I
FAUSTO W. LIMA, ALClDlO ABRAO, and CONSTANCIA P A G A N 0 lnstituto d e E n e r g i a Atbrnica,
Sao Paulo, Brazil
Removal and Recovery of Cesium-137 from Swimming Pool Reactor Water The use of ethyl alcohol solutions simplifies the operations for removing cesium-137 from the waste of nuclear reactor plants
T H E importance of removing and recovering fission products from the waste of nuclear power reactors and fuel reprocessing plants has been stressed (7: 8, 9 ) . Although on a less extensive scale, removal of the radioactive fission products from the recirculating water of swimming pool research reactors is also of vital concern, particularly if corrosion or inadvertent rupture of the fuel elements becomes a problem, or external uranium-235 contamination of the fuel elements occurs ( 5 ) . The water of swimming pool reactors is usually kept in circulation through a n ion exchanger system to reduce the amount of soluble ionic substance to as low a level as possible-i.e., to limit the build-up of hazardous induced activities. When fuel element corrosion or rupture occurs, the liberated fission products (normally descendants of the fission gases krypton and xenon) will accumulate in the resins of the ion exchange unit. At the time of regeneration of the resin bed, the elutrient solution may be sufficiently radioactive to prevent disposal in public sewers. As a consequence of corrosion affecting some elements of the Brazilian Sivimming Pool Reactor, the problem has been of some concern (5). During attempts to improve the water-treatment methods, a process was developed for removing and concentrating cesium-137. The procedure is simply executed and affords high radioisotope yields (and/or high decontamination of the aqueous resin effluent-i.e., about lo6) and is believed to be more economical than similar established processes. Processes for cesium recovery have been reviewed by Fisher and Ragenbass (7)-e.g., precipitation by tetraphenylboric acid, by the ferrocyanides of nickel or iron, or by phosphotungstic acid, and co-crystallization with alums. The latter process has been described by Rupp (8) and Gresky (3) and is now used for the recovery and purification of cesium137, which is being extensively utilized
as a y-radiation source in medical and industrial radiology. Briefly, the process consists of adding ammonium (or potassium) alum to the cesium-containing aqueous solutions (generally highly salted) at an adjusted pH, heating the mixture to about 80' C., and finally cooling to room temperature to lower the solubility of the alums and permit them to crystallize from the liquid phase. The less soluble cesium-137 alum is quantitatively co-crystallized on the insoluble ammonium alum fraction, and sequential use of the latter carrier crystals effectively permits very significant final cesium product concentrations, with pure cesium-137 alum as the limiting case. Where only occasional and smallscale utilization is required, it is advantageous to reduce the solubility of the ammonium alum by the addition of about '/4 volume of ethyl alcohol to the aqueous effluent from the resin bed. Because of the lower solubility (about 30 grams per liter at room temperature) in the water-alcohol mixture, as opposed to about 150 grams per liter in water ( 4 ) ,it is believed that the cost savings of alum utilization more than compensate for the increased cost of the alcohol addition, particularly if based on prices in Brazil. A saving in total cost may also be achieved by evaporation of water from the resin effluent before the co-crystallization step. I n general, the lower alum solubility in the wateralcohol mixture also obviates the need for the 80' C. temperature preceding the co-crystallization step. I n general, such utilization of alcohol may afford either economic or operational advantages ovc'r existing processes for cesium-137 recovery and purification.
Experimental The repurification system for the water of the Brazilian Swimming Pool Reactor is composed of two sets of ion exchangers and activated carbon units in
stainless steel tanks. In each set there is one activated carbon unit, followed by a mixed-bed ion exchanger which repurifies 6,000,000 liters of water after each regeneration. The total volume (for regeneration solution and resin washes after regeneration) is about 520 liters. During regeneration those volumes are collected in stainless steel tanks, where ferric chloride is added to the solution. Ferric hydroxide is then precipitated by adding sodium hydroxide and sodium carbonate. The mixture is stirred and left for decantation; 24 hours later the supernatant solution is separated from the precipitate (which has collected a great part of the fission products). A4tthis stage the pH of thr solution is about 10 and a majority of the fission products is entrained with the precipitate. Cesium remains in the liquid along with traces of strontium, rare earths, and ruthenium. Sodium sulfate, formed when the resins were regenerated, also remains at a concentration of about 38 grams per liter. This liquid, which has a gamma activity of 30 c.p.m. per ml. (counted in a thalliumactivated sodium iodide scintillator), was the source for all experiments discussed. Experiments were carried out with 1, 2, 10, and 100 liters of solution.
Procedure To 1 liter of the filtrate from the carbonate-hydroxide precipitation, sulfuric acid was added (to the end point of methyl orange). At this point 60 grams of ammonium alum were added and the liquid was stirred until dissolution of the alum gave a clear solution. The pH was adjusted to 3.6 with sulfuric acid and 250 ml. of technical grade ethyl alcohol were added with continuous stirring. The temperature in this operation usually increased from 22' to about 29' C. The ammonium alum starts crystallizing and is completely settled in about 10 to 1 3 minutes. The precipitate settles rapidly and is very VOL. 52, NO. 2
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Cesium Removal from Solution Expt, K O
Agitation Standing Ammonium Alum, Grams Time, Time, Added Crystallized pH Hours Hours 1-Liter Solution, 250 M1. of Alcohol Added
1 2 3 4 5 6 7 8 9 10
20 30 40 50 60 60 60 60 60 60
1 2 3 4 5 6 7
60“ 30 30 30 30 30 30 30 30 30 30
0 3 14 24 35 37 25 34 34 29
3.70 3.70 3.70 3.70 3.70 3.70 3.90 3.70 2.10 1.75
1.0 1.0 1.0 1.0 1.0 1.2 1.5 1.5 1.5 1.5
24.0 24.0 24.0 24.0 24.0 2.0 16.0 16.0 16.0 16.0
cs Recovered, % 0 0
13.5 97.5 99.5 100.0 98.0 98.0 100.0 98.0
By Sequence of Precipitation
8 9
10 11
...
... ... ... ... ... ...
... ... ... ...
3.35 3.70 3.70 3.65 3.60 3.32 3.36 3.64 3.60 3.60 3.50
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
16.0 0.3 0 0 16.0 0.5 0
0 1.0 0.3 15.0
100.0 100.0 100.0 100.0 99.5 100.0 100.0 99.0 99.1 98.0 100.0
a In each experiment the alum crystallized in the previous o n e was used. 250 ml. of alcohol added per liter of solution in each step. Initial alum, 60 grams per liter; 30 grams added in each operation.
easy to filter, and its volume is only about 370 of the solution volume. The precipitate is filtered and 98 to 100% recovery of cesium is achieved with only one precipitation. The table gives a typical set of operational data from a 1-liter solution scale. This same process can be applied by using precipitated alum from a first decontamination and adding it to a fresh solution of cesium to be recovered, adjusting the amount of alum to 60 grams, and repeating the process. The table also gives results for the experiments carried out in this way. Experiment 1 is started with 60 grams of alum already used in a cesium-recovery step. Experiment 2 is carried out by using the alum filtered from experiment 1 (about 30 grams), adding 30 grams of fresh alum, and so on. At the end of 11 operations the total amount of alum used was 360 grams and the alum recovered was 58 grams, giving a consumption of 302 grams of alum per 11 liters of cesium solution. [This same set of operations, in water and without alcohol, can be carried out ( 3 ) by using 350 grams of alum to start; adding 200 grams of alum per operation; heating the solution in each step to about 60” C. ; and then cooling it to 25’ C. The total amount of alum used for the 11 recoveries would be 2350 grams of alum, 150 grams of which would be lost in each operation, giving a total loss of 1650 grams of alum per 11 liters of cesium solution.] The alum is filtered and the activity in the liquid is determined in a scintillometer and compared with the activity of
148
the same volume of liquid before the carrying of cesium by alum. Decontamination factors up to lo6 have been obtained. The crystallized alum is dissolved in hot water and ammonium hydroxide is added to the solution to precipitate aluminum hydroxide. After filtration, the solution is concentrated by heating until ammonia is eliminated, leaving ammonium sulfate and cesium sulfate in solution. The ammonium salt can then be removed by adding aqua regia and distilling off ammonia gas. The liquid containing cesium sulfate is passed through an anionic resin (in the hydroxide form). Cesium remains in the resin effluent in the form of hydroxide. Excess ammonium is eliminated by boiling, and the cesium hydroxide solution is finally neutralized with sulfuric acid and concentrated. Aliquots of the solution were taken and sources for counting were mounted. The gamma spectrum for the recovered cesium was compared with the spectrum of a cesium-137 standard furnished by Tracerlab, Inc. ; no gamma impurity was detected. Feather analysis from aluminum absorption curves indicated one beta component of 0.5 m.e.v. and a harder one of 0.8 m.e.v. It is known (70) that aluminum absorption curves do not afford a highly precise method of indicating the 8% 1.2-m.e.v. beta component for cesium-137, which can be resolved only by p-ray spectrometry (7). Aluminum absorption curves made by Glendenin and Metcalf (2) have the same shape as those obtained by the present authors, indicating the 0.5-
INDUSTRIAL AND ENGINEERING CHEMISTRY
m.e.v. beta component and another one with 0.8 m.e.v. as determined by a Feather’s anlyzer. One impurity which might contaminate the cesium-137 would be strontium89 and strontium-90, since traces of these two isotopes might pass into the filtrate from the ferric hydroxidecarbonate precipitate. For this reason the 1.46-m.e.v. P-rays from strontium-89 ( 5 3 days), 0.61-m.e.v. p-rays from strontium-90 (28 years), and 2.229-m.e.v. prays from yttrium-90 (64.8 hours, strontium-90 descendent) were looked for in the aluminum absorption curves; none were found. The activity determined every day for about 4 months showed the absence of any beta impurity with a half life of 53 days that might be ascribed to strontium-89 ; the absence of strontium-89 indicates no contamination by strontium-90 or its descendent yttrium-90. No contamination from rare earths and ruthenium was found. Even if the concentration of cesium after several operations reaches a macro level, the efficiency of the process is not hampered, because the solubility of cesium alum is low (6). The use of alcohol-water mixtures and lower operating temperatures in this process is considered possible advantages over the existing processes for cesium-137 recovery.
Acknowledgment The authors express appreciation to Alan T. Gresky for his kindness in improving the English manuscript.
literature Cited (1) Fisher, C., Ragenbass, A., “Radioisotopes in Scientific Research,” UNESCO Intern. Conf., Paris, September 1957, p. 694. (2) Glendenin, L. E., Metcalf, R. P., “Radiochemical Studies. The Fission Products,” Book 2, ed. by C. D. Coryell and N. Sugarman, p. 1062 McGrawHill, New York, 1951. (3) Gresky, A. T., Atomic Energy Comm., AECD-2999 (1 950). (4) “Handbook of Chemistry and Physics,” Hodgman, C. D., ed., 13th ed., p. 370, Chemical Rubber Publishing Co., Cleveland, Ohio, 1948. (5) Lima, F. W., AbrHo, A., Pagano, C., Tognoli, L., 2nd Intern. Conf. on Peaceful Uses of Atomic Energy, Geneva, September 1958, Paper 2256, vol. 10, p. 532. (6) Mellor, J. W. A , , “Comprehensive Treatise on Inorganic and Theoretical Chemistry,” vol. 5 , p. 35, Longmans, Green, London, 1952. (7) Peacock, C. L. T., Mitchell, A. C. G., Phys. Rev. 75, 1272 (1949). (8) Rupp, A. F., Intern. Conf. on Peaceful Uses of Atomic Energy, vol. 14, p. 68, 1955. (9) Saddington, K., “Radioisotopes in Scientific Research,” UNESCO Intern. Conf., Paris, September 1957, p. 707. (10) Townsend, J., Owen, G. E., Marshall, Cleland, Hughes, A. L., Phys. Reu. 74, 98 (1948). RECEIVED for review July 9, 1959 ACCEPTED September 21, 1959
