I
ELI I. GOODMAN Nuclear Science and Engineering Corp., Pittsburgh 36, Pa.
Selection’and Handling of
...
A Radio Tracer for Studying Sewage Distribution done to ensure against beach pollution when an expanded sewage plant in Los Angeles, Calif., is placed in operation. The over-all study provided information to correlate bacterial disappearance in sewage effluent directly with physical dilution as well as time and distance from the point of entrance into the bay. Aside from instantaneously measuring dilution, the radioactivity method can “date” a particular mass of sewage. Salinity and chlorinity measurements can determine the ratio of fresh water to sea water, bxt it cannot determine how much of the fresh water is new and how much is old. I t was concluded that only a small percentage of sewage a t a particular point in the bay is “young.” Radioactivity in the effluent, even a t the point where it entered bay waters, was substantially less than the concentration permitted for drinking water as established by the National Bureau of Standards. Scientists aboard the laboratory ship, Velcro ZV, tcok radicactive measurements at variocs depths and positions to determine dilution and rate and direction of diffusion. This phase covered about 36 hours and an area of 25 square miles. Selection of scandium-46 was made after a thorough search for a suitable tracer. I t was considered satisfactory because it meets all requirements for radiological health standards, has a short half life, and its cost is not prohibitive. The 20 curies required for the experiment were obtained from the Oak Ridge National Laboratory. T h e sanitary engineering ( 3 ) and radioactivity detection ( 7 ) aspects of this experiment have been described (2, 4, 5). Five criteria were used for the tracer: T I J I s WORK W A S
Concentration of radioactivity in the sewage effluent a t the point of discharge into Santa Monica Bay should be no greater than the maximum permissible concentration (MPC) for drinking water as established in the National Bureau of Standards Handbook 52. The tracer had to be chemically compatible with both the effluent and sea water, and the radioactivity used had to permit in situ measurements when dilution of sewage by sea water to 1 part in 10,000. Also, its half life had to bc a t least several days, and cost reasonable. The in situ measurement criterion limited. the choice to gamma emitters only; thus, beta-only activities are not listed in Table I. The required activity for each tracer in this table is established from the criterion far detecting a dilution of l part in 10,000. This corresponds to 2.3 X lo3disintegrations per minute per milliliter for scandium-46 in the effluent when consideration is given to determination in these laboratories that 0.23 (10,000-fold less than in the effluent) such disintegrations show a count significantly above background, using a scintillation probe immersed in water. Thus 20 curies of scandium-46 introduced during a 1hour pericd was sufficient to meet the dilution criterion. Half lives shorter than several days were undesirable because of difficulty in predicting weather. Inclement weather might necessitate postponement of the experiment and such isotopes might have decayed to a point where they would be useless. Also, shipping time for the 20 curies of activity was about three days and could not be forecast or controlled accurately. From these considerations, all tracers with half lives less than several days or less
METERING TANU ( 5 0 GAL.)
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/LSTIRRER
T h e equipment NEEDLE VALVE
used for adding
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ROTAMETER
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sewage SEWAGE STREAM
SHlELDlNQ VALVE EXTENSION
O P E R A T ~ ~ POSITION S
2 10
INDUSTRIAL AND ENGINEERING CHEMISTRY
than molybdenum-99 were eliminated. Also iodine-131 was eliminated because the discharge concentration is a factor of 50 above the tolerance concentration. Rhenium-186, molybdenum-99, chromium-51! and selenium-75 are eliminated because solubility of their chemical compounds in sea water had to be established. This narrows the list down to rubidium-86, scandium-46, cesium134, and cesium-137. At this point cesium-137 was selected because of cost. However, because of its long half life. 85-day scandium-46 was considered, which had been eliminated because the O R N L catalog price for 20 curies was $40,000. From a radiological health standpoint, scandium-46 is more satisfactory than cesium-137 because in addition to its shorter half life, its discharge concentration is 300 fold lower than its tolerance concentration. The discharge concentration of cesium-1 37 would be somewhat higher than its tolerance value (Table I). This means that less than 40 curies of cesium-I37 would have had to be used, resulting in a loss of sensitivity in detection. Sensitivity of detection considerations make scandium-46 more desirable than rubidium-86 which has approximately 0.1 gamma per disintegration compared with 2 for scandium-46. Therefore, 400 curies of rubidium-86 is required to obtain the same sensitivity as that obtained with 20 curies of scandium-46. However, some reservations for scandium-46 were its possible incompatibility with sea water or its adsorption on particulate matter. If uniform mixing of the scandium (specific activity, 1 curie per gram) is assumed, concentration of scandium prior to discharge into the ocean is 10-9 gram per ml., about ll/a times the total concentration of lanthanum, yttrium, and scandium in sea water. T o minimize adsorption of scandium chloride, a sodium Versene complexing agent was used. Solubility of scandium chloride in the sewage was high. Injection Equipment and Procedure
The equipment comprises a 50-gallon head tank (oil drum) manual paddle for stirring, needle valve for flow control as well as cutoff, rotameter, and a ‘/*-inch piping with fittings. A head of 10 feet
was maintained between the tank bottom and the discharge pipe to assure sufficient flow. T h e needle valve was operated from behind 30 inches of packed sand as shielding; from this position, the operator was able to read the rotameter. T o minimize adsorption of scandium chloride on the equipment and piping, the system was conditioned with a solution of 100 grams of lanthanum chloride in ‘/zgallon of sodium Versene solution made up to 10 gallons. This was used in a dry run to work out various operating procedures and obtain experience in controlling the flow a t 10 gallons per hour. The scandium-46 came from the Oak Ridge National Laboratory in five separate bottles in a single shielded container. Each bottle cap was loosened on the day before the introduction. On May 22, the date of the experiment, the loosened caps were removed and contents of the bottles were poured into the 50gallon metering tank using a 4-foot tool. T h e empty bottles were flushed once with sodium Versene solution which was then added to the tank. Prior to adding the tracer to the tank, sodium Versene solution and 10 pounds of sodium hydroxide made u p to a volume of 10 gallons less the flushing solution, were added. After mixing with the manual stirrer, the contents of the tank was metered into the sewage a t 10 gallons per hour for l hour. Injection of the activity began a t 6:59 A.M. and was detected a t the sewage outfall in Santa Monica Bay a t 7 :47 A.M. Radiological Safety in the Field
All personnel were provided with film badges and self-reading dosimeters. Also, an ionization chamber-type survey meter (Nuclear Chicago Model 2586 “cutie pie,” 0 to 1000 m r per hour) was used to monitor all the operations. While one operator, using a 4-foot long-handled tool, transferred the scan-
dium-46 to the metered tank, another followed with a bucket of Versene flushing solution under the scandium-46 bottle. This bucket, held with a 4-fOOt pole, was provided in the event the scandium-46 bottle was dropped during the transfer operation. Most personnel exposures occurred during this transfer and pouring operation. This job was distributed among all participants to limit dosage received by a single individual to a nominal value. The self-reading dosimeters were useful in this aspect of the work. This transfer method is considered adequate for nonrepetitive experiments. However, for frequent experiments in the future, performance of this transferring operation from behind shielding is being considered. Introduction of the tracer from the metering tank into the sewage was performed from behind 30 inches of packed sand as shielding placed on three sides of the tank with a 60 X 120 foot exclusion area on the fourth side. T h e unshielded side provided convenient access to the metering equipment and piping before and after the 20 curies were in the metering tank. While the 20 curies were in the metering tank, all operations such as stirring and working the control valve were from behind the shielding. Decontamination
After the bulk of the scandium-46 was metered into the sewage, contamination of about 3 r per hour was measured a t the surface of the metering tank and 1 r per hour a t the piping and flowmeter surfaces. Repeated flushing of the equipment with cold water, cold water with sodium Versene, and boiling wafer with EDTA [ (ethylenedinitrilo) tetraacetic acid] and Versene brought the radiation level down by approximately a factor of 3. When suhequent flushings became ineffectual, the equipment was dismantled, and its individual components, placed on sepa-
Table 1.
Tracers Considered for Hyperion Dilution Study pc./Ml. Required Half Life, Gamma Activity, cost, Discharge Hr. Curies” Ed $103~ concn. M.P.C. I
12.4 15 26.8 36 2.F 3.9 8.1 19.5 27.8 85 127 2.3e 30
0.2 2
200 20 57 40 37 200 29 400 500 20 20 31 40
0.7 1 1.2 0.2 1.4 0.1 0.08 2 2 1.3 1
400 400 57 200
50 150 12 80 1,000 3d 20 31
10
x
10-8 1 2.8 2 1.6 10 1.5 20 25 1 I
10
x
10-8
8 200
... 14,000 ...0.03 3 500 300
... ...
3 1 2 1.5 a Based on sensitivity of detection corresponding to the 20 curies of scandium-46 (detectable in 1:10,000dilution). Effect of gamma energy neglected. ORNL catalog prices. Days.
Special quotation for quantity order.
e
Years.
rate pieces of absorbent paper with waxed paper backing, were further decontaminated with live steam and boiling EDTA. Contaminated wash solutions were poured into the plant sewage stream where the scandium-46 concentration was diluted to approximately 10,000 times below the drinking water tolerance concentration. After decontamination operations, all items considered contaminated were wrapped in absorbent paper, taped, and labeled according to their levels as measured a t the surface. These items were put into containers and turned over for disposal by a local contractor authorized by the Atomic Energy Commission for such disposal. Decontamination and disposal involved only a minor exposure to radiation by personnel because most of it was handled from behind shielding or a t a distance. In the future, it is hoped to minimize decontamination operations by using easily disposable or decontaminatable material-e.g., plastics, plastic coatings, and stainless steel. When flushing the scandium-46 bottles, some dripping had occurred which contaminated parts of the sand and wooden shielding structure. According to AEC regulations, a surface is adequately decontaminated for most beta and gamma emitters if its level is brought down to 1 m r per hour or less. Strontium-90, calcium-45, zirconium-95, bismuth-210, and radium-226 contamination must be reduced to 0.1 m r per hour. The wood and sand which were not considered contaminated were retained for re-use. T h e balance of the wood and sand was disposed of as mentioned previously, After disposal of the contaminated components, a comprehensive area survey was done, which indicated that lumber, sand, and iron stays which remained at the Hyperion plant were safe. One spot reading 18 mr per hour a t the surface was found on the concrete near decontamination operations. This was reduced to a safe level by repeated scrubbings. literature Cited (1) Ely, R. L., IRE Trans. Nuclear Sci. NS-4, 49-50 (March 1957). ( 2 ) Kaufman, W. J., and Orlob, G. T., Trans. Am. Geophys. Union 37, 297306 (June 1956). ( 3 ) Public Works (July 1956). f4) Thomas. H. A,. Jr.. Proc. Intern. Conf. Peaceful Uses’ Atomic Energy Geneia, 7955, 15,42-6 (August 1955). (5) Watkins, J. W., Mardock, E. S., “Use of Radioactive Iodine as a Tracer in Water Flooding Operations,” A m . Inst. Mining Met. Engrs. Tech. Publ. 3894 (February 1954). RECEIVED for review March 19, 1957 ACCEPTEDOctober 14, 1957 \
