has been used in the plutonium-recovery cycles. Corrosion Studies. Monel and nickel are excellent materials of construction for fluorination equipment, but corrode badly at elevated temperatures (2) in a 5% solution of aluminum nitrate at p H 0.5. Because the plutonium-bearing residues must be dissolved in hours, thus requiring temperatures near 100' C., stainless steels. types 302 and 347, known from redox corrosion studies to be satisfactory with acidified aluminum ni-
Table l. Stainless Steel, Types 302 and 347, Shows No Attack at 80" C. after 6 to 10 Days' Exposure UFO, Wt.
HF,
%
7'0
%
100 50 45 20
50 50 65
5 15
ClFs,
Soln. No.
n-t .
1
2 3 4
..
wt.
.. ..
.---
I
r -
I
I
I
L
I I
D ISSO LV E R SOLUTION
I
4000 9. C I F 3 500g. HF
I
I
----------
I DISSOLVER S O L U T I O N ~ 1 MAKE-UP I
1
_________--
lesoo
CHARGE
"
-
,
J
S T I L L FEED 3700 9. uF6
4000 9. CIF3 500 g. H F
-
I I I
I I I
I I I
I I I
I
I
I I
SOLUTION 300 9. AI(NO,),
I
2 g. P u - 2 g . F.F? 300 9. A I ( N o 3 1 3 370g. H N O 3 6 0 0 5 g . HZO 44 g. F e ( N H 2 S 0 3 1 2
59109. H20 449. F e ( N H 2 S 0 3 ) 2
T O Pu RECOVERY
t
I I
c
I
I I I I
[
I
L -DISSOLVER
500g. HF-
J
IC/ I
SOLUTION RECYCLE S T R E A M
I I
Figure 10.
i (
STILL
Pilot plant experiments involved dissolution, distillation, and recovery
trate solutions a t 100" C., were tested for corrosion during the fluorination cycle in various fluorination mixtures. IVhile the corrosion resistance demonstrated (Table I) was satisfactory, more extensive corrosion studies will be required before the length of life of a plant
made from these types of stainless steel can be estimated. More recent work at Argonne National Laboratory, which indicated that sulfamic acid is an effective corrosion inhibitor for attack of Monel by aqueous aluminum nitrate, makes use of Monel most attractive. VOL. 51, NO. 2
FEBRUARY 1959
183
Stability of Chlorine Trifluoride to Radiation (74). Limited investigations
diameter, is estimated as 19 hours. When the reaction was complete, as indicated by cessation of fluorine consumption, the reaction mixture was transferred as a vapor, to the fractionating column. I n the simple distillation from the dissolver a beta-decontamination factor of lo4 to 105 and a gamma-decontamination factor of about lo3 were achieved. Distillation. The distillation column was constructed of a 4l//~-footlength of 3j4-inch Monel tubing packed with Podbielniak Heli-Pak. Under total reflux and a throughput of 200 ml. per hour, the column had about 23 theoretical plates, giving an HETP (height equivalent to a theoretical plate) of 1.93 inches. The over-all beta-decontamination factor for the process varied from 6 X 106 to 20 X 106, whereas the gamma-decontamination factor was about 10' (Table 11). During the first run a ICW product yield of 5370 was obtained, because a completely adequate distillation technique was developed only after completion of this run. The last two runs here made with the column head operating a t 66' C. during the entire distillation. This ensured that all components of the system were liquid a t all times. Because the column was a batch column and the holdup was about 20 to 25y0 of the total amount of uranium hexafluoride charged, a product fraction containing 75 to 80% of the uranium charged would be the largest obtainable. Table I1 also shows decontamination effected for individual fission products. Separation of chlorine trifluoride from uranium hexafluoride by distillation is
have been made on the rate of decomposition of gaseous chlorine trifluoride under a-particle irradiation at room temperature. Chlorine monofluoride and fluorine are the predominant decomposition products. Calculation indicates that less than one molecule is decomposed per ion pair or that the yield is in the order of one molecule of chlorine trifluoride decomposed per 100 ev. energy absorbed by the gas. Pilot Plant Experiments Using Irradiated Uranium Metal
Essentially three unit operations were involved in the pilot plant experiments (Figure 10) : conversion of irradiated uranium to the hexafluoride; distillation of uranium hexafluoride from the dissolver solution, plutonium, and fission products; and recovery in a n aqueous solution of the plutonium and fission product fluorides. Dissolution. I n a typical dissolution about 2.5 kg. of irradiated uranium, decayed 90 days, was dissolved in 4.5 kg. of dissolver solution at 30 ' C. The pressure of fluorine was allowed to vary from 30 to 50 p.s.i.g. About 5 days were required to dissolve, at 30' C., all of a batch of uranium slices varying from to inch in thickness with a dissolver solution containing about 2 moles of chlorine trifluoride per mole of hydrogen fluoride. Using a dissolver solution having a chlorine trifluoride-hydrogen fluoride mole ratio of 0.3 to 1 a t 80' C., the dissolution time for an unsliced slug, 1.43 inches in
Table II. Distillation and Decontamination Data for High Activity Level Runs Run No. 1 2 3 Throughput, ml./hr. 155 200 200 Reflux ratio 50 ' 13.7 7.5. HETP, inches 2.7 9.0 18 Theoretical plates 20 6 3 Still charge 2930 327 Beta activity" Gamma activitya 3.37 x 105 1 x 106 6.4 0.66 Pu, p.p.m. Product 5 1.5 5 Beta activitya 21 96 73 Gamma activity" 6 5
