I
ROBERT P. MILFORD Chemical Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tenn.
Engineering Design of Oak Ridge Fluoride Volatility Pilot Plant N e w facility will demonstrate uranium recovery from spent zirconiumenriched uranium reactor fuels by volatilization of uranium hexafluoride and absorption on sodium fluoride
Tm
fluoride volatility process appears promising as a method of recovering the uranium from nuclear reactor fuel elements of the zirconium type. This nonaqueous process consists of the following steps:
1. Dissolution of the elements with anhydrous hydrogen fluoride in a sodium fluoride-zirconium tetrafluoride melt at
4HF
+U
NaF-ZrF4 650° C.
UFd
(2)
2. Formation and volatilization of uranium hexafluoride from the molten salt by contact with excess fluorine at 600' C.: UF4
+ Fz
NaF-ZrFd 600" C.
UFat
(3)
Further purification of the ura-
3.
F2 D I S P O S A L Y
Z r - U FUEL ELEMENTS
Na F - Zr F4
+ 2Hz t
4NHYDROUS
1
H2
+ HF
t
F6 PRODUCT
UF6 t F2
c
HYDRON ~ F - Z ~ F ~ - U FUSED F~ SALT FLUORINATION FLUORINATION 650' C 600 - 6 5 0 ° C
-
t
WASTE NaF-ZrF4
1
WASTE NoF
Uranium i s recovered from spent fuel elements by a fused salt fluoride volatility process
nium hexafluoride by absorption on sodium fluoride at 100' C., followed by desorption a t 100' to 400' C. with fluorine. 4. Collection of decontaminated uranium hexafluoride in cold traps. The chemistry of this process has been described by Cathers (7, 2). A similar process, in which the final purification is effected by fractional distillation of the uranium hexafluoride from bromine pentafluoride, the fluorinating agent, is being studied a t Argonne National Laboratory ( 5 ) . A pilot plant has been constructed a t , Oak Ridge National Laboratory to demonstrate this process. I t is equipped to study the process from the fluorinator through fluorine disposal with a synthetic radioactive fuel solution. After the hydrofluorination equipment has been installed, the plant should be capable of processing high-level zirconiumcontaining fuel elements such as are used in the Nautilus. The hydrofluorination step is not discussed in this article. Location The facility is located in cells 1 and 2 of Building 3019 a t Oak Ridge National Laboratory. This building was constructed during the early days of the Manhattan Project to house the pilot plant for demonstrating the bismuth phosphate process for the recovery of plutonium from irradiated natural uranium slugs. The cells are shielded with 5 feet of concrete on the operating gallery side and between cells, and with 4 feet of concrete elsewhere. EquipVOL. 50, NO. 2
FEBRUARY 1958
187
ment containing molten salt is located in cell 1, which is 11 X 19 X 27 feet A concrete wall 15 inches high. thick across cell 1 forms a 3l/2 X 11 foot subcell, which houses the waste salt discharge nozzle and carrier. Both items are unit-shielded to keep the subcell accessible. Cell 2 is 20 x 19 x 27 feet high and contains the absorbers, cold traps, and productreceiving equipment. The incoming uranium hexafluoride line and the first absorber are unit-shielded to make cell 2 a relatively free-access area. The fluorine-disposal system is located above cells 1 and 2 in a free-access area.
deflected back down into the melt by a cone above the draft tube and mounted on the 1-inch pipe. The vessel is also equipped with a drain line, a 4-inch gas outlet, high and low-pressure bubbletype liquid-level probes, thermowells, and provisions for introducing five corrosion specimens ' 1 4 inch in diameter. To eliminate the possibility of damage to furnace heating elements by a molten salt leak from the fluorinator, an Inconel liner is installed between the outside of the fluorinator and the furnace elements. To melt salt spray from the upper walls of the fluorinator, rod-type electric heating elements are installed on the portion of the fluorinator outside the furnace. The heating elements are clipped to the vessel and the entire vessel is wrapped with strips of stainless steel shimstock to provide more uniform heat transfer. A layer of diatomaceous silica-asbestos fiber insulation 4 inches thick is used over the stainless steel strips. After the uranium hexafluoride has been volatilized from the molten salt, the fluorinator is isolated from the rest of the system and the barren salt is forced out of the fluorinator and into a shielded waste can by nitrogen gas pressure.
Fluorinator
The fluorinator is fabricated from L-nickel (0.02% carbon maximum) and is 14 inches in outside diameter, 53l/2 inches straight side, and has a lj4-inch wall thickness. The bortom of the vessel is a flanged and dished head 14 inches in outside diameter, with 3/8inch wall thickness. The top closure consists of a 150-pound standard Incone1 blind and matching slip-on flange, both lvith ring-type joint facing for a copper ring gasket. The lower 19 inches of the vessel is heated to give it an effective capacity of approximately 1.4 cu. feet of salt. The heat is supplied by a vertical pot-type furnace rated at 30 kw. A draft tube or airlift is used to bring the molten salt in contact Lvith fluorine. Fluorine is introduced into the vessel through a I-inch schedule 40 nickel pipe whose lower end is surrounded by a 4-inch schedule 40 pipe, 18 inches long, to form the draft tube. Molten salt is pumped through the annulus between the 1- and 4-inch diameter pipes and
y
Molten Salt Handling
One of the many interesting problems encountered in the design of this plant was that of handling fused salts. Although rod-type electrical heaters banded with stainless steel shimstock are used in a few cases to maintain the temperature of molten salt transfer lines above the melting point of the salt, the principal method of heating was "autoresistance" -a process of heating pipes or tubes by passing electrical current through the pipe or tube walls. Inconel is used be-
cause of its high electrical resistivity, coupled with its corrosion resistance to the fused fluoride salts being transferred. The I/a-inch schedule 40 pipe used in the pilot plant, when insulated with 2 inches of diatomaceous silica-asbestos fiber, requires 245 amperes of current and 0.5 volt per foot. Freeze valves effect a gas-tight seal in a molten salt transfer line. The valve is merely a loop in the Inconel transfer line, heated and insulated, but vented ar the inlet and outlet ends of the loop. The vent lines are surrounded by a tubular furnace which is normally not operated. However, if molten salt is accidentally forced into the vents, it will freeze and can be melted out of the vent lines with the furnace. Although these valves have been thoroughly tested against leakage and rupture of the pipe, blowing the salt out of the seal as a vessel becomes empty presents a problem. Decontamination of Uranium by Absorption on Sodium Fluoride
Although excellent decontamination from the nonvolatile fission product fluorides-e.g., cesium, strontium, and the rare earths-is obtained in the fluorination step, the volatile fission product fluorides must be removed by another method. In the plant described this is done by passing the uranium hexafluoride and volatile fission product fluorides (mostly niobium and ruthenium in the case of 120-day or longer decayed uranium) into the first of a pair of absorbers, which is a t 100" C. Both absorbers are charged with 1 cubic foot of ',/g-inch sodium fluoride pellets. The uranium hexafluoride forms a complex with sodium fluoride a t 100' C. according to the reaction
Q
3 0 3 6 9
M
INCHES
q # -! &Fz \/k-ir
I
MATERIAL L-NICKEL ~
4 Fluorination vessel and
$6 in I
furnace
NLET
P-ATE
Ln 0
Freeze valve effects a gas-tight seal in molten salt line SALT TRANSFER PIPE SCrl 40 INCONEL
'/?-In
L BARSTOCK
188
INDUSTRIAL AND ENGINEERING CHEMISTRY
NU CLEAR TECHNO LOGY
+
UFe(g) 3NaF(s) +. UF6.3NaF(s) (4) which is the basis for the sodium fluoride absorption step. Most of the niobium is absorbed on the sodium fluoride in the first absorber along with the uranium hexafluoride, but a large part of the ruthenium activity passes through the first sodium fluoride bed for collection in a second trap, which has not yet been installed in the plant. After the uranium hexafluoride has all been absorbed on the first bed, the setting of valves A and B (shown in diagram of apparatus on page 187) is reversed, and the uranium hexafluoride on the first bed is desorbed to the second bed with a stream of fluorine as the temperature of both beds is raised from 100' to 400' C. An equilibrium partial pressure of uranium hexafluoride exists over the complex. The dependence of the equilibrium partial pressure on temperature accounts for the effectiveness of absorbing uranium hexafluoride on sodium fluoride a t 100' C. and the ease of desorption at 400' C. The equilibrium pressures of uranium hexafluoride at 100' and 360' C. are IOF3 and 760 mm. of mercury, respectively. The absorbers as now designed are shown in Figure 1. It was difficult to scale u p the absorbers from laboratory to pilot plant size and the present design may be far from optimum. The outside diameter of each vessel is 10 inches and the straight side height is 24 inches. Gas enters through a diffuser ring a t the bottom of the sodium fluoride bed and leaves through an exit line in the top flange. Each absorber is located in a 15-kw. pit-type furnace. At present the ultimate capacity of the absorbers for fission products is not known, but the sodium fluoride will undoubtedly have to be dumped periodically. Consequently, the flanges connecting the absorbers to the rest of the system are located outside the shielding barricade for disconnection with a minimum of exposure to radiation. A lifting lug enables the absorbers to be removed with an overhead crane. Both absorbers
1
1
1
---
ABSOHBtR MATERIAL INCONEL
6
0
6
I2
18
SCALE IN INCHES
c,, Figure 1.
L
Absorber and furnace
uranium hexafluoride either drops out as "snow" or collects on a cold surface as "frost." In the ORNL Volatility Pilot Plant two cold traps are used. The C., primary trap operates a t -40° hhere 'the vapor pressure of uranium hexafluoride is 0.37 mm. of mercury, and the secondary trap operates at
are fabricated from Inconel, and copper ring gaskets are used with all flanges.
Cold Traps
.
After uranium hexafluoride has been further decontaminated in the absorbers, it is collected in cold traps, where
AL
cold trap
&
/THERMOCOUPLE
WELL
---
-1
--
1-
-
6-in IPS SHELL
CENTER TUBE
Figure 2.
Primary cold trap
8 - i n OD JACKET
AB''
REFRIGERANT TUBES (41
5-10 SPS COPPER PIPE
&' v
-
4 ~ C A L R O DHEATERS 16)
. ... .. . .
20 l-
12 SCALE IN INCHES
24
0
4
WELL
8
INCHES
VOL. 50, NO. 2
FEBRUARY 1958
189
4Figure 4.
-60" C., where the vapor pressure is 0.03 mm. of mercury. Uranium hexafluoride can be removed conveniently from cold traps because it liquefies when heated above 64" C. a t a pressure of 1134 mm. of mercury (the triple point). In practice, the cold trap and product receiver are isolated from the rest of the system, and both vessels are heated to above the triple point. The liquid formed then drains Eeadily into the receiver. After the liquid uranium hexafluoride has drained from the cold trap, the receiver is cooled, so that the remaining vapor In the trap and line will migrate to the lower vapor pressure space in thr receiver and freeze out there.
The primary cold trap, which is identical with some now in service in certain gaseous diffusion plants, is an 8-foot 8-inch length of 5-inch deoxidized copper pipe of standard pipe size and regular weight (Figure 2). The gaseous inlet and outlet connections are 2-inch standard pipe size. A 3/pinch standard pipe size nozzle is provided for draining liquid Llranium hexafluoride from the
Basic Reqiirements for a Uranium Hexafluoride Cold Trap. A heatremoval medium. Circulating liquid Freon 11 is used in the plant described. A method of adding heat Electric heaters at the inlet and outlet ends during the cooling cycle prevent plugging bv deposition of solids. The bodies of the traps are also heated electrically to effect liquefaction. Heavily baiffed trap internals of high surface area. Material of construction with high thermal conductivitv and resistance to uranium hexafluoride and fluorine.
Figure 5. Fluorine disposal system
190
INDUSTRIAL AND ENGINEERING CHEMISTRY
-
Fz AQ
KOY
Installation of cold traps
trap. The inside of the trap is heavily baffled with "a-inch copper sheets assembled into "egg crate sections." These baffles vary from two sheets at right angles a t the inlet end to six sheets by six sheets near the outlet end. The last 12 inches of the trap is packed with nickel wire-mesh filter cartridges to help collect entrained uranium hexafluoride dust. Coolant is circulated through four 1inch standard pipe size, extra-heavy, copper pipes, which are brazed to the full length of the shell. Heat is supplied by stainless steel-sheathed tubular resistance heaters applied in three zones-shell, outlet head, and inlet head. The secondary cold trap (Figure 3) was designed at Oak Ridge National Laboratory as a composite of several experimental traps used previously at the Oak Ridge Gaseous Diffusion Plant. The trap consists of a 5-foot 4-inch length of 6-inch schedule 40 pipe closed at each end with butt-welded pipe caps. It is designed for operation in a vertical position. Seventy copper bames at inches spacings varying from '/4 to are brazed to a center tube through which Freon 11 is circulated. Freon 11 is also circulated through four jacketed sections on the outer side o l the trap. When the temperature is to be raised, heat is supplied by five pairs of strip heaters bent to a snug fit with the trap shell exterior. The trap is fabricated from Monel, except for the center tube baffles, which are copper. Both the primary and secondary traps are equipped with vacuum jackets, partly for insulation and partly to collect any leakage that might occur during operation of the traps and indicate its source. Both traps are insulated with
NUCLEAR TECHNOLOGY
,
4 inches of glass-fiber low-temperature insulation. Figure 4 shows both cold traps during installation, with vacuum jackets. A separate refrigeration unit is provided for chilling the circulating liquid Freon 11 coolant for each cold trap. Each unit is a cascade system in which evaporating Freon 22 from a watercooled, high-stage compressor condenses the Freon 13 from the low-stage compressor. Temperatures to -100’ F. are obtainable with such systems. A canned rotor pump circulates liquid Freon 11 through the Freon 13 lowstage evaporator, the cold trap, and back to the pump. Uranium Hexafluoride Handling
To prevent freezing in transfer lines, uranium hexafluoride must be kcpt above its triple point temperature of 64’ C. during all transfers. Where possible, piping and valves are enclosed in ducts fabricated from ‘/,-inch-thick asbestos fiber-diatomaceous silica boards. This material was chosen for its relatively low thermal conductivity of 0.77 (B.t.u.) (inch)/(sq. ft.) (’ F.) (hr.) a t 200’ F. ( 4 ) ,coupled with fire resistance and ease of fabrication. The temperature inside the ducts is maintained a t about 100’ C. by tubular electric heaters, zone-controlled by thermostats. Air is moved through the duct system and through a small tower packed with sodium fluoride at a few cubic feet per minute to provide a more uniform distribution of heat within the ducts and to assist in detecting any uranium hexafluoride leaks and collecting the leakage. The air inside the ducts can be monitored for uranium hexafluoride and fluorine leaks, and the sodium fluoride trap will quantitatively absorb any uranium hexafluoride which escapes into the ducts. Uranium hexafluoride lines that cannot be run through ducts are heated by units consisting of 50 feet of No. 20 Nichrome wire with asbestos insulation and Monel braid, Each unit is rated at 450 watts and 120 volts. Units were coiled on the pipe to be heated and wrapped with stainless steel shimstock before being covered with thermal insulation. Each unit is controlled by a variable-voltage autotransformer in conjunction with an on-off temperature controller actuated by a thermocouple on the pipe. Lines u p to 3/~-inch diameter for handling uranium hexafluoride are seamless copper tubing; lines of ‘/,-inch diameter and larger are fabricated from Monel. Hand-operated valves for controlling uranium hexafluoride are Crane Type SMMD in accordance with the practice
at the Oak Ridge GasPous Diffusion Plant. These valves are all-Monel, metal-to-metal seat and disk, bellowssealed, and demountable, and are supplied with welding ends. The numerous remote, air-operated valves required in the plant have similar specifications, but they have given considerable trouble. They have been modified several times in attempts to secure and maintain more nearly leaktight shutoffs. Fluorine Supply and Disposal
Fluorine required for the Volatility Pilot Plant is obtained from the Oak Ridge Gaseous Diffusion Plant’s 25pounds-per-hour fluorine production facility ( 3 ) . Steel tanks of 146-cubic foot capacity, mounted on semitrailers, transport the fluorine a t 75 pounds per square inch gage pressure between the gaseous diffusion plant and Oak Ridge National Laboratory. Excess fluorine is disposed of by reaction of the off-gas stream with aqueous potassium hydroxide in a spray tower (Figure 5). The hydroxide is purchased in 55-gallon drums as 45% potassium hydroxide and diluted with water to 10%. From the make-up vessel the 10% potassium hydroxide is centrifugally pumped to a 280-gallon surge tank. A progressing cavity-type pump pumps the aqueous hydroxide from the surge tank to the spray tower. A potassium hvdroxide charge can be used until its strength has dropped from 10 to approximately 5% potassium hydroxide. Figure 6 shows the spray tower, which is 12 inches in outside diameter and 10 feet high, with */s-inch wall thickness and flanged and dished heads. The fluorine enters the vessel through a nozzle mounted on the upper head at 30’ from the vertical axis. Six spray-nozzle openipgs are located on 15-inch centers in the upper part of the vessel. The gas discharge line is a 1inch schedule 40 pipe near the bottom of the tower, Nozzles of Monel and Teflon with various sprav patterns and characteristics are available for testing in the spray tower. Monel is used for the tower and piping, and gaskets are Teflon-jacketed compressed asbestos. Acknowledgment
The advice of J. H. Pashlev and S. H. Smiley on the technology of UFa and fluorine and of G. J. Nessle, Jr., on the handling of molten salts is gratefully acknowledged, Detailed design, review of the design, and construction of the pilot plant were accomplished by personnel from several groups a t Oak Ridge National Laboratory. T h e author
Figure tower
6.
Fluorine
disposal
spray
wishes to acknowledge in particular the efforts of F. N.Browder, W. A . Bush, W. H. Carr, Jr., G. I. Cathers, S. L. Coulter, L. H. Chase, G. A. Cristy, C. L. Fox, W. H. Lewis, J. T. Long, W. A. Pate, and S. H. Stainker. Literature Cited (1) Cathers, G. I., “Uranium Recovery from Spent Fuel by Dissolution in Fused Salt and Fluorination,” American Nuclear Society Winter Meeting, Washington, D. C., Dee. 10-12, 1956. (2) Cathers, G. I., Leuze, R. E., “Volatilization Process for Uranium Recovery,” Nuclear Engineering and Science Congress, Cleveland, Ohio, Dec. 12-16, 1955, Preprint 278. (3) Dykstra, J., Thompson, B. H., Paris, W. C., IND.ENG. CHEM.50, 181 (1958). (4) Johns-Manville Corp., New York, N. Y . , “Marinite-Physical and Thermal Properties,” Tech. Data Sheet IN-4036 (h4arch 1948). ( 5 ) Steunenberg, R. K., Mecham, W. J., Vogel, R. C., ‘‘Recovery and Decontamination of Irradiated Reactor Fuels by Processes Using Uranium Hexafluoride,” Division of Industrial and Engineering Chemistry, 131st Meeting, ACS, Miami, Fla., April 1957.
RECEIVED for review April 20, 1957 ACCEPTED September 6, 1957 Division of Industrial and Engineering Chemistry, Symposium on Nuclear Technology in the Petroleum and Chemical Industries, Joint with Division of Petroleum Chemistry, 131st Meeting, ACS, Miami, Fla., April 1957. VOL. 50, NO. 2
FEBRUARY 1958
191
