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23 Analysis and Public Health Aspects of Environmental Tritium A. A. MOGHISSI

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Downloaded by UNIV OF CINCINNATI on May 30, 2016 | http://pubs.acs.org Publication Date: January 1, 1970 | doi: 10.1021/ba-1970-0093.ch023

U. S. Department of Health, Education, and Welfare, Southeastern Radiological Health Laboratory, P.O. Box 61, Montgomery, Ala. 36101 An advanced dioxane-based liquid scintillation solution is developed which incorporated 5 ml. of water and yields a Y value of approximately

A new detergent, Triton-101, in association with p-xylene is used for the suspension of 10 ml. of water with a Y value of approximately 0.5 nCi/liter. The application of a new instrumental technique with three photomultipliers decreases further the Y value of both mixtures. A Teflon cylinder with a volume of 250-300 ml. is used as sample container for low level counting with a Y value of approximately 0.2 nCi/liter. Selected results of samples collected during 1967 are reported, and the radiation dose to the population of the United States from tritium is estimated to be approximately 0.2 mrem./year. ' " p r i t i u m is a radionuclide with considerable public health significance. Starting with the atmospheric thermonuclear tests, tritium concentration in the Northern Hemisphere has increased considerably above the natural background of approximately 10 p C i / l i t e r of water (5). Since the cessation of these tests, environmental tritium concentrations have decreased gradually. Tritium is, however, produced i n every nuclear reactor to some extent as a product of fission ( J ) or the activation of deuterium. In particular, reactors with heavy water as the moderator or cooling agent produce a large amount of tritium. Inasmuch as no Present address: Southwestern Las Vegas, Nev. 89114.

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Radiological Health Laboratory, P.O. Box 15027,

419 Freiling; Radionuclides in the Environment Advances in Chemistry; American Chemical Society: Washington, DC, 1970.


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economically feasible process is known for separating tritium i n nuclear waste water, the expanding nuclear industry w i l l introduce tritium into the environment. In addition, k C i amounts of tritiated compounds are produced annually for various applications. A portion of this tritium escapes into the environment as waste water. Consequently, a constant and careful observation of the tritium concentration i n the environment, and research to understand its effect i n biological systems are necessary. This paper summarizes some of the recently developed methodology used at the Southeastern Radiological Health Laboratory to determine environmental tritium concentrations. Since some of the methods used are not yet available in the literature, their development and essential features are discussed. Selected data on environmental tritium concen trations are presented and interpreted. In recent years, several developments have generated new possi bilities i n liquid scintillation counting. The most important event was the development of the bialkali photomultipliers with considerably i m proved quantum efficiency. In addition, several new scintillators and solvents of higher purity have become available. W i t h the resulting counting systems it has been possible to achieve sensitivities comparable with gas counters, with much less time and effort involved. Criteria

for Comparing

Counting

Systems

The selection of an appropriate technique for low level counting has been discussed by Moghissi et al. (9). If M ml. of water with a specific activity of Y n C i / l i t e r are counted with an efficiency of Ε c.p.m./d.p.m. for t minutes, the number of counts obtained are as follows: (Ai Χ Ι Ο " ) Χ Y X (2.22 Χ 10 ) 3

3

X £ X i

If background ( Β in c.p.m. ) is counted for t minutes, its standard devia tion w i l l be y/ Bt. The minimum limit of detection at 1σ confidence level and one-minute counting time requires: (Ai Χ ΙΟ" ) 3

Χ Y X (2.22

Χ Ι Ο ) Χ EXt 8

= y/~Bt

where 2.22 is c.p.m./pCi. Rearrangement of this equation yields:

r-

^*

Χ Ε Χ M This equation is applicable to background dominant counting sys tems and cannot be used to compare counters such as those used i n tracer studies. In addition, it disregards the importance of convenience and time requirements in using a particular procedure. However, if internal 2.22

Freiling; Radionuclides in the Environment Advances in Chemistry; American Chemical Society: Washington, DC, 1970.

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gas and liquid scintillation counters are compared, these factors should also be considered. The manufacturer rates the performance of his instrument by a figure of merit, F , defined as S / B , where S is reported as the counting efficiency of an unquenched sample. Unfortunately, F is inadequate for evaluating low level counting systems. This value, for an instrument using S l l type photomultipliers, for example, is only slightly lower than those with bialkali photomultipliers. The use of the latter instruments for low level counting, however, reveals that the Y value is decreased by a factor of 3-4. The reason is the necessity of applying highly quenched samples i n low level counting where the majority of radioactive decays produce only 2-3 photons. The difference i n the quantum efficiency of the two photomultiplier types, i n this case, affects the counting efficiency considerably more than an unquenched sample where more photons are available.


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Materials A Beckman liquid scintillation counting system was used to study scintillation solutions and to analyze environmental samples. A l l scintillators were obtained from Pilot Chemicals, Boston, Mass. p-Dioxane, spectroscopic grade, and p-xylene (m.p. 12°-13°C.) were received from Matheson, Coleman, and Bell, Rutherford, N . J . Scintillation

Solution

The first attempt to develop an efficient scintillation method for low level counting consisted of evaluating the dioxane-based solution and studying the many factors influencing the efficiency and the background (9). The resulting solution consisted of 5 m l . of water and 20 m l . of dioxane containing 7 grams 2,5-diphenyloxazole ( P P O ) , 1.5 grams bis(o-methylstyryl) benzene ( b i s - M S B ) , and 120 grams naphthalene/liter dioxane. The Y-value of this solution was approximately 1 n C i / l i t e r . The presence of oxygen in this solution as in all dioxane-based scintillators adversely affects the efficiency and background. Also, light excitation produces phosphorescence. Therefore, it is necessary to use high purity compounds and avoid contact with air and exposure to light. Many successful suspension counting systems have been reported where water is suspended in a scintillation mixture by a detergent. V a n der Laarse ( 8 ) reported a system consisting of 4 ml. of water and 16 m l . of a mixture of toulene containing 4 grams P P O , 0.3 gram P O P O P / l i t e r mixed with Triton X-100 in a ratio of 55:25. H e obtained a counting efficiency of 2 1 % and a Y-value of 1.3 n C i / l i t e r .


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Williams (12) compared suspension and homogeneous scintillation for tritium low level counting. H e showed, i n agreement with van der Laarse, that higher £ · M values can be obtained using Triton X-100 as detergent. H i s most sensitive solution consisted of toluene (7 grams P P O , 0.5 gram P O P O P / l i t e r ) and Triton X-100 mixed i n a volume ratio of 65:35. Water (7.5 ml.) was counted with an efficiency of 15.5% and a background of 10 c.p.m. resulting i n a Y-value of 1.2 n C i / l i t e r . A n important factor i n using suspension counting is the counting vial. Glass vials usually decrease the counting efficiency b y 10-20% and increase the background b y approximately 10 c.p.m. as compared with polyethylene vials. Unfortunately, aromatic solvents migrate through polyethylene vials with considerable speed (120-180 m g . / d ) , placing limitations and inconvenience on the application of these vials i n suspension counting. The new nylon vials (Nuclear Chicago) are resistant to aromatic solvents. Their resistance to water, however, is low, and owing to the resulting change i n their shape, they cannot be used for water counting i n instruments with automatic sample changers. These factors should be weighed carefully against the small decrease in Y value before a decision is made regarding the application of the reported procedures. Recognizing this fact, many commercial detergents were tested i n the author's laboratory. A Y value of 0.5 was obtained with several detergents; for example, Triton N-101 ( Rohm and Haas Co., Philadelphia, Pa.) using p-xylene as solvent and a ratio of xylene to detergent of 2.75:1. The optimum concentration of water was 10-12 m l . i n a 25 m l . mixture. It was of considerable interest to find that the instabilities reported by Benson (2) and confirmed b y others (12) were absent i n room-temperature operated liquid scintillation equipment. O n the other hand, it was found that phosphorescence and chemiluminescence were severe i n suspension counting. The possible temperature effect is under study i n this laboratory. F r o m the preliminary results it can be concluded that the increased sensitivity is to some extent responsible for the detection of the mentioned undesirable effects i n suspension counting. Large Volume Liquid Scintillation Counting The lowest Y value obtainable using the above-mentioned liquid scintillation counting systems is limited by the volume of the vial used (25 ml.). F o r lower Y values, a larger volume is necessary (3,7). To determine the optimum volume, a system was designed as shown i n Figure 1. A Teflon tube with a 5-cm. internal diameter was used as the container and reflector. T w o Teflon sheets (0.3-mm. thick) closed both ends of the tube by two O-rings and two aluminum devices pressed


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• T e f l o n

O - R i n g

A l u m i n u m

T e f l o n


S h e e t

Figure 1.

R i n g

T u b e

Large volume cell for low level counting of tritium by liquid scintiUation

Figure 2. Block diagram of the three photomultiplier system for low level counting of tritium together by four long screws. This system permitted volume variation by using tubes of different lengths. A volume of 250-300 ml. was found to have a minimum Y value of approximately 0.4 n C i / l i t e r for dioxane and 0.2 n C i / l i t e r for the suspension system. The latter figure approaches the lowest Y value of any tritium low level counting system, including the most sensitive gas counters (4). A n anticoincidence ring is being prepared for this system.


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Tritium Counting with Three Pbotomultipliers A triple-coincidence technique has been described by Schwerdtel (11) for determining counting efficiency of quenched samples by using coincidence rules. A similar system was designed independently and used i n this laboratory for the same purpose and, contrary to Schwerdtel's method, also for low level counting. The block diagram is shown i n Figure 2. During this study it was found that if the system were designed i n such a manner that a pulse were registered when any two of the three photomultipliers observe an event (simultaneous double coincidence), the counting efficiency increases by approximately 10-20% as compared with the usual two-photomultiplier system. The Y value i n this case is correspondingly lower. This is explainable by the increased surface of photomultipliers by 5 0 % . Environmental Levels of Tritium during 1967 The Bureau of Radiological Health ( B R H ) of the Public Health Service for many years has been analyzing and evaluating environmental tritium. Several networks comprising various media have been estab lished. Surface waters, rain, urine, and food are analyzed by the South eastern Radiological Health Laboratory, and the results are reported periodically i n Radiological Health Data and Reports. During 1967 the dioxane-based liquid scintillation solution was used to analyze tritium samples. The minimum limit of detectability of this system at 2 σ confidence level was 0.2 n C i / l i t e r . A n example of the net works' operating during 1967 is the one carried out i n cooperation with the South Carolina State Board of Health. Samples are collected from the Savannah River near Beaufort, S. C , and Augusta, G a . ; and the Broad River near Columbia, S. C. These three stations are part of the environmental surveillance program of the South Carolina State Board of Health and include two stations located near the Savannah River Plant operated by Ε. I. d u Pont de Nemours and Co. for the Atomic Energy Commission. Figure 1 shows the results of the analysis of the collected samples from these stations and the difference between the tritium levels of the samples from the two latter stations. Inasmuch as the Savannah River plant is located between these stations, the difference can be attributed to the operation of this major nuclear facility (Figure 3). Radiation Dose from Environmental Tritium The ultimate purpose of the surveillance activities was to determine the radiation dose to the population. Moghissi and Porter (JO) reported



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Figure 3. Tritium concentration of samples collected during 1967 from the Broad River at Columbia, S. C, and the Savannah River at Beaufort, S. C, and Augusta, Ga., as well as the difference in tritium concentration of samples from the latter two stations the following equation for correlating dose equivalent (DE) concentration of tritium ( C ) i n body water:

and the

DE (mrem./α) = 0.17C (nCi/liter) This equation is based on several assumptions as discussed by Moghissi and Porter (JO) requiring further study and, as such, should be used with caution. Among several media analyzed during 1967, urine is probably the best for calculating DE. The tritium concentration i n urine during 1967 averaged approximately 1 n C i / l i t e r , corresponding to 0.2 mrem./α. I n terms of Federal Radiation Council guidance (6) this represents 0.1% of the radiation protection guide ( 170 mrem. ) for an average dose to a suitable sample of the population. The average tritium concentration i n surface waters is 5 to 6 n C i / l i t e r . If it is assumed that the specific activity of tritium i n surface water is identical to that i n the body, this corresponds to 1.0 mrem./α. This discrepancy between DE values calculated from urine and surface waters is explained by the fact that the stations where surface waters are collected are not necessarily representative for the



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areas monitored b y urine analysis and are generally located around nuclear facilities. Often the relatively high concentrations are diluted by rain and other waters before the river water is consumed by the popu lation. Food results do not follow any pattern because of the composite nature of the samples. It is particularly interesting to evaluate the contribution of a major nuclear facility, such as the Savannah River Plant, to the radiation dose to the population. Inasmuch as Beaufort, S. C , receives its drinking water primarily from the Savannah River, the expected body burdens may be extrapolated from tritium concentrations of the river at Beaufort. As mentioned previously, the difference between the concentrations at Augusta, Ga., and Beaufort, S. C., may be attributed to the operation of the Savannah River Plant. The difference averages approximately 4 n C i / l i t e r and corresponds to 0.8 mrem./α. This represents 0 . 5 % of the radiation protection guide for an average dose to a suitable sample of the population. It should be emphasized that there are a number of factors i n the public health aspects of tritium which are not well established and whose discussion is beyond the scope of this paper. Studies should continue to develop more sensitive techniques for tritium measurement and to increase present knowledge i n this field of radiological health. Acknowledgment The support of many staff members of the Southeastern Radiological Health Laboratory, i n particular my co-workers, is greatly appreciated. Literature Cited (1) Albenesius, E. L., Phys. Rev. Letters 3, 274 (1959). (2) Benson, R. H., Anal. Chem. 38, 1353 (1966). (3) Boyce, I. S., Cameron, J. F., "Tritium in the Physical and Biological Sciences," Vol. I, p. 231, IAEA, Vienna, 1962. (4) Cameron, J. F., "Radioactive Dating and Methods of Low-Level Count ing," p. 543, IAEA, Vienna, 1967. (5) Kaufman, S., Libby, W. F., Phys. Rev. 93, 1337 (1954). (6) Federal Radiation Council, Background Material for the Development of Radiation Protection Standards, Rept. No. 1, 1960. (7) Kaufmann, W. J., Nir, Α., Parks, G., Hours, R. M., "Tritium in the Phys ical and Biological Sciences," Vol. I, p. 249, IAEA, Vienna, 1962. (8) van der Laarse, J. D., Intern. J. Appl. Radiation Isotopes 19, 337 (1967). (9) Moghissi, Α. Α., Kelley, H. L., Regnier, J. E., Carter, M. W., Intern. J. Appl. Radiation Isotopes 20, 145 (1969). (10) Moghissi, Α. Α., Porter, C. R., Rad. Health Data Rept. 9, 337 (1968). (11) Schwerdtel, E., Atomkernenergie 11, 324 (1966). (12) Williams, P. H., Intern. J. Appl. Radiation Isotopes 19, 377 (1968). RECEIVED July 22, 1968. Mention of commercial products used in connection with work reported here does not constitute an endorsement by the Public Health Service.


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