Washing a n d Drying of Precipitate. Before the cake is dried, i t must be washed free of nitrate to prevent spontaneous oxidation. The trifluoride cake contains 15 to 25YGmoisture by weight after it is washed with 0.8M HF and dried by aspiration of ambient air through the cake. Anhydrous plutonium trifluoride is produced by drying with warm air to a moisture content of 2 to 370. The cake is then heated a t 600' C in an atmosphere of argon to remove remaining volatile impurities. The moisture content can be reduced to 2 to 3y0by washing the cake with alcohol before drying with ambient air; how-
ever, this procedure is not recommended for the normal production process. literature Cited (1) Mandleberg, C. J., Francis, K. E., Smith, R., J . Chem. Soc. 1961, 2464-8. (2) Orth, D. A., IND.ENG.CHEM.PROCESS DESIGN DEVELOP. 2,
121-7 (1963).
RECEIVED for review October 9, 1963 ACCEPTED September IO, 1964 Information developed during work under contract A T (07-2) -1 with the U. S. Atomic Energy Commission.
DISSOLUTION OF SODIUM FROM SODIUMBONDED URANIUM CARBIDE FUEL PIECES L O U I S S I L V E R M A N AND E D W A R D L. REED
Atomics International, A Dioision of North American Auiation, Inc., Canoga Park, Calif,
Several procedures were investigated for the dissolution of sodium from sodium-bonded uranium carbide nuclear fuel pieces. Two are described. In the first, the clad uranium carbide pieces are immersed in ethylene glycol n-monobutyl ether until sodium solution (hydrogen evolution) has ceased. The slugs are then washed in clean glycol, suspended in a trichloroethylene vapor bath, cooled, and dried in vacuum at 100' C. and then at approximately 500" C. Sodium-bonded uranium carbide, cleaned in this manner, has been rebonded successfully. In the alternative procedure, butyl bromide reacts with the sodium a t 80" C. to form sodium bromide, some of which i s highly adherent to the carbide surface. Dimethylformamide, at 140" C., dissolves all of the adhering sodium bromide, and in turn i s completely evolved in vacuum. The butyl bromide technique i s particularly attractive in related processes where evolution of hydrogen i s not permissible.
of its highly favorable physical properties, uranium monocarbide is considered one of the most desirable materials for solid nuclea'r fuels. In the sodium-cooled reactor concept, a group of 16-foot "fuel elements" operate in a large bath of liquid sodium a t moderately high temperatures. The sodium-filled annulus in the fuel element provides excellent heat transfer from the surfaces of the uranium carbide fuel pieces to the stainless steel-encased fuel element and to the liquid sodium bath. In one design system, the uranium carbide is cast into slugs approximately 8 inches long by 0.5 inch in diameter. About 2 5 or 26 slugs are encased linearly in a stainless steel fuel element and liquid sodium is introduced under vacuum a t elevated temperature to effect the bonding. After this operation, the fuel rods are tested for bond integrity and over-all soundness. If fuel rods fail the test, the fuel must be declad and the sodium metal removed for subsequent rebonding. This paper describes the effort to find an economical process, less hazardous than use of butyl alcohol or methanol, to remove sodium metal from sodium-bonded uranium carbide without physical or chemical injury to the carbide. Each 8-inch slug is bonded with approximately 2 grams of sodium metal. BECAUSE
Experimental
Water. Sodium-bonded uranium carbide slugs may be dipped into water to remove the sodium. However, because of the great danger of fire and deleterious reaction with the carbide material, the use of water is not considered satisfactory for cleaning uranium carbide. 32
l&EC PROCESS DESIGN AND DEVELOPMENT
Lower Aliphatic Alcohols. Fires have been encountered frequently when methanol is used. Normal or isobutyl alcohols readily dissolve sodium, and both are comparatively safe, but they leave insolubIe white deposits on the uranium carbide. None of the lower alcohols or common solvents dissolved the deposit. Ethylene Glycol. A temperature of about 80' C. was required to initiate the reaction, which then became very vigorous with the formation of excessive fumes. This discouraged its use. Diethylene Glycol Monobutyl Ether. Butyl carbitol (Chemicals Division, Union Carbide Corp., b.p. 195" C., flash point 205' F., molecular weight 134.18) reacted with sodium very nicely. I t was comparable to butyl Cellosolve, but insufficient work was done to justify its adoption for sodium removal. Ethylene Glycol n-Monobutyl Ether. Butyl Cellosolve (Chemicals Division, Union Carbide Corp., or Dowanol-EB, Dow Chemical Co., b.p. 168-71 " C., flash point 165" F., viscosity 3.15 centistokes a t 25" C., sp. gr. 0.900, water content O.ly,,molecular weight 118.2) reacted rapidly and smoothly with sodium. In repeated experiments, small pieces of sodium metal covered with oxide and carbonate fell to the bottom of the liquid, reacted, and then floated to the surface. In this action, the sodium was "cleaned" of oxide and the floating metal was bright and shiny and quickly reacted to completion. Pieces of sodium metal dropped into adjacent baths of butyl alcohol and butyl Cellosolve showed the latter to be more active in dissolving sodium metal. In a succession of tests, the butyl Cellosolve presented less of a fire hazard than
the butyl alcohol. A procedure based upon butyl Cellosolve is given below. Chlorinated Hydrocarbons. Ethylene dichloride and chloroform were slow in reacting with sodium even a t their bailing points. Because of unfavorable published information relating to potential explosive reactions, carbon tetrachloride was not tested. Iodinated Hydrocarbons. Iadobutane and iodopentane react with sodium above 80" C. (Wurtz-Fittig synthesis) to give sodium iodide and hydrocarbons as end products. Initial cost3 of these reagents are a deterrent t o their use. Ethylene Dibromidc (no flash point). The reaction with sodium metal is slow. The liquid must be heated above the melting point of sodium (99' C.) to obtain a usable rate of reaction. n-Butyl Bromide (bailing point 100-0l0 C., sp. gr. 129). The reaction starts at about 80' C. and goes rapidly to the "blue" or intermediate stage (sodium butyl) of the WurtzFittig synthesis. The reaction to the "white" stage (sodium bromide) is completed by further heating. The white salt adhered to the uranium carbide surface. A search for a nonaqueous solvent for sodium bromide revealed dimethylformamide. Solution takes place at about 100" C. An effective procedure based upon these reagents is given below. Selected Procedures. Of the processes examined, the butyl Cellosolve and the butyl bromide-dimethylformamide methods were chosen as most suitable for the dissolution of sodium from sodium-bonded uranium carbide fuel pieces and for use in similar sodium dissolution processes. Butyl Bromide-Dimethylformomide PFoeedure
Apparatus. Copper cooling coils are wound around the vertical sides and soldered to a stainless steel tank, 14 X 14 X 4 inches deep with flanged top, which rests on a heating coil consisting of eight 500-watt heating elements set in Transite. The cover is 14 X 14 X 6 inches high, with flanged bottom to rest on the tank. The gasket is Silastic rubber wrapped in aluminum foil. The tank and lid are fastened by eyebolts attached to the tank, just below the flange. The bolts swivel upward just beyond the edge of the flange, and washers and wing nuts seal the tank and lid. The top of the lid is covered with cooling coils soldered to the lid. A hole, centrally located, is drilled through the top of the lid, through which passes a 3/4-inch SS pipe, 3 feet high, welded to the lid. The pipe acts as an air condenser. One wall of the lid is pierced to admit a thermocouple. A wire tray, made of '/s-inch steel rods spaced inch apart, fits snugly into the tank and keeps the uranium carbide '/2 inch away from the bottom of the tank. Operation. The stainless steel cladding of the fuel element is removed by a roll-stripping process, and the sodium-bonded slugs are transferred to the wire tray in the reaction tank (Figure 1). Butyl bromide (technical grade) is poured into the tank to cover the slugs with approximately I/z inch of liquid (about 1 gallon). The cover is set in place and sealed with the wing nuts. The thermocouple or a metal thermometer is inserted and adjusted, to within inch of the bottom of the tank. The cooling water to the coils an the lid is turned on'and the heaters are connected and allowed to heat (15 to 20 minutes) until the temperature reaches 80' C. The heaters are disconnected, and when the walls of the lid become warm to the touch the cooling coils on the tank are turned on. When the temperature falls to 6O0 C. or below, the lid is removed, the tray is drained, and the slugs are transferred to a second, duplicate tank. Dimethylformamide (b.p. 152" C., technical grade) is added to cover the slugs by 1 inch and the cover is sealed. Water is turned into the coils on the lid only and the heaters are connected. The temperature is then allowed to rise to the boiling point ofdimethylfarmamide and is maintained far 15 minutes. At this time the heaters are disconnected and the tank cooling coils are turned on. When the temperature falls to 100' C., the cover is removed, the tray is drained, and the slugs are moved to a work bench,
brushed, and then transferred to a vacuum desiccator. Slugs are dried overnight a t 300' F. under vacuum, then cooled and examined. They are wrapped in aluminum foil and stored in a tightly sealed can with desiccant. Butyl Cellosolve-Tri~hloroethylene Procedure
Apparatus. The stainless steel tank is 14 X 14 X 4 inches deep. A wire tray, made of '/a-inch steel rods spaced inch apart, fits snugly into the tank and keeps the slugs approximately inch off the bottom of the tank. A basket handle, about 5 inches high, is an integral part of the tray. The tray carrier is made of 'jz-inch steel strips of size such that the wire tray fits into the carrier. The handle is about 2 feet high, so that the apparatus may be easily transported and suspended conveniently in a trichloroethylene degreasing tank. The trichloroethylene tank is about 3 X 5 X 5 feet tall with roll-back cover. The liquid is heated so that the vapor rises about halfway up the tank. Operation. The stainless steel Gladding of the fuel element is removed by a roll-stripping process, and the sodium-bonded slugs are transferred to the wire tray on a table top and allowed to stand until the bright sodium color has darkened. T h e tray and contents are then placed in the tank and butyl Cellosalve is poured to a level 0.5 inch above the slugs. Hydrogen is evolved, and action continues for about 12 minutes, accompanied by a temperature rise of approximately 1S0 C. (Figures 2 and 3). Each slug is then rotated 180°, and examined far residual sodium. (Cracked slugs require more reaction time.) The liquid is drained, and the tray and slugs are moved to tank 2 containing clean butyl Cellosolve. The tray is dipped and raised several times, drained, and moved to tank 3, where this operation is repeated. The tray and contents are placed in the tray carrier and immersed as soon as possible in trichloroethylene vapor for 30 to 60 minutes. The slugs are removed from the carrier and placed in a vacuum desiccator at about 100' C. to remove the last traces of any trichloroethylene decomposition products, after which the temperature is raised to 1000O F. for approximately 1 hour. The slugs are cooled to room temperature under vacuum, wrapped in aluminum foil, and stored in airtight cans containing desiccant. Discussion
The two procedures are based on different chemical reactions. Butyl Bromide Procedure. The butyl bromide reaction is based on the Wurtz-Fittig synthesis for hydrocarbons:
--
+ 2Na NaBr + CnHBNa(blue) NaBr + CaH, "C4HJJa + C 4 H &
C+HgBr
Figure I . tank
Uranium carbide pieces lowered into reaction
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Figure 2. Uranium carbide pieces after 2 minutes in ethylene glycol monobutyl reagent
The intermediate product is blue sodium butyl, and the final products are sodium bromide and octane. The sodium is so well "wetted" to the uranium carbide that, in the second reaction, the sodium. bromide adheres tightly to the uranium carbide surface and the white salt is not easily removed by light brushing. Far this reason, the slugs are placed in a second tank and the sodium bromide is dissolved with a,hot solution of dimethylformamide. Assuming that each slug contains a n average of 2 grams of sodium, a charge of 26 slugs (52 grams of sodium) will cansume about 300 grams (115 ml.) of butyl bromide. At the same time, about 125 grams qf octane (b.p. 126O.C.) will be formed and will remain with the butyl bromide. Thus, the butyl bromide may be decanted from the sodium bromide (200 grams) and re-used, a t least once. The technical grade dimethylformamide deteriorates after use. On its second use, its boiling point (152' C . ) falls about 15' C. For this reason the solvent (containing 'sodium bromide) is discarded. Butyl Cellosolve Procedure. This reaction is based upon the displacement of alcoholic hydrogen by sodium metal: 2ROH
+ 2Na
2C,H,0.C2H40H
--
+ 2Na
2RONa f H1
+ H,
2CIHU0.C2H40Na
The reagent, as purchased, contains less than 0.1% water, and the reagent, dried overnight with Drierite, reacts essentially as rapidly as the undried reagent. Before the sodium-bonded slugs are dipped into the reagent, the sodium is allowed to remain in air and tarnish. In this manner the initial attack by the reagent is slowed and the initial hydrogen evolution is less rapid. However, the reagent has sufficient penetrating action to remove the oxide coating, as shown in early experiments. A feature in the use of butyl Cellosolve is its "solubilizing" property, an obvious advantage over butyl alcohol. I t is for this reason that the sodium salt of ethylene glycol n-butyl ether does not appear as an insoluble deposit on uranium carbide pieces. It is, however, necessary to wash the "dirty" Cellosalve (containing dissolved salt) with two extra washes of butyl Cellosolve. The trichloroethylene vapor wash is used to remove butyl Cellosolve and higher boiling impurities present in the reagent. Thc low boiling halide is easily evaporated under vacuum and 34
I S E C PROCESS D E S I G N A N D DEVELOPMEN1
Figure 3. solution
Uranium carbide pieces after complete dir-
the vacuum retort treatment further volatilizes any traces of sodium and decomposes any residual carbonaceous maierial. Comparisons. The sodium-butyl Cellosolve reaction is smooth but it does produce hydrogen, a potential hazard. T h e action is not violent, as with water, or too rapid, as with methanol. I n the procedure, the slugs are allowed to surfaceoxidize for a short period, so that the start of thk reaction with the butyl Cellosolve will be somewhat slower. Hydrogen is evolved smoothly in the form of small bubbles. I t is not hazardous if the action takes place in a high face-velocity hood. The main advantage of the butyl bromide process is its nonhazardous nature. T h e use of covered tanks with cooling coils minimizes losses to the air, and flammability of the reagents is the only hazard. The over-all butyl Cellosolve process takes less time and the chemicals used are lower in cost than for the bromide process. The stripping, washing, and vapor cleaning may be completed in 1.5 hours, while ihe butyl bromide procedure requires at least 3 hours. Other' Techniques. Confining the problem to sadiumbonded uranium carbide materials precludes the use of cold water or steam because of the chemical reaction between the carbide and water. Methanol presents a fire hazard even with superior ventilation. T h e butyl alcohols present less of a fire hazard than methanol, but residuals of sodium butylate are not soluble in organic solvents. (In the intermittent destruction of waste residues of clean or exposed metallic sodium, field workers found ethylene glycol manabutyl ether far superior to methanol or the butyl alcohols.) Liquid Ammonia. Cleaning with liquid ammonia app'ears to have many good features ( 7 ) . At low temperatures, liquid ammonia dissolves melallic sodium (endothermic) with meager evolution of hydrogen, and a facile test far complete removal of sodium is the absence of the blue calor. However, liquid ammonia does not penetrate, cut through, or remove sodium oxide, hydroxide, or carbonate surfaces, layers, or space blocks, as does the glycol. Furthermore, the ammonia may be trapped and rztained on these surfaces ( 7 ) , both as absorbed ammonia and as dried sodamide, and be discovered in subsequent washing operations. In the case of uranium carbide, aqueous washes would be required to remove residual sodium oxide, carbonate, and amide.
Economically, the liquid ammonia process faces the requirements of a separate facility, a relatively remote location, cooling equipment, trained operators, extra safety protection, and a disposal problem. Extended Usage. The two processes-i.e., glycol and but)rl bromide-might be used in the recovery of spent fuels. I n these events, remo.te control facilities, collection of gas spray, conservation, and storage of wash liquids would be required Ruptured fuel rods are expected to be handled in 18-foot casks. preheated to 60' to 70' C., with suitable vents and drains. I a other than fuel piece cleaning, butyl bromide has been used for the removal O F residual sodium from deep, open retorts! whereas HB-40 [a mixture of partially hydrogenated terphenyls) (2) is preferred for enclosed systems.
Acknowledgment
T h e authors thank G. H. Hayes and J. P. Mills, Jr., for the construction and operation of the equipment. literature Cited (1) Robb, R. L., North American .Aviation, Atomics International, Canoga Park, Calif., Spec. Rept. 8314 (1963). (2) Silverman, L., Sallach, R. A , I n d . E n g . Chem. 52, 231 (1960).
RECEIVED for review November 4: 1963 ACCEPTED May 6, 1964 Research supported by Empire State .Atomic DeLelopment Associ-
ates (ESXDX), a nonprofit corporation composed of Sew Yorkbased, investor-owned utility companies : Central Hudson Gas 8;
Electric Corp., Consolidated Edison Co. of New York, Inc., Siagara Mohawk Corp., Orange and Rockland Utilities, Inc., and Rochester Gas & Electric Corp.
NITRIC OXIDE DISTILLATION PLANT FOR ISOTO PEI SEPARATION B. B. M c l N T E E R AND R O B E R T M . P O T T E R Los Alamos Scientijc Laboratory, Cnizerstty of Calzforma, L o s Alamos. 5.M .
A plant for enriching oxygen and nitrogen isotopes b y NO distillation, consisting of a purifier unit, a staged column 575 cm. long for continuous flow, and a uniform column for final batchwise enrichment, has been built and successfully operated. Over 600 (STP) liters of enriched NO have been produced, with an average and composition of 1.5% O", 21 % 01',and 25% N". Enrichments as large as 8.3% Oi7, 98,2% 01', 93.9y0 N15have been attained. Technological developments in low temperature distillation and purification of nitric oxide! are described. Exchange reactions among the isotopic species are negligibly slow in these columns.
and cokzorkers (7. 2: 4) in 1959 announced the measurement of the vapor pressure differences of the isotopic forms of nitric oxide (SO) for the t\vo nitrogen isotopes S I 4 and Sij and the three oxygen isotopes OI6>01',and Ol8. These differences \\-ere so large that distillation of N O became attractive as a possible method for separation of these isotopes. In 1961, Clusius, Schleich, and Vecchi ( 3 ) reported the results obtained \vith a packed 'column distilling S O . -At the time of the first announcement this laboratory had been interested in a lo\+;temperature distillation project using liquid oxygen for separation of the oxygen isotopes and changed its plans to include the study of S O distillation. The resulting plant has produced over 600 (STP) liters of enriched NO with a n average compositioii of 1.5% 0 ' 7 : 217, Ole, and 2570 XI5 a t a rate of 4 (STP) liters per day under steady operation. The description of this plant and its performance is the subject of this paper.
C
LUSIUS
The Flow Scheme
The plant consists of a purifier column to remove the chemical impurities found in commercial S O ; a composite column of three stages, column I, which is operated in a continuous flow fashion for the primary enrichment; and a second uniform column, column 11, \vhich operates batchwise, for further enrichment.. The enri'ihed product may be used as nitric oxide or. more often, is made to react chemically to produce water and nitrogen or ammonia. These isotopes are used by research groups in the L,os ,4lamos Scientific Laboratory.
Purifier Flow Systems. Commercial nitric oxide contains N20. and S O 2 as impurities. These impurities have boiling points of 77', 185", and 295' K.: respectively, as compared with 121' K. for S O . The X2 and SO2 are thoroughly removed by freezing the raw material to 77' K., pumping thereon. and subsequently boiling off the maior body of the mix at 121' K. However. the NCO is removed by this step to only a few tenths of 1%. If subsequently enriched in the plant, this impurity would deposit as an ice: clogging the column and associated lines. T o correct this, the material is passed through an auxiliary distillation column of a few plates in order to remove the SZO. T h e following scheme for gas purification is used in this plant. A s shown in Figure 1, commercial YO is condensed in a 2-liter stainless steel trap at 77' K. and the N2 impurity fraction is quantitatively removed by pumping to a few microns pressure. T h e trap is then warmed to no mofe than 153' K. and the gas transferred to a bank of gas cylinders (S-tanks) of 220-liter volume and stored at 5 to 15 atm. Residual gas in the trap is removed, carrying away the higher boiling impurities. The purifier column is continuously supplied with material from the S-tanks at the rate of 600 (STP) liters per day. T h e purified S O is removed from the top of the condenser. By adjusting the condenser temperature, as described subsequently, the NO pressure in the column is adjusted to 2.0 a t m . ; this gas pressure is reduced by a pressure-regulating valve to 1.5 atm. and bled at a controlled flow rate through a throttle needle valve to column I, which has a pressure of 0.75 atm. The S20 impurity builds u p as a solid on the surfaces of the boiler. After about 2 weeks' operation the NpO deposits plug the purifier. I t is then warmed to room temperature, its contents are pumped away, and it is rechilled to resume operaS1.
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