I
LOUIS SILVERMAN and ROBERT A. SALLACH Atomics International, A Division of North American Aviation, Inc., Canoga Park, Calif.
Removal of Molten Sodium from Reactor Coolant Systems Molten sodium may be removed from oxygen-free iniccessible locations in pot or piping containers by flotation with HB-40, a liquid hydrogenated terphenyl
THE
favorable physical properties of sodium (and NaK) invite the use of sodium as a high temperature heat transfer agent. .4n example is the use of molten sodium as coolant in the Sodium Reactor Experiment, a nuclear power project conducted by Atomics International at Santa Susana, Calif., for the Atomic Energy Commission. As always in such experiments, plans are made for new designs to be investigated, and the coolant must be removed, nelv structures added, and the sodium returned. For the Sodium Reactor Experiment equipment the molten sodium in a large enclosed pot could be pumped down to the last inch, leaving a covered bottom surface of such area that about 400 pounds of metal remained. T h e plan described here is a method of removing this sodium without chemical action on the bulk of the metal and without corrosion effects and the dangerous formation of hydrogen gas. Another use of this procedure is removal of metallic sodium in manually inaccessible locations, low spots in piping or intricate apparatus (fuel elements). T h e plan is to pump in hot (120' to 140' C.) HB-40 continually, which displaces the liquid sodium by gravity, and to pump the liquid sodium away. I n exceptional cases when hot sodium (300' C.) has bonded to the stainless steel surface, the thin layer of sodium
can be removed with butyl alcohol without excessive corrosion. A cover gas of nitrogen may be used to dilute the hydrogen evolved. T h e properties of HB-40 and the ideal liquid for this operation are compared in the table. HB-40 is a mixture of partially hydrogenated terphenyls (hlonsanto Chemical Co. trade-mark) .
Comparison of Properties of HB-40 and an Ideal Medium Specific gravity
>1
Coe5cient of cubic expansion Flash point, O C. Fire point, ' C. Vapor pressure at 150' C., mm. Distillation, below 300' C. Pour point, C. Water content, p.p.m. Odor Reaction with Na metal Rate of oxidation at 150' C. Toxicity
Near Na
HB-40 1.004 at 25O/ 15" 0.000741/°C.
>I65 > 165 Very low
170-180 190-200 2
None Very low
Starts at 340° C. -25 125 max.
None None
Faint None
Low
Stable
Nontoxic
Nontoxic
Ideal
0
Flotability
An 8 X "4 inch test tube was half-filled with HB-40 and the liquid was main-
tained under a blanket of nitrogen. About 2 grams of clean sodium were added and the test tube was insrrted into an oil bath and heated. The sodium melted below 100' C., rose to the top of the HB-40, and floated as an immiscible liquid (100' to 150" C.). At 160" C., the liquid sodium sank to the bottom of the test tube. T h e heating \vas stopped at 175' C., and during cooling the sodium again rose to the. surface at 140' C. T h e solidified metal was removed from the test tube, and sermed to have a protective (to air) coating of oil. Laboratory Mock-up
Without Stirring. A hollow 304 stainless steel cylinder was welded to a bottom plate of the same material. The rubber stopper was drilled to accommodate a stirrer, a thermometer, an inlet and an outlet tube, and a tube to lead in nitrogen. The receiver was a tall-form 200-ml. beaker with a two-hole rubber stopper; the outlet tube of the receiver was attached to a vacuum line. Clean sodium (about 150 grams) was transferred to the nitrogen-flushed pot, which was quickly stoppered and blanketed with nitrogen. T h e sodium in the pot was heated to 300" C. so as to duplicate working conditions and permit the sodium to wet the metal of the con-
4
At 90" to 140" C. sodium floats on HB-40
In
the
laboratory
model sodium flushing apparatus outlet pipe is above the original sodium level VOL. 52, NO. 3
MARCH 1960
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tainer. The liquid sodium was cooled to 130” C., and 150 grams (150 ml.) of HB-40, preheated to 130’ C., were added. The mixture in the pot was allowed to remain quiescent at 130” C. for 20 minutes, then suction was applied, T h e left-hand bottle of the group captioned “without stirring” shows recovery of sodium for the first batch of HB-40. A second batch of 150 grams of preheated HB-40 was added and treated in similar manner. T h e second bottle “without stirring” was clear, and it was evident that the molten sodium was out of contact with the suction outlet. For this reason, the third batch of HB-40 was increased in volume and gently stirred to spread the sodium. The third and fourth recoveries are shown. The fifth pumping was clear, and the job was considered complete. The pot was opened and disclosed the expected small residuum of sodium dross with HB-40. T h e dross may be treated with butyl alcohol, which mixes with the HB-40 and destroys any metal enclosed in the dross. The sodium butylate dissolves in excess butyl alcohol. With Stirring. A second run was made, similar to the previous run, except that vigorous stirring was used to homogenize the sodium and reduce it to about ‘/Ifi-inch globules. T h e sodium was disseminated throughout the entire medium; for this reason, sodium cannot be completely removed in this manner. Plant Scale Mock-up
I n a 2y0 plant scale mock-up of the operation the central piece was a welded 304 steel pot with tight-fitting cover. Four to 20 pounds of sodium were added per trial run. T h e first batch was heated above 300” C. to wet the surface. T h e left-hand pot contained heated HB-40, which was forced into the center
pot by nitrogen and blown over to the receiver (right-hand pot). Much of the sodium was emulsified, as shown by the beakers. The receiver (right-hand pot) contains a stainless steel filter to filter the cooled sodium pieces, and the HB-40 is re-used. I n this manner the bulk of the sodium was removed. Butyl alcohol, under nitrogen pressure; was added to remove the last amounts of adhering sodium metal.
Discussion Other Methods for Removing Sodium. The usual method is by suction, bur when the molte‘i material cannot be reached by the inlet end of a suction pump, the sodium may be allowed to freeze and be destroyed by water or alcohols. Hydrogen and sodium hydroxide or alcoholates are formed, and the material is subsequently removed by steam flushing. Anothr: less attractive possibility is the formation of sodium alkyls (7), in which event very reactive. flammable, insoluble compounds are formed. (An ether must be present to catalyze the combination.) T h e objection to chemically destroying the sodiuin in location is that uncontrolled corrosion of the walls and floor of the por or pipe will take place. In sodium reactor core tanks of intricate configuration the risk of damage would be too great. HB-40 METHOD.T h e proposed reagent, HB-40, not only takes into consideration the lifting of the molten sodium from a location not accessible to a suction unit, but may “dewet” a stainless steel-sodium metal contact. The lifting action of the HB-40, due to a specific gravity differential with the molten sodium, is possible over only a limited temperature range (120’ to 140’ C.). In the laboratory mock-up run, after
the sodium had been in contact with the 304 stainless steel container at 300” C. for about 5 minutes, and after subsequent treatment with the HB-40 a t 130’ C., the metallic sodium was almost completely removed. I t was assumed that HB-40 had dewetted the stainless steel-sodium contact. Examination of the large pot indicated that some sodium was still in contact with the wails of the container. In this event, butyl alcohol may be added to remove the thin layer of sodium, without extensive corrc sion. Butyl alcohol may not be requ-red when clean sodium is used throughout an experimental run and the temperature does not exceed 300’ C. Most of the sodium is removed more quickly in bulk by the “without stirring” technique. If operational conditions are such that the sodium is partly emulsified, the effhent HB-40 niay be filtered from the nonsettling sodium and the HB-40 re-used. Thus, if a 7-foot pot is to be cleaned, the volume of HB-40 may be limited to one or two drums
I n the proposed procedure, no flammable or explosive gases, liquids, or solids are formed. The sodium removed is covered with a protective coating of oil. The HB-40 may be re-used, whether the sodium has been emulsified or not. I t does not seem economical to recover and re-use the sodium. If the HB-40 does not break the steel-sodium contact, the thin layer of sodium may be removed with butyl alcohol. Even in clean operations, some foreign material like sodium carbonate may accumulate and completely envelop small amounts of sodium metal. This material can be removed only by steam, or by a selected dilute acid. In all operations, a precautionary blanket of nitrogen is recommended, in case of error in the operations.
Acknowledgment
T h e plant scale mock-up was built and the series of tests was performed by Harvey Chapman. literature Cited (1) Sittig, M., “Sodium, Its Manufacture,
Properties and Uses,” Reinhold, New York, 1956.
RECEIVED for review July 6 , 1959 ACCEPTED December 1, 1959 In the large simulated plant-scale run, sodium flushing apparatus outlet tube is 2 inches above floor of sodium pot
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Based on studies conducted for the U. S. Atomic Energy Commissidn under Contract AT-11-1-GEN-8.
