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ORIGINAL ARTICLE
Year : 2021  |  Volume : 44  |  Issue : 2  |  Page : 92-97  

Natural and fallout radioactivity mapping of Kakrapar Gujarat site, India


1 Environmental Survey Laboratory (ESS, EMAD, BARC), Surat, Gujarat, India
2 Environmental Monitoring and Assessment Division, BARC, Mumbai, Maharashtra, India

Date of Submission18-May-2021
Date of Decision06-Aug-2021
Date of Acceptance11-Aug-2021
Date of Web Publication23-Oct-2021

Correspondence Address:
S S Wagh
Environmental Survey Laboratory (ESS, EMAD, BARC), Kakrapar Gujarat Site, P.O. Anumala, Surat - 394 651, Gujarat
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/rpe.rpe_16_21

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  Abstract 


This study presents the activity and outdoor gamma absorbed dose rates (terrestrial and cosmic) due to the naturally occurring (226Ra, 232Th, and 40K) and anthropogenic (137Cs) radionuclides around Kakrapar Gujarat site. The activity level (Bq/kg) in the soil ranged 5.7–48.4 for 226Ra, 9.7–28.0 for 232Th, 83.9–585.7 for 40K, and 0.2–4.6 for 137Cs, respectively. The mean concentration levels measured in Kakrapar soil from the naturally occurring radioisotopes are lower than the corresponding global average values. 137Cs activity in the soil is comparable with the preoperational period. The total effective dose rates in air outdoors ranged 17.2–78.1 μSv/y with the mean value of 32.8 μSv/y. The mean value of absorbed dose rate due to cosmic components was 37.5 nGy/h which is comparable to the worldwide reported values.

Keywords: Cosmic, gamma dose rate, Kakrapar, natural radioactivity


How to cite this article:
Wagh S S, Patra A K, Saradhi I V, Kumar A V. Natural and fallout radioactivity mapping of Kakrapar Gujarat site, India. Radiat Prot Environ 2021;44:92-7

How to cite this URL:
Wagh S S, Patra A K, Saradhi I V, Kumar A V. Natural and fallout radioactivity mapping of Kakrapar Gujarat site, India. Radiat Prot Environ [serial online] 2021 [cited 2021 Dec 7];44:92-7. Available from: https://www.rpe.org.in/text.asp?2021/44/2/92/329132




  Introduction Top


Human beings are exposed to background radiation that stems both from natural and human-made sources. Natural background radiation, which is equivalent to 2.4 mSv/person, makes up approximately 80% of the total radiation dose a person is exposed in a year.[1] The natural radioactivity in soil comes from 238U, 232Th, and from natural 40K. Some other terrestrial radionuclides, including those of the 235U series, 87Rb, 138La, 147Sm, and 176Lu, exist in nature, but at such low levels, their contributions to the dose in the humans are small. The radiological implication of these radionuclides is due to the gamma-ray exposure of the body and irradiation of lung tissue from inhalation of radon and its daughters. Therefore, the assessment of gamma radiation dose from natural sources is of particular importance as natural radiation is the largest contributor to the external dose of the world population.[2]

Measurement of radioactivity in soil is very important to determine any changes in activity with time as a result of radioactive release. During accidental conditions, it is necessary to monitor the release of radioactivity into the environment in order to manage appropriate radiological protection of humans.[3] Predominant parts of the natural radioactivity in soils derive from the members of the radioactive decay series of 238U and 232Th, along with 40K. Anthropogenic radionuclides (137Cs) is present in the atmosphere resulting from nuclear weapon test fallout, Chernobyl and Fukushima nuclear power plant accidents.

The primary objective of the present study is to determine the natural (226Ra, 232Th, and 40K) and anthropogenic (137Cs) radioactivity concentration in soil samples collected around Kakrapar Gujarat site environment and its contribution to dose rate around this environment. In addition, the gamma radiation dose rate at 1 m in air, coming from terrestrial and cosmic radionuclides, was also investigated.


  Material and Methods Top


Study area

This study was carried out at Kakrapar Gujarat site [Figure 1], situated on the southern bank of Moticher lake, which is about 85 km by road from Surat city, which is a southern region of Gujarat state (latitude – 21° 14' 8.37” N and longitude – 73° 21'1.3” E).
Figure 1: Map of the investigated area showing sampling locations

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Soil sample collection and estimation of natural and anthropogenic (137Cs) radioactivity

A total of 55 soil samples were collected from cultivated areas around this site, as shown in [Figure 1]. Soil samples were dried in an oven at 105°C for 24 h and then crushed, ground to fine powder, and homogenized by passing through 180-μm test sieve. About 500 g of the soil samples were sealed and stored in a Marinelli beaker for 1 month to allow the attainment of radioactive equilibrium with the decay chain. Natural radionuclides of relevance for this work are mainly gamma-ray emitting nuclei in the decay series of 232Th and 238U and single occurring 40K. 137Cs and 40K are measured directly by its own gamma-rays 662 keV and 1461 keV, respectively. The activity of 226Ra was determined by taking the weighted mean of their decay products: 214Bi (609 and 1120 keV) and 214Pb (352 keV). Similarly, the activity of 232Th was determined by taking the weighted mean of their decay products: 208Tl (583 and 2614 keV), 212Pb (238 keV), and 228Ac (911 keV). After attainment of secular equilibrium between 232Th, 226Ra, and their daughter products, the samples were subjected to gamma-ray spectrometric analysis. The measurements were carried out using gamma-ray spectrometry with a P-type coaxial high-purity germanium detector of 100% relative efficiency and a resolution of 1.9 keV at the 1332 keV gamma energy of 60Co. The system was calibrated for energy and efficiency using spiked soil sample and liquid standards containing 137Cs, 60Co, 133Ba, and 40K. The efficacy of the sample counting was verified by the analysis of certified reference material such as IAEA-375 soil and IAEA-444. The results agreed within ±5% of the certified values.

Ambient gamma dose rate measurement

The gamma absorbed dose rate in air was measured using environmental thermoluminescence dosimeters (TLDs) placed in the different locations around Kakrapar Gujarat site on a quarterly basis. Simultaneously, gamma dose rate tracers were also used for gamma exposure rate measurement of respective TLD locations. The measurement points were in the same locations as the soil samples were collected. Gamma dose rate tracer is an autonomous measurement probe used for continuous surveillance of the environmental gamma dose rate. At each location, gamma dose rate tracer was kept above 1 m ground level in air. Two measurements were taken for 10 min in each location, and the average was calculated. After the exposure period, gamma dose rate tracer data were retrieved through inbuilt IR receiver present in DataGate embedded web server to computer. Gamma dose rate tracer and environmental TLD data provide the terrestrial and cosmic ray components of gamma radiation.

Calculation of gamma dose rate in air outdoors and effective dose rate

The total absorbed dose rate D (nGy/h) in air at 1 m above ground level due to the presence of natural radionuclides in the soil samples was estimated using the following formula:[4]



where CRa, CTh, CK, and CCs are the activity concentrations of 226Ra, 232Th, 40K, and 137Cs (Bq/kg dry weight), respectively.

In order to make a rough estimate for the annual effective dose outdoors, one has to take into account the conversion coefficient from absorbed dose in air to effective dose and the outdoor occupancy factor. In the UNSCEAR reports,[4],[5] the committee used 0.7 Sv/Gy as the conversion coefficient from absorbed dose in air to effective dose received by adults and 0.2 for the outdoor occupancy factor. Effective dose rate outdoors (μSv/y) were calculated by the following formula:




  Results and Discussion Top


Distribution of natural (226Ra, 232Th, and 40K) and anthropogenic (137Cs) radioactivity in cultivated surface soil samples

The activity levels of 226Ra, 232Th, 40K, and 137Cs in cultivated surface soil samples collected from different locations around Kakrapar Gujarat site are given in [Table 1]. The activity of 226Ra in the soil ranged 5.7–48.4 Bq/kg, 232Th ranged 9.7–28.0 Bq/kg, 40K ranged 83.9–585.7 Bq/kg, and 137Cs ranged 0.2–4.6 Bq/kg, respectively. The 137Cs activity in the soil samples collected around Kakrapar Gujarat site during the preoperational period (1992–1993) was observed to be in the range of 0.5–4.8 Bq/kg dry weight with an average of 2.2 Bq/kg dry weight. The present level of 137Cs activity in the soil sample is comparable with the preoperational period. The variation of 226Ra, 232Th, 40K, and 137Cs activity in cultivated surface soil samples is shown in [Figure 2]. It is of interest to compare the natural radioactivity data of Kakrapar soil with the worldwide soil data, as shown in [Table 2]. It is observed that the mean concentration levels measured in Kakrapar soil from naturally occurring radioisotopes such as 226Ra, 232Th, and 40K are lower than the corresponding global average values obtained in worldwide soil. Linear regression analysis was carried out for 40K/226Ra versus 40K/232Th and 40K/226Ra versus 226Ra/232Th. The correlation coefficient was found to be 0.84 and 0.52, as shown in [Figure 3].
Figure 2: Variation of 226Ra, 232Th, 40K, and 137Cs activity in cultivated surface soil

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Figure 3: Linear regression analysis of 40K/226Ra vs. 40K/232Th and 40K/226Ra vs. 226Ra/232Th

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Table 1: Natural (226Ra, 232Th, and 40K) and anthropogenic (137Cs) radioactivity in cultivated surface soil collected from different locations

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Table 2: Comparison of natural and anthropogenic radioactivity in soils of Kakrapar, Gujarat, with different parts of the world

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Measurement of absorbed dose rate and effective dose rate

The absorbed dose rate and effective dose rate in air outdoors were calculated from concentration of radionuclides of 238U series, 232Th series, 40K, and 137Cs using Equations 1 and 2 and are tabulated in [Table 1]. The absorbed dose rate in air outdoors was found to be in the range of 14.0–63.7 nGy/h with a mean and standard deviation of 28.0 ± 9.1 nGy/h, which is below the corresponding world median value of 51 nGy/h and Indian value of 41.5 nGy/h.[16] The total effective dose rates in air outdoors for soils ranged 17.2–78.1 μSv/y with the median value of 32.3 μSv/y. [Figure 4] illustrates the measured relative contributions to total effective dose in air outdoors due to 232Th, 226Ra, 40K, and 137Cs content in soil.
Figure 4: Relative contributions to total absorbed dose in air outdoors due to 226Ra, 232Th, 40K, and 137Cs content in soil

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Tzortzis et al.[17] reported the dose rate and effective dose rate in air outdoors at Cyprus as 0.1–50 nGy/h and 0.1–61.4 μSv/y. Ramli et al.[18] reported the dose rate in air outdoors at Malaysian state of Johore as 9–1262 nGy/h. Mohanty et al.[19] reported the dose rate and effective dose rate in air outdoors at the eastern coast of Orissa as 650–3150 nGy/h and 0.78–3.86 mSv/y, respectively. Singh et al.[20] reported the dose rate in air outdoors at some areas of Punjab and Himachal Pradesh as 33–126 nGy/h. Malanca et al.[21] reported the dose rate in air outdoors at Rio Grande do Norte (Brazil) as 15–179 nGy/h. Anagnostakis et al.[22] reported the dose rate in air outdoors in Greece as 40 nGy/h.

Absorbed dose rate was measured using environmental TLD and gamma dose rate tracer around 17 locations in and around Kakrapar Gujarat site, and the results are tabulated in [Table 3]. It is observed that the absorbed dose rate measured using TLD and gamma dose rate tracer varied 316–679 μGy/y with an average value of 513 ± 19 μGy/y and 428–964 μGy/y with an average value of 611 ± 50 μGy/y, respectively. The absorbed dose rate observed using TLD and gamma dose rate tracer is due to the terrestrial and cosmic components.
Table 3: Measurement of absorbed dose rate using environmental thermoluminescence dosimeters and gamma dose rate tracer

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The absorbed dose rate due to cosmic component was computed based on the absorbed dose rate measured using TLD and gamma dose rate tracer data and by using the activity of 238U series, 232Th series, 40K, and 137Cs in soil samples collected from Kakrapar Gujarat site, as shown in [Table 3]. It is observed that the absorbed dose rate due to cosmic components around Kakrapar Gujarat site was found to be in the range of 20.4–58.2 nGy/h with an average of 37.5 ± 9.2 nGy/h which is comparable to the value of Japan (30.1–59.4), Indonesia (21.1–61.9), Taiwan (25.7–58.0), Iran (33.0–57.6), Sweden (32–50), and Italy (32–54).[16]


  Conclusions Top


The mean concentration levels measured in Kakrapar Gujarat site soil from the naturally occurring radioisotopes such as 226Ra, 232Th, and 40K are lower than the corresponding global average values obtained in worldwide soil. The 40K activity in the Kakrapar soil is higher than that of 232Th and 226Ra. The absorbed dose rate in air outdoors was found to be in the range of 14.0–63.7 nGy/h with a mean and standard deviation of 28.0 ± 9.1 nGy/h, which is below the corresponding world median value of 51 nGy/h and Indian value of 41.5 nGy/h. The total effective dose rates in air outdoors for soils ranged 17.2–78.1 μSv/y with the median value of 32.3 μSv/y. The absorbed dose rate due to cosmic components around Kakrapar Gujarat site was found to be in the range of 20.4–58.2 nGy/h with an average of 37.5 ± 9.2 nGy/h. The present level of 137Cs activity in the soil sample is comparable with the preoperational period.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest



 
  References Top

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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4]
 
 
    Tables

  [Table 1], [Table 2], [Table 3]



 

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