I
R.
P. MILFORD, SYDNEY MANN,
JOHN B. RUCH, and WILLIAM
H. CARR, Jr.
O a k Ridge National Laboratory, Oak Ridge, Tenn.
Recovering Uranium Submarine Reactor Fuels Equipment for a fluoride volatility pilot plant has been modified to handle this new feed
ONE
OF THE MAJOR EFFORTS of the Chemical Technology Division a t Oak Ridge National Laboratory is the development of the fluoride volatility process through the pilot plant stage. The status of the process as of 1958 was summarized by Cathers and others (2). The chemistry, certain engineering developments, the engineering design of the first pilot plant, and its operation have been described in more detail by Cathers (3, 4 ) , Long (5), Milford (7), and Carr (7). The ORNL fluoride volatility process (below) consists basically in separating irradiated uranium from nonvolatile fission product fluorides by converting UFI in a fluoride salt melt to volatile UFB by reaction with elemental fluoride; absorbing the UF6 and volatile fission product fluorides on NaF pellets a t 100' C. and desorbing the U F G (leaving most of the volatile fission product fluorides on the NaF) as the temperature is raised to 400' C.; and finally collecting the UFe in cold traps as a solid, liquefying it with heat
and the resulting pressure, and collecting the product. The uranium in the fluorinator may originate from either homogeneous reactor fuels, such as the NaF-ZrFpUF4 from the Aircraft Reactor Experiment (ARE) or the LiF-BeFz-ThFd-UF1 from a proposed molten salt reactor, or from heterogeneous fuel elements thus far of the zirconium-uranium type from naval reactors or the seed elements frcm the Pressurized Water Reactor (PWR). In 1958 the ARE fuel was processed in the volatility pilot plant. Plans to dissolve Zr-U nuclear submarine fuel elements in molten fluoride salts with H F have made necessary the principal modifications described here. These include the addition of hydrofluorination equipment, a redesigned fluorinator, improved absorbers, and an alternate temperature-zoned, movable bed combination complexible radioactive products (CRP) trap and two-stage absorber. Some of the significant problem areas expected during operation of the
modified pilot plant are dissolution and corrosion rates, formation of off-gas solids, and handling of NaF pellets. A problem related to the entire process will be decontamination and direct maintenance of a nonaqueous radiochemical processing plant in a highly radioactive environment. Process Details and Equipment
Charging the Hydrofluorinator. Irradiated zirconium - uranium fuel elements (subassemblies) are currently being stored in the O R N L graphite reactor storage canal. A charger-carrier is provided for transferring single subassemblies from the canal to the processing area. A zirconium wire will be threaded through one of the s1o:s in a subassembly under water with special tools, and the subassembly will be placed in the 6.5-ton charger-carrier while horizontal, removed from the canal with a hoist located over the canal, transferred by truck to the processing area, and moved to the vertical charging
Uranium is recovered, from spent zirconium-uranium nid e a r submarine fuel elements b y a fused salt fluoride volatility procc45s 1
VOL. 53, NO. 5
M A Y 1961
357
H F OUT
,,-SLUG
CHUTE
SALT INLET
7 .yc)
e:
L
!-in. O D 0.253- in. WALL
t
- 318-1n. NPS 0.091-in. WALL
c L
(0
I i
5'12417. O D -
'/q in.-
MATERIAL INOR-8
HF INLET
--c
===
-
SALT OUTLET
1
Hydrofluorinator is fabricated of INOR-8 (also known as Hastelloy N or lnco 806),which has a nominal composition of 770 Cr, 17% Mo, 5% Fe, balance Ni. Note bottom HF inlet and salt outlet lines, charging chute and baffled outlet line at top, and increased wall thickness a t tapered section near center of vessel. Latter feature was adopted to compensate for higher corrosion rates obtained a t salt-vapor interface during laboratory corrosion studies
position, above the first processing cell, with the building crane. The NaF-LiF-ZrF4 mixture required for the dissolution will be blended and purified a t the ORNL Reactor Chemistry Division's molten salt purification facility. An electric resistance furnace is provided in which to melt the salt in the transfer vessel prior to forcing it into the hydrofluorinator with nitrogen gas pressure. Subassemblies will be charged and the hydrofluorinator heated and purged with helium before the barren salt is transferred to the hydrofluorinator. Hydrofluorination (Dissolution). The 41.6 kg. of zirconium and 400 grams of uranium, per charge, are converted to ZrF4 and UF4, respectively, in the hydrofluorinator (above) with HF while the subassemblies are submerged in 31 liters a t 650' C. of molten XaF-LiF-ZrF4
358
Lower three fourths of hydrofluorinator (left) is heated by vertically split, tubular 57-kw. electrical resistance furnace mounted on casters and tracks. Each half of the furnace can b e moved away from or toward the hydrofluorinator b y remotely operated air cylinders, or it can b e removed from the cell semiremotely with building crane
(37.5-37.5-25 mole ye; melting point, 605' C.). The resulting melt is 51 liters a t 500° C. of 27.5-27.5-45 mole 7 0 NaFLiF-ZrFd (melting point 453' (2.). Deentrainment (upper) section of hydrofluorinator is equipped with two zones of tubular electric heating elements, each zone consisting of six 3-kw. elements. The elements are clipped to the vessel, wrapped with stainless steel shim stock, and covered with asbestos fiber-diatomaceous earth insulation. These two heating zones can be operated to melt down salt splatter, or for any other purpose as required. HF Supply and Recycle. The hydrofluorination step requires introduction of anhydrous H F into the bottom of the hydrofluorinator a t a design rate of 10 kg. per hour (15 kg. per hour maximum). Anhydrous HF is received from the vendor in 91-kg.-capacity cylinders as a
INDUSTRIAL AND ENGINEERING CHEMISTRY
liquid. In accordance with published recommendations (6), H F is transferred by nitrogen pressure from the original cylinder to a process storage vessel, the 13O-kg.-capacity H F supply vaporizer (p. 359, top). A cylinder hoist is provided to elevate and invert the cylinders so that a liquid transfer can be made. Three 1000-watt strip heaters mounted on the walls of the supply vaporizer should vaporize HF into the system a t a maximum rate of 30 kg. per hour. The HF enters the recycle system immediately before the vapor inlet to the water-cooled H F condenser. As it condenses, it collects in the 95-kg.-capacity H F surge tank and is further chilled by a fluorocarbon refrigerant (Freon-11) circulating through the jacket a t SAMPLER NOZZLE
CORROSION SPECIMEN NO2
FURNACE LINER
-FLUORINATiON CHAMBER (16-in.OD)
WFLUORINE INLET
WASTE SALT OU
0
5
-
10 15 INCHES
20
I FURNACE
VOL. 53, NO. 5
M A Y 1961
359
L A D L E INLET CHUTE
RMOR PLATE
C U B I C L E WINDOW
ARRIER SAMPLE
L A D L E TON
L A D L E VISE
MA N I PU L A T O R
7
L A D L E ROTATOR
4 0 4 812 INCHES
1
Sampling tube is introduced into molten salt sample cubicle through inlet chute. Tube i s an open-top copper cylinder 0.5 inch in diameter (0.035-inch wall) and 1 inch high. Two ‘/*-inch-diameter holes located near top allow engagement with weighted Inconel-X spring clip. Latter i s attached to 0.01 0-inch nickel wire for lowering clip and sampling tube about 25 feet into molten salt through 0.75-inch-diameter lnconel pipe connecting top chamber o f fluorinator with sample cubicle. W i r e i s coiled on grooved reel rotated b y motorized gear train. Calibrated counter indicates elevation of sampling tube. Ball-type manipulator i s used for all handling functions within cubicle; air vise and turntable, with external controls, are used to hold and orient container during engagement and disengagement with spring clip. Cubicle has two lead glass windows 8 inches square and 4 inches thick (density 6.2 grams per cc.) for viewing. Inner surface of lead glass i s protected with fluorine-resistant 0.25-inch-thick plate of clear Homolite plastic. Water-tight cubicle is equipped with spray nozzles and drain for use in decontamination, and cubicle liner and internal components are fabricated from stainless steel and plastics suitable for aqueous decontamination
The fluorine supply system is essentially the same as previously described ( 7 ) , fluorine being transported from the Oak Ridge Gaseous Diffusion Plant in steel tanks mounted on semitrailers. T o minimize the danger involved in handling fluorine, several safety features have been added since the system was originally designed. To prevent a sudden release of a large quantity of fluorine, an automatic shutoff actuated by a high fluorine flow rate was installed during the ARE fuel recovery program. Whenever the fluorine florv rate exceeded 56 standard liters per minute [Z standard cubic feet (SCF-0’ C., 760 mm. of mercury, dry) per minute], flow switches actuated a solenoid valve in the air supply to close the shut-off valve. A “dead man” switch is provided to by-pass manually the high-flow control
360
valve while the sy-stein is being filled. Since a nitrogen blanket system is also used in conjunction with the fluorine supply system, an interlock is provided to stop fluorine flow automatically whenever the fluorine line pressure rises to within 0.5 p.s.i. of the nitrogen pressure and/or the fluorine pressure exceeds a set value, usually 4.5 p.s.i.g. Three emergency remote manual shut-off switches are also located strategically about the plant. Since the available fluorine contains u p to 5y0 HF contamination which could eventually either contaminate the product UFe or interfere with UFs absorption on the NaF absorbers or chemical traps (described later), an H F removal tower is installed in the fluorine supply line. This tower is a column 4 feet 7 inches high with a 4.5inch inside diameter packed with I / a -
INDUSTRIAL AND ENGINEERING CHEMISTRY
inch NaF pellets. The gas inlet section is heated to 100’ C. to avoid plugging, and the exit section is cooled to 25’ C . for more complete HF removal. The column is designed to lower the HF content from 5% to less than 0.01%. Unreacted fluorine is disposed of in an aqueous KOH spray tower as previously described (7). Molten Salt Sampling. Sampling of the molten fluoride salt in the fluorinator is required to establish the initial uranium charge and to check the completeness of uranium removal prior to waste salt disposal. A 9.5-inch-thick steel cubicle (left) over the first processing cell encloses and shields this operation. Salt samples will be transferred to the analytical laboratory in a 6-inch lead-shielded carrier. Because of the high radiation level involved, remote handling will be used throughout. In preparation for loading the full sampling tube into the shielded carrier, the carrier is positioned on the cubicle cover. An extension rod is used to lower a recessed plug through the carrier and into position in the cubicle to receive the full sampling tube. The plug and tube are raised back into the carrier and clipped and locked in place. Waste Salt Disposal. When the fluorination step has been com?let:d, the residual salt, containing the nonvolatile fission product fluorides, is forced out of the fluorinator, through a freeze valve, and into a waste salt transfer can by nitrogen pressure. A special heated nozzle is designed to prevent molten ralt plugs at the end of the fluorinator waste-salt discharge line (p. 361) The shielded waste carrier is designed with a removable cover and a bottom drawer so that the waste can may be filled from the top and later dropped into a burial vault. The design of the carrier makes it an integral part of the waste nozzle shield when in position. Accessory equipment is provided for remotely lowering the waste container into the cell. positioning the carrier under the waste nozzle, and, after filling the waste container, reversing the cycle, installing the carrier cover, and removing the shield waste container from the cell. UFB Sorption-Desorption. The distinguishing feature of rhe ORNL fluoride volatility process is the hTaF sorptiondesorption cycle in which most of the volatile fission products fluorides are separated from the UFs stream leaving the fluorinator. This contrasts with the Argonne Sational Laboratory process in which this further purification is by distillation. Uranium hexafluoride (vapor pressure approximately 10-3 mm. of mercury) is sorbed on NaF pellets in the first absorber a t 100’ C. with the evolution of heat: UFa 3NaF a UF6 3NaF 23 kcal./mole UF,
+
+
RECOVERING U R A N I U M During desorption, the reaction is reversed, the UFe vapor pressure being >760 mm. of mercury. Both absorbers are heated to 400’ C., and the UFg is evolved from the first absorber and swept through the second with a stream of fluorine. Significant quantities of volatile fission products are retained on the two NaF beds. Above 400’ C., in the absence of fluorine, a back reaction takes place to form a nonvolatile uranium complex :
UFe. 3NaF + UFs. 3NaF
+ 1/2F2
Problems of distributing the UFe as it is introduced, of NaF disposal, and of temperature control to suppress the back reaction all complicate this step. Two operational techniques, each requiring different equipment, will be tested in forthcoming runs. The more conservative technique, and the first to be studied, involves replacement of the two absorbers (IO inches in diameter and 25 inches high) used in previous runs with two smaller, redesigned units (p. 362, left). The number of absorption-desorption cycles per charge of NaF pellets is still unknown. During the ARE fuel processing, four runs were made with the same absorber beds. Later, nine spiked runs were made with a single filling of the absorbers. I n both series of runs the NaF pellets were vacuumed from the absorbers, but in high radioactivitylevel runs this technique will not be feasible. The absorbers will be removed to the burial grounds for semiremote dumping of the NaF pellets, or a so-called “movable bed” or “temperature-zoned)’ absorber (p. 362) may be used. This latter technique has been only partially tested, but in the equipment a n attempt is being made to combine a complexible radioactive products (CRP) trap and the two absorbers into a single unit from
which NaF can be intermittently discharged directly to the fluorinator. T h e C R P trap was developed previously to remove certain volatile fluorides, principally ZrFe and CrF6, by reaction with NaF a t 400’ C. UF6 Collection. After UF, has been further decontaminated in the absorbers, it is collected in coId traps where it either drops out as “snow” or collects on a cold surface as “frost.” Two cold traps designated “primary” and “secondary” and operating a t approximately -45’ and -55’ C.,respectively, are used (5). Because of the small quantity of uranium per unit weight of zirconium, the UF6 product from several hydrofluorinations and fluorinations will be collected on the absorbers before being transferred to the cold traps. However, to obtain decontamination data from individual hydrofluorinations and fluorinations, a small supplementary cold trap is installed just ahead of the primary cold trap. This unit is a 2.7liter UFG cylinder, about 15 inches high and 43/8 inches in diameter, which will be cooled by being submerged in a dry ice-solvent bath. A NaF bed is also used between the secondary cold trap and the fluorine disposal tower as a final UFe trap. T h e new trap is identical with the fixed bed absorbers previously discussed except that it is neither heated nor cooled. Any U F O ever trapped in this vessel may be recovered by substituting the trap for one of the absorbers and desorbing as part of a regular run.
data accumulation followed by automatic data processing. T o this end, an available automatic data acquisition system will be installed. This device will digitize 96 electrical signals (temperature readings) and 24 pneumatic variables (such as flow, level, density, and pressure) and punch a paper tape for computer input. T h e computer used will be either the Oracle, ORNL’s digital computer, or the IBM 7090, located a t the Oak Ridge Central Data Processing Laboratory. Thus, the determination of any imaginable interrelation between these 120 variables should be possible. Much of this information will take the lorm of curves plotted automatically by a photographic curve plotter and available within a few days after the completion of a run. T o facilitate optimum utilization of the data logger, the following priority has been established for inclusion of a signal in the logger system: Per-
centage (Approx.)
of
Priority I 2
3
Use of Signal Process calculations and curve plotting Equipment temperature history for corrosion studies Operational assistance (minimize manual data taking)
Logger Capacity 50 25
25
Needless to say, the data logger system will be only an adjunct to the existing conventional control system, a portion of which is a rather elaborate graphic panelboard arrangement.
Data-logger Computer Study
Studies will be made during the submarine fuel processing runs to determine the value of large scale data reduction to a batch radiochemical processing plant. Such studies will require automatic
Cell Ventilation A i r Scrubber
Since the volatility pilot plant was first operated, a n air filtration system
OFF-GAS
n
WASTE SALT LINE
OFF-GAS
End of fluorinator waste-salt discharge line i s a built-up assembly which serves as a container for bronze casting in which four 3.5-kw. tubular heating elements are closely wound. Two heating elements are connected, leaving other two as spares. Assembly i s also designed to serve as hood for collecting fumes and to be upper half of flanged seal through which aqueous decontaminating reagents are introduced into system
VOL. 53, NO. 5
M A Y 1961
361
r N o F CHARGING CHUTE
l’/z-in NPS, SCHED-40 (AIR COOL-
I N G AND THERVOCOUPLES)
TO UFs TRAPS DESORPTION C Y C L P ?
COOL N: .
--’
1
NFS -
ABSORBER NO 1CO‘CR 400°C
5 - n NPS SCIED-40 INCONEI
; “
174
&”
-7p-A
i _ .
’/?-in
AIR INLET
-OUTLET
INLET-
TO CHEMICALTRAP
NaF P E L L E T S 6 - i n NPS
SCHED 404’12-ln OD x ‘&-in
WALL-
‘/e-in
WALL-
5-in NPS, SCHCO-BO INCONEL FYDRAU-IC CYLlNDEST
I
THERMOCOUPLE M,ELLS-2’-
~
Each new absorber i s fabricated from 2-foot length of 6 inch-diameter NPS schedule 40 lnconel pipe. A 2.5-inch outside diameter (l/s-inch wall) center tube contains tubular electric heaters and air line for cooling. A 4.5inch outside diameter (‘/&ch wall) baffle attached to covet flange makes U-tube path for gases passing through vessels. Each absorber i s surrounded b y light-weight low heat-capacity electric furnace hinged vertically. Furnace can b e opened slightly and stream o f compressed air introduced into center o f first absorber for cooling during sorption, i f required. System is designed to handle 6.3 kg.of UFe
with glass fiber-roughing and highefficiency filters in series has been installed in the building ventilation system. To avoid the possibility of HF or fluorine from an accidental release or poorly operating gas disposal unit damaging these filters, with subsequent release of radioactivity to the stack, an aqueous K O H scrubber is being installed. T h e scrubber is designed to decrease the fluorine concentration, in the event of an accident, from 1520 to
