A N ION E X C H A N G E UNIT FOR RADIOCHEMICAL SEPARATION W. J . MOTTEL
J.
F.
PROCTOR
Three units for large-scale separation, based on the uniticed concept of equipment arrangement, have operated successfully for more than two years
Figure 1. The cquipmnt frame of hcoy s ~ t u r o stccl l pouidcs m m ' m m rigidity and ~ ~ i n i mstrcsscs i ~ ~ on pcrmoncntly mounted piping and tankage
used extensively in laboratory radiochemical Although separations, ion exchange is not often applied in large production facilities, mainly because of difficulties in handling the resins by remote control. Therefore special equipment was designed, based on the unitized concept of arrangement, to adapt small-scale processing to remote operation in the large, shielded facilities of the Savannah River fuel-processing plants. After building a complete prototype unit to test feasibility of the concept, three units were constructed which have operated successfully for more than two years. The unitized concept which basically means installing a number of small equipment pieces in a steel frame which is movable can be used for processes other than ion exchange, such as solvent extraction, or precipitation where throughputs and volumes are small. Also it is not limited to large separations plants, but can be used in small-scale facilities which require process versatility.
F i p e 2. The compIe
.
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exchange columnr the cq I is ty, , rcnwtcly mnintained inrtollotiomi n the canyon
,-..d
r ion
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for
AUGUST 1 9 6 3
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Equipment Frames
Plutonium-238 is used in space power generators. Using the equipment described here, it is separated at Savannah River from neptunium-237 and fission prcducts. The equipment also is used to recover from spent VOL 5 5
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reactor fuel neptunium-237, the raw material for making plutonium-238. Gamma radiation levels in the process are comparable to those encountered in processing spent reactor fuels, and shielded space within the Savannah River canyon huildmgs is used. The term canyon refers to the heavily shielded buildings which house and supply services to the process equipment for recovering plutonium and uranium from spent reactor fuels. The canyons at Savannah River provide a module 10 feet square by 17 feet high for each vessel having standard servicei.e., inlet and outlet piping, steam, cooling water, electricity, sampling, and instruments. Each section of four modules, isolated by a low curb to contain liquid spills or leaks, has a sump and permanently installed pumpout jet for transferring spilled liquid or leakage to a rework system. Transfers between vessels are usually made via a pipe rack in which the piping is remotely removable. Transfers between adjacent vessels can be made by direct jumpers. Compared to other operations in the plant, throughput of the iron exchange process is small (4, and therefore small equipment had to be developed, particularly ion exchange columns, to operate in a building originally designed for the large equipment. Also, the new facilities had to be completely separate from the fuel processing operations. This was one of the governing considerations in designing the units. F'revious design philosophy for m o t e maintenance (3, 6) requires a prohibitive amount of shielded space and holdup of excessive volumes of solutions in process piping. Therefore,a unitized design containing a number of small individual equipment pieces was mounted in a frame of structural steel (Figure 1) which is removable from the canyon as a unit. All process piping and tankage for each frame are permanently mounted within the frame, and only those few critical items subject to periodic failure are individually removable. This technique, the minimum departure from the accepted practices of a remotely maintained plant, is equivalent to completely integrated relatively complex processing units contained in a space originally designed for a single process vessel. A comparable approach is described in the literature ( I , 7). Three equipment frames were built, each of which occupies one module. One frame contains a dissolver, a filter, two ion exchange columns, and five solutionadjustment tanks. The second frame contains three ion exchange columns and six tanks, and the third contains three columns and five tanks. Except for the columns, the process vessels are equipped with agitators and probes for measuring liquid level and specific gravity; most are equipped also with heating or cooling coils, samplers, and temperature instrumentation. Solutions are transferred between vessels either by steam jets or hy air lifts. The steam jets, air lifts, ion exchange columns, motor-driven agitators, several resistance thermometers, and the filter can be removed from the frame individually. Each frame (Figure 1) contains internal support 28
INDUSTRIAL A N D ENGINEERING CHEMISTRY
members for each piece of equipment. Panels on the top are for mounting the removable piping and connectors that span the gap between the frame and the supply piping embedded in the shielding walls. All steel in the frame is coated with Amercoat, a protective paint, and the bottom is completely enclosed with stainless steel to form a sump for collecting tank overflow and incidental spillage. This sump complete the chemical isolation of the process from the remainder of the canyon operations. Pipe sizes were chosen to provide minimum process holdup consistent with reasonable head requirements for solution transfers. Where feasible, lines were sloped to ensure complete drainage. With the exception of the ion exchange columns, the equipment is typical of that normally used for remotely maintained installations in the canyon. 7A11 10 Al11Y
Figure 3. Ion exchangi column is a right onlicul cylinder 12 i d e s in diamctn and 2 feet high, in which the resin is nmRwNy contained a ( a fircd bcd submcrgcd in a pool of process soiution. AN mlm'al is Typc 304L skinless steel Ion Exchange Columns
The principal new piece of equipment required was an ion exchange column where all operations could be performed remotely. The resin must be replaced frequently because of the cumulative effects of chemicaland radiation-induced degradation. Also, the normal cyclic operations of absorption, washing, elution, and regeneration must be done without complicated valving. For charging and discharging the columns, the resin is suspended as a free-flowing slurry in a solution having a
specific gravity comparable to that of the resin. Valving is done by a set of two weirs; air pressure can be applied preferentially to either or both weirs, causing them to act as a three-way valve for feed, wash, elution, and resin removal operations. Because the column can clear itself of accumulated solids, it is useful where filtration is impractical. The agitator can be operated during the feed or absorption cycle and therefore performance does not follow the typical breakthrough characteristic of fixed bed ion exchange. Nevertheless, the resin can be used efficiently. Operation of this column has been described as an agitated bed and operating details are available (2). Service Piping and Process Vessels
Services including water, steam, electric power, chemical solutions, service air, and instrument air are supplied to the frames through existing lines embedded in the canyon shielding wall. Connections to the frames were made by removable pipe and electrical jumpers that connect nozzles on the embedded pipe with corresponding nozzles mounted on panels at the top of the frame (Figure 2 ) . From the nozzle panels services are distributed by permanently installed piping and electrical conduits. The number of service lines was increased by drawing bundles of as many as six small ( l / ~inch) tubes through the 3-inch pipes embedded in the canyon shielding walls. Some were through pipe sections up to 15 feet long that contained several irregular bends. This technique has found a variety of other applications. All solution-adjustment and hold tanks are flatbottomed and rectangular. Consistent with canyon practice, none has bottom drains and therefore operate with small nonremovable liquid heels. T o minimize the number of welded junctions, all lines enter through the top of the tanks. With few exceptions, tanks are equipped with agitators, probes to measure liquid level and specific gravity, sample lifts, temperature-measuring devices, and coils for cooling and heating. All tanks except those handling resin slurries are agitated with air spargers. Those handling resin discharge are agitated electrically. The dissolver, a scaled-down version of one used in processing natural uranium reactor fuels, is also equipped with liquid level and specific gravity probes, pressuresensing taps, and an off-gas condenser. A filter, individually removable from the frame, removes solids, principally silica, from the dissolver solution. Solution is transferred between tanks with both air lifts and steam jets. Where air lifts are used, a 5-foot, 4-inch diameter well is mounted in the bottom of the tank to provide the necessary submergence for operating the lift. Steam jets are used for transfers between frames, W . J . Mottel and J . F. Proctor are Research Engineers in the SefJarations Engineering Division of the Savannah River Laboratory, E. I. du Pont de Nemours @ Co., Inc., Aiken, S. C. Work developed under contract AT (07-2)-7with the U. S. Atomic Energy Commission. AUTHORS
for recycle of material within a frame, and for transfers of waste solutions. Air lifts feed all columns and vacuum lifts transfer product solutions and samples. Air lifts
Feed rate of solution to an ion exchange column is gaverned principally by kinetics of absorption, size of the column, and concentration of the absorbable species. Typically, these factors restrict feed rate to about 1 to 3 liters per minute. Air lifts were chosen for this service because they are simple (no moving parts) and they can handle resin slurries without pluggage (5). Steam or air jets are susceptible to pluggage in the nozzle throat and mechanical pumps are subject to wear and leakage. Air flows required to produce liquid flows of 1 to 3 liters per minute range from 0.5 to 2 standard cubic feet per minute, and changes in liquid level and specific gravity of the feed produce no more than a 10% change in liquid flow at any given air flow. Liquid flow rate, monitored by tank depletion, is indicated by the liquid level instrumentation. The receiving chamber at the top of the lift provides a disengaging space for liquid entrained in the air stream as well as a small liquid holdup to damp surging. Vessel Venting and Entrainment
All process equipment within each frame is connected to the vessel vent system of the canyon which maintains a 1- to 2-inch negative pressure differential between process equipment and the canyon atmosphere. This minimizes release of airborne activity. Further, to reduce the number of connections required, each process vessel is connected to a common vessel vent header, mounted permanently within the frame, which makes a single connection with the common vent system of the canyon. Liquid seals were included in all process lines that entered the frame from a service area of the building. This technique considered mandatory in operating separations plants, reduces the hazard of active materials being released in areas occupied by personnel. REFERENCES (1) Bresee, J. C., Long, J. T., “Design Philosophy for DirectMaintenance Radiochemical Processing Plants,” Nuclear Congress, New York, N. Y., June 4-7, 1962; Preprint No. 80 Engineers Joint Council, 345 East 47th Street, New York 17, N. Y . (2) Caracciolo, V. P., “Anion exchange in a large-scale, agitated bed,” USAEC Rep. DP-624, E. I. Du Pont de Nemours & Co., September 1961. (3) Harty, W. M., “Design Philosophy of Remote Operation and Maintenance of Separations Facilities,” Chem. Eng. Progr. Symp. Ser. 50, No. 13, 115-21 (1954). (4) Joyce, A. W., Peery, L. C., Sheldon, E. E., “Design us. Performance of Process and Equipment in a Large-Scale Radiochemical Separations Plant,” Clzem. Ene. Prom. Symb,. . _Ser. 56, No. 28, 21-6 (1960). (5) Kearslev. G . W. T., “Use of an air lift as a metering Dump for radioactive solutibns, USAEC Rept. ORNL-2175,- 6nioA Carbide Gorp., October 1956. (6) Rehrmann, C. A., “Process Engineering at the Hanford Separation Plants,” Nucleonics 14,No. 6, 66-8 (1956). (7) Schneider, H., others, “A Study of the Feasibility of a Small Scale Reprocessing Plant for the Dresden Nuclear Power Station,” USAEC Rept. IDO-14521, p. 39, Philips Petroleum Co., April 1961.
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