|Year : 2015 | Volume
| Issue : 1 | Page : 14-22
Estimation of annual effective dose rate due to the ingestion of the primordial radionuclide 40K for the population around the Kalpakkam nuclear site, Tamil Nadu, India
Pew Basu, R Sarangapani, K Sivasubramanian, B Venkatraman
Radiological Safety Division, Radiological Safety and Environmental Group, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu, India
|Date of Web Publication||14-Aug-2015|
Radiological Safety Division, Indira Gandhi Centre for Atomic Research, Kalpakkam - 603 102, Tamil Nadu
Source of Support: Nil., Conflict of Interest: None
A study was carried out to estimate the ingestion dose for the general population residing around Kalpakkam nuclear site due to 40K activity in the fresh and cooked food samples collected from the surrounding areas. For the estimation of specific activity of 40K, food samples consisting of 31 numbers of market basket samples (MBS) and 33 numbers of duplicate diet samples (DDS) were collected, conditioned, and analyzed by gamma spectrometry. The annual effective dose (AED) received by the population was estimated based on 40K activity in the food samples, food consumption data, and ICRP model. Uncertainty associated with the estimates was quantified based on the guide to the expression of uncertainty in measurement framework approach. The 40K specific activity in MBS ranged from 10.44 ± 1.11 to 129.00 ± 13.64 Bq/kg fresh weight. Similarly, the 40K specific activity in DDS ranged from 10.85 ± 1.10 to 60.71 ± 6.15 Bq/kg fresh weight. The AED due to the ingestion of 40K estimated based on MBS was 93.81 ± 7.30 μSv/year. Similarly, the AED due to the ingestion of 40K estimated based on DDS was 33.47 ± 0.79 μSv/year among males and 26.31 ± 0.62 μSv/year among females.
Keywords: 40K, annual effective dose, expression of uncertainty, Kalpakkam, natural radioactivity, specific activity
|How to cite this article:|
Basu P, Sarangapani R, Sivasubramanian K, Venkatraman B. Estimation of annual effective dose rate due to the ingestion of the primordial radionuclide 40K for the population around the Kalpakkam nuclear site, Tamil Nadu, India. Radiat Prot Environ 2015;38:14-22
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Basu P, Sarangapani R, Sivasubramanian K, Venkatraman B. Estimation of annual effective dose rate due to the ingestion of the primordial radionuclide 40K for the population around the Kalpakkam nuclear site, Tamil Nadu, India. Radiat Prot Environ [serial online] 2015 [cited 2018 Mar 20];38:14-22. Available from: http://www.rpe.org.in/text.asp?2015/38/1/14/162827
| Introduction|| |
The natural radionuclide concentration in environmental samples varies based on geological and geographical factors. Studies on natural radioactivity could provide present levels of dose to the population in the area due to them. The dose to the population is from internal and external exposure. The components contributing to external dose are from cosmic rays, cosmogenic radionuclides, primordial radionuclides, and nuclear fallout radionuclides. The primordial radionuclides found in the earth's crust are thorium, uranium and actinium series, and some specific radionuclides like 40K and87 Rb. The internal dose is mainly due to the ingestion from the dietary intake of these primordial radionuclides. The factors that influence the level of radionuclides in foods are the chemical behavior of soil, physico-chemical forms of the radionuclide in the soil, radionuclide uptake by the plant and finally the degree of accumulation by particular foodstuffs. Among these radionuclides, 40K is an important radionuclide from radiological perspective since it is the largest contributor to the dose received by humans due to its spatial distribution in the environment. The natural potassium is an essential element in the body and its average mass concentration is 2 g/kg of body weight, and the amount of potassium in the body is maintained through homeostatic control. Potassium is an essential element to maintain biological processes, and its mean biological half-life is 30 days. The natural abundance of 40K is 0.012%, and the average activity of 40K in the human body is about 60 Bq/kg. Therefore, the estimation of average individual exposure to the public is possible based on measurements of activity in foodstuffs and use of appropriate models.
In the present study, 40K ingestion dose to the general population residing around Kalpakkam nuclear site is reported. The measurement of activity in the samples is affected by the statistical fluctuations and normally quantified through uncertainty analysis. The uncertainties associated with the estimated values were quantified by the guide to the expression of uncertainty in measurement framework (GUF) approach. The present study covers about 10 Km2 area around Kalpakkam. 40K ingestion dose is estimated based on analyzing two kinds of food samples viz., market basket samples (MBS) and duplicate diet samples (DDS). MBS are raw samples from the local markets in each village which includes green leafy vegetables, nonleafy vegetables, fruits, fish, milk, meat, and rice. DDS are cooked food samples collected from different houses from the villages around Kalpakkam. 40K activity is estimated by gamma spectrometric measurements after sample conditioning. The analysis of obtained gamma spectra was performed using the Microsoft Excel, Microsoft Office 2007 System, Microsoft inc., Version 1.0, 2011, Microsoft Corporation, Redmond, Washington, USA. The food intake values for the population were based on a report released by National Nutrition Monitoring Bureau of Indian Council of Medical Research. ICRP methodology was followed for the estimation of ingestion dose. The 40K ingestion dose is reported for the general population around the Kalpakkam nuclear site comprising of local inhabitants and migratory population from other states of India.
| Materials and Methods|| |
Kalpakkam site (12°33'N and 80°11'E) is situated about 65 km south of Chennai City on the East Coast of India. Kalpakkam site is a major nuclear complex having Madras Atomic Power Station (MAPS) with two units of pressurized heavy water reactor with a capacity of 220 MWe each, fast breeder test reactor. The villages selected for the sample collection were Kokkilimedu, Anupuram, Sadras, Pudupattinam, Natham, and Vitalapuram. Among them Kokkilimedu is in sector-A, Anupuram is in sector-F, Sadras and Pudupattinam are in sector-I and Vitalapuram is in sector-H as shown in [Figure 1]. All the villages chosen were within 10 Km2 area around the plant MAPS, Kalpakkam. Among the villages Kokkilimedu and Sadras are in Sterilized zone (Within 5 km radius of MAPS) and other villages are in Emergency planning zone (Within 16 km radius of MAPS). So the samples collected from various location around MAPS, Kalpakkam are based on the emergency preparedness plan and hence the samples are the representative samples.
Sample collection and analysis
64 dietary intake samples were collected for study. Among them, 31 were MBS and 33 were DDS. The foodstuffs (raw materials) were grown and collected from the local main markets of each village. These are the major consumed foodstuffs by the critical group of the population. They are representative samples since more than 70% of these foodstuffs are consumed by these population. As already mentioned, the group chosen for survey consists of people living near to Kalpakkam site and having maximum consumption of the particular foodstuffs collected for analysis. Besides, these villagers used to take only these particular foodstuffs for their daily diet. Hence, the group is the representative group for the study, and they are regularly monitored.
Market basket sample preparation
The MBSs were brought from the nearby local market and washed with distilled water for removing any external contamination on the surface of the items. The nonedible portions were removed from the samples and weighed. The samples were dried in hot air oven at 60°C for 24 h. The oven dried samples were weighed, homogenized, and stored in an airtight container for analysis. When ashing the food samples, the temperature was increased slowly until the upper limit of the temperature (150°C–325°C) was reached. After reaching the upper limit, the temperature was raised rapidly to 450°C for about 16 h to remove any organic matter.
Duplicate diet sample preparation
The DDSs were collected from the volunteers in the form ready for consumption. DDS are the mixture of breakfast, lunch, and dinner in various families, and they were combined for analysis. These are the cooked food samples. The cooking process involves various stages as required for each item such as boiling, frying, etc. After collection, samples were weighed, labeled, and stored at −20°C prior to analysis. The frozen samples were thawed at room temperature. Once again, the samples were weighed and homogenized. The homogenized samples were transferred to a precleaned freeze drying attachments like petri-dishes and lyophilized in the lyophilizer. Once the freeze drying was completed, the dry weight of the samples was recorded and stored in an airtight container for the analysis.
Analysis of the samples
40K activity in the samples was determined by gamma spectrometry analysis. Gamma spectrometry system was a p-type coaxial high purity germanium detector (Model No. PGC– 3018) shielded with 5-cm-thick lead and having 30% relative efficiency. Resolution of the system at 1333 keV of60 Co line was 2 keV. The gamma spectra were acquired using Fastcom 4K MCA and were analyzed using APTEC@ software, Multi Channal Analyser, Aptec MCA Application, Aptec-NRC, Version 6.3.1, 1998-99, New York, USA. The IAEA standard source ( 152Eu), which is a multi γ emitter with energies varying from 39 keV to 2 MeV and of intensity 4810 Bq was used for energy and efficiency calibration. The standard has the same geometry (250 ml) as that of the samples. The minimum detectable activity of the system was 4.30 Bq/kg for 40K, with a counting time of 2000 s.
The conditioned food samples were placed on the detector and counted for a period of 2000 s. The net area under the 1460.52 keV peak in the energy spectrum was computed by subtracting counts coming due to compton scattering and from other background sources from the total peak area. Specific activity estimation of 40K was directly measured by its γ line of 1460.52 keV. From the net area of the peak, the specific activity of 40K in the samples was obtained using the equation.
Where N = the net peak area of the sample spectrum, m = Sample mass in kg, ε = Photopeak efficiency at 1460.52 keV, γ = Emission probability of the gamma line corresponding to 1460.52 keV, K1 = Nuclide decay correction factor, K2 = Nuclide decay correction factor for the duration of counting, K3 = Correction factor for the self-attenuation in the measured sample compared with the calibration sample and estimated using Point Kernel-based IGSHIELD code,K4 = Correction factor for the loss due to random summing, K5 = Coincidence correction factor.
Estimation of annual effective dose due to 40K
40K activity concentration indicates the amount of radioactivity present in the diet. The annual effective dose (AED) is very useful from the radiation effect perspective as it would give an idea of the proportion of the total dose received by the subject in the year. The calculation of the AED is based on activity in food samples and consumption rate defined in the NNMB report. AED, D (µSv/year) due to the radionuclide 40K in various food samples was calculated using the following expression.
Where, Cw = (Cd × h) is the 40K content in fresh food Bq/kg, Cd is the activity content in dry food, h is the ratio of the dry weight to wet weight of the sample, I is the annual consumption of the food (kg/year), DCF is the dose conversion factor (Sv/Bq). The value of DCF for 40K is 6.2E-09 Sv/Bq.,,
Uncertainty analysis of activity and dose rate using guide to the expression of uncertainty in measurement framework approach
In addition to the measurement of activity and estimation of dose rate, the statistical uncertainty associated with these quantities provides information about the spread of the data. In this study, uncertainty analysis was carried out based on GUF approach. Measurement of uncertainty using this approach is simple as computation is possible very easily, and no software is needed for this calculation and also this approach is very effective to detect the outliers in the data set. The GUF approach estimates the standard uncertainty of the output by combining the standard uncertainties of the inputs using the "error propagation law" of Gauss. In general, the output quantity is dependent on many parameters. Hence, the uncertainty of output is measured by a linear combination of the uncertainty of the inputs.,
The activity, A, is a function of various input parameters as given in eq. (1). The output uncertainty is not sensitive to the uncertainty of all of the input parameters, and hence quantification of uncertainty is required to know the individual contribution of each input uncertainty component to the combined output uncertainty. Furthermore, there is some correlation between some of the input parameters and that could affect the uncertainty in output. The relative combined standard uncertainty uc (A)/A of the activity concentration is given by the expression:
Where u (N) = uncertainty in net counts, u (m) = uncertainty in sample mass, u (ε) = uncertainty in photopeak efficiency, u (γ) = uncertainty in emission probability, u (K3) = uncertainty in K3, r1 = correlation coefficient between Nand K3, r2 = correlation coefficient between m and K3, r3 = correlation coefficient between Nand m.
The expanded uncertainty ur (A) for 3 σ confidence level was obtained by multiplying uc (A) with the coverage factor 3.
| Results and Discussion|| |
40K specific activity in various dietary components of market basket samples and duplicate diet samples
Market basket samples were grouped into seven categories, namely green leafy vegetables, nonleafy vegetables, fruits, fish, meat, milk, and cereal. All the raw materials are locally grown and collected from the local markets. The measured specific activity in various samples collected nearby Kalpakkam nuclear site is given in [Table 1]. The activity present in various MBSs varies from 10.44 ± 1.11 Bq/kg fresh weight to 129.00 ± 13.64 Bq/kg fresh weight.
From [Table 1], average 40K activity in dietary components from the Kalpakkam environment varies in the order of rice > fruits > milk > leafy vegetables > meat > fish > nonleafy vegetables. The present study shows that the specific activity in vegetables varies from 10.44 ± 1.11 (snake guard) to 75.46 ± 8.15 Bq/kg fresh (Curry leaves), for fruits the value ranged from 13.13 ± 1.37 (yellow squash) to 129.00 ± 13.64 Bq/kg fresh (coconut), for fish the value ranged from 27.76 ± 2.92 (Sankara fish) to 78.32 ± 8.34 Bq/kg (Oora fish). The average values of specific activity in milk and meat were found to be 52.5 ± 4.63 Bq/kg fresh and 44.72 ± 3.45 Bq/kg fresh, respectively. The value for rice was found to be 79.08 ± 8.25 Bq/kg fresh.
It is observed that a lot of work has been carried out to estimate natural radionuclide content in environmental samples especially for 40K as it is more abundant and easily measurable from its characteristic gamma peak., In the Indian context, estimation of natural radioactivity and associated external dose rates in soil samples from Kalpakkam and Thanjavur were performed and the reports shows the 40K specific activity as 434.1 ± 131.1 Bq/kg in Kalpakkam and 149.5 ± 3.1 Bq/kg in Thanjavur and the external effective dose rate as 91.5 ± 37.8 μSv/year in Kalpakkam and 53.1 ± 11 μSv/year in Thanjavur. Shukla et al., estimated natural and fallout radioactivity in milk and diet samples and population dose rate in Bombay and reported the value between 79 and 177 μSv/year with an average of 110 μSv/year. James et al. estimated the internal dose to members of the public at the Kaiga site, India due to the ingestion of primordial radionuclide 40K and reported the average annual dose was 136 μSv/year. International studies have also been reported. Abbady, estimated the level of natural radionuclides in foodstuffs and reported the annual ingestion dose due to radium, thorium, and potassium as 214.8 μSv/year in Egypt. Makon et al. investigated the gamma-emitting natural radioactive contents in three types of vernonia (Vernonia is an asterceae with bitter leaves; it generally occurs in three species namely: Amygdalina, Calvoana,and Richardiana) consumed in Cameroon and reported the average value of 40K activity as 302 ± 36 Bq/kg and the average AED due to 40K as 0.15 μSv/year. Thus, it can be seen that the levels of 40K depend on many factors including geological and hence a study has been undertaken in Kalpakkam, India to estimate the annual ingestion dose.
Shanthi et al. reported the 40K activity for MBS and estimated the 40K specific activity in rice, milk, fish as 120.2 ± 15.8 Bq/kg fresh, 34.35 ± 5.2 Bq/kg fresh and 88.91 ± 6.7 Bq/kg fresh respectively. Activity in vegetables ranged from 29.67 ± 9.1 (Cucumber) to 78.7 ± 9.3 Bq/kg fresh (Drumstick), for fruits the value ranged from 33.4 ± 4.5 (Guava) to 136.2 ± 8.3 Bq/kg fresh (Banana). James et al. reported the 40K activity around Kaiga nuclear site, activity for rice was found to be 45.4 Bq/kg dry, for leafy vegetables 167 Bq/kg edible, for nonleafy vegetables 73.1 Bq/kg edible, for milk 19.1 Bq/l, for meat 61 Bq/kg edible, for fish 89.9 Bq/kg edible. The activity value for leafy vegetables, nonleafy vegetables, meat, and fish are higher than the estimated values in Kalpakkam. The estimated values of activity for rice and milk in Kalpakkam are higher than the value reported by James et al. This difference could perhaps be due to variation in the irrigation methods such as well/river. Because well and lake (stagnant source) type of irrigation is practiced in Kalpakkam whereas in Kaiga, river (flowing source) type of irrigation is practiced. Ramchandran et al. estimated the 40K activity in dietary intake (MBS) for Indians ranged from 45.9 to 649 Bq/kg. Nair et al. estimated the 40K activity in DDS for an Indian population with eight samples and reported the mean value as 86.3 ± 19.8 Bq. Except Nair et al. all others have carried out the estimation through survey, not through DDSs.
Internationally, few authors have carried out this study mainly with MBS. Changizi et al. reported the average 40K activity in vegetables as 187.4 Bq/kg. In typical Korean foods, the 40K activity for milk was found to be 52.94 ± 0.19 Bq/kg fresh, in meat the value ranged from 58.54 ± 0.36 to 96.13 ± 0.48 Bq/kg fresh, in various grains 1.18 ± 0.12–632.7 ± 3.93 Bq/kg fresh, in different vegetables 15.0 ± 0.1–96.88 ± 0.44 Bq/kg fresh, in fruits 13.08 ± 0.07–34.65 ± 0.37 Bq/kg fresh, in fish 15.45 ± 0.68–168.3 ± 2.46 Bq/kg fresh. In Tehran-Iran, the activity in milk samples varies from 11.4 to 42.8 Bq/kg fresh with an average value of 31.0 ± 6.1 Bq/kg.
In the light of the above discussions, our investigation consisted of 33 DDSs from different areas within 10 km. DDS are the mixture of breakfast, lunch, and dinner collected from different households from various villages located around Kalpakkam nuclear site. The major component in all the samples was found to be rice. The estimated specific activity in DDS is given in [Table 2]. 40K activity in various DDSs varies from 10.85 ± 1.10 to 60.71 ± 6.15 Bq/kg fresh weight.
Mean 40K specific activity in DDS varies from 10.85 ± 1.10 to 60.71 ± 6.15 Bq/kg fresh with a minimum at Anupuram village and maximum at Kokkilimedu village.
Annual consumption rates of market basket samples and duplicate diet samples
For the computation of AED rate, the consumption rates for different kinds of MBS and DDS were taken from the values given in NNMB report. The annual intake of various dietary components is given in [Table 3]. The values are given for Tamil Nadu, India, and also for few other states.
Among various dietary components, consumption of rice is the highest in Tamil Nadu. The consumption rate of rice is about 64% of the total consumption rate. The contribution of milk, nonleafy vegetables and fruits are 17%, 7.5%, and 7%, respectively. The total contribution of fish, leafy vegetables, and meat was found to be 4.5%. It is inferred from the table that the dietary pattern is different in different states. Hence, the annual internal dose received by the population among various states will also be different which is graphically explained in [Figure 2]. The maximum consumption rate of green leafy vegetables is at Odisha, the maximum consumption of nonleafy vegetables is at Gujarat, fruits at Andhra Pradesh, fish at Kerala. Consumption rate of meat is found to be same in Andhra Pradesh and West Bengal and higher compared to other states. Milk consumption rate is highest in Gujarat and cereal consumption rate is highest in West Bengal.
|Figure 2: Variation of ingestion dose for various states and India from market basket samples|
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Similarly, consumption rate of DDS is given in [Table 4]. For the estimation of AED, the age and sex specific standard values were taken from National Nutrition Monitoring Bureau report. [Table 4] shows that the maximum consumption rate is for the age group ≥18 years irrespective of sex for both Tamil Nadu and India.
|Table 4: Age and sex specific consumption rate of DDS for Tamil Nadu and India|
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Calculation of annual effective dose based on market basket samples and duplicate diet samples
Annual effective dose computed using equation (2) based on specific activity present in MBS for the population residing around Kalpakkam is given in [Table 5].
|Table 5: Annual effective dose due to 40K to Kalpakkam population based on MBS|
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Dose from various dietary components decreases as in the order of rice > milk > fruits > nonleafy vegetables > meat > fish > green leafy vegetables. Of the total effective dose rate, 74% is contributed by rice, 13% by milk, 6% by fruits, and 4% by nonleafy vegetables. Green leafy vegetables, fish, and meat contribute about 3%. The maximum AED is due to the consumption of rice. Lenka et al. estimated the annual ingestion dose from cereals between 109.4 and 936.8 µSv/year to the Odisha population, which is a naturally very high background radiation area because of the presence of Uranium mines. James et al. reported the annual dose to Kaiga population ranged from 114 to 152 µSv/year with an average of 136 µSv/year. In Midalam, Kanyakumari the dose due to 40K was reported as 460 µSv/year. In Kudankulam, the annual dose was found to be 143 μSv/year. The annual dose received due to ingestion of typical Korean foods range from 115 to 136 μSv/year with an average of 101 μSv/year. The AED due to the ingestion of 40K for Japanese adults varies between 160 and 210 μSv/year with a mean of 180 μSv/year and for Bangladeshi adults the value reported was 100 ± 25 μSv/year. In Egypt, the annual dose due to ingestion of vegetables was found to be 80 μSv/year. The annual dose from 40K received by the adults of Tehran-Iran was reported as 14 μSv/year. Frissel et al. reported that people receive 180 μSv/year dose from ingestion of 40K. Narayanan et al. reported the average annual ingestion dose from the ingestion of 40K is 189 μSv/year. Thus, it can be seen that our study revealed as already mentioned that the total annual dose value was close to that of Korea. Furthermore, the geology of the region and the food processing methods has a profound influence on 40K concentration. Further, it should be noted that most of the Indian study is based on the survey reports compared to actual DDS.
It is also interesting to compare the estimated AED to the Indian population. The AED was also estimated for various states based on the activity given in [Table 1] and dietary intakes of individual states as given in [Table 3]. The computation of effective dose is necessary since the working population of Kalpakkam nuclear site belong to Tamil Nadu and many other states and hence they have different dietary pattern. The estimated AED rate for several Indian states and India are graphically shown in [Figure 2].
Within our study area, AED estimated based on DDS is given in [Table 6]. The annual average dose to an adult person in Kalpakkam from DDS is estimated as 29.89 ± 0.50 µSv/year.
|Table 6: Annual effective dose due to 40K to the adult population around Kalpakkam based on DDS|
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The age wise effective dose rates estimated based on DDS intake for Kalpakkam, and the Indian population is shown in [Figure 3]. The effective dose rate is higher for age groups 16–17 and ≥18 years because of the highest consumption of diet in this age. The effective dose rate is more or equal for Kalpakkam population compared to Indian population for all groups excepting for ≥18 years persons because of the higher intake of rice by the Kalpakkam population during this age compared to India. The estimation was based on mean activity given in [Table 2] and age-specific dietary intake given in [Table 4].
|Figure 3: Comparison of age-specific ingestion dose for Kalpakkam and India based on duplicate diet samples|
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| Conclusion|| |
40K is an important radionuclide of interest because of its long half-life and one of the predominant radionuclides present in the human body. It is an essential biological element present in daily diets. It is reported that 40K has high solubility and a very high transfer factor from soil to the food chain. The major dietary components of the adult population around the Kalpakkam site are rice, fish, nonleafy vegetables, leafy vegetables, fruits, milk, and meat. 40K specific activity in MBS ranged from 10.44 ± 1.11 to 129.00 ± 13.64 Bq/kg fresh weight and in DDS ranged from 10.85 ± 1.10 to 60.71 ± 6.15 Bq/kg fresh weight. The 40K AED received by the Kalpakkam population from MBS was found to be 93.81 ± 7.30 μSv/year. Similarly, the AED due to the ingestion of 40K estimated based on DDS was 33.47 ± 0.79 μSv/year among males and 26.31 ± 0.62 μSv/year among females. The ratio of dose estimates based on MBS to DDS is about 2.8. The estimates based on DDS are considered to be more realistic and appropriate. The maximum annual dose received by the Kalpakkam inhabitants is found to be due to rice consumption.
Specific dose estimates are required for the locations where nuclear installations are proposed to be commissioned to understand the variations in the natural background. The annual dose received by the local population residing around Kalpakkam nuclear site is in agreement with the reported results by some other investigators. The present study provides site-specific data which are very useful if any epidemiological or environmental impact survey is contemplated in the vicinity of Kalpakkam nuclear site in future.
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[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]