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ORIGINAL ARTICLE
Year : 2021  |  Volume : 44  |  Issue : 1  |  Page : 12-18  

Studies on natural and fallout radioactivity mapping of the proposed Mithivirdi Atomic Power Project Site in Bhavnagar District, Gujarat, India


1 Environmental Survey Laboratory (Environmental Studies Section, Environmental Monitoring and Assessment Division, Bhabha Atomic Research Centre), Surat, Gujarat, India
2 Environmental Monitoring and Assessment Division, BARC, Mumbai, Maharashtra, India

Date of Submission26-Feb-2021
Date of Decision16-Mar-2021
Date of Acceptance08-Apr-2021
Date of Web Publication07-Jun-2021

Correspondence Address:
Akhaya Kumar Patra
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_5_21

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  Abstract 


The activity and gamma-absorbed dose rate due to the naturally occurring (226Ra, 232Th, and 40K) and anthropogenic (137Cs) radionuclides in the terrestrial environment were determined in soil samples collected around Mithivirdi Atomic Power Project Site, Bhavnagar District, Gujarat, by using gamma-ray spectrometry. The mean concentration levels measured in soil from the naturally occurring radioisotopes 226Ra, 232Th, and 40K are lower than the corresponding global average values reported worldwide. 137Cs activity in the soil sample is comparable with the preoperational baseline level activity of other nuclear power plant sites in India. The total outdoor effective dose rates due to soil ranged 16.7–79.5 μSv/y with the median value of 34.3 μSv/y. The absorbed dose rate due to cosmic components around Mithivirdi site was found to be in the range of 44.4–90.6 nGy/h.

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


How to cite this article:
Patra AK, Jaison T J, Wagh S S, Jha M K, Saradhi I V, Kumar A V. Studies on natural and fallout radioactivity mapping of the proposed Mithivirdi Atomic Power Project Site in Bhavnagar District, Gujarat, India. Radiat Prot Environ 2021;44:12-8

How to cite this URL:
Patra AK, Jaison T J, Wagh S S, Jha M K, Saradhi I V, Kumar A V. Studies on natural and fallout radioactivity mapping of the proposed Mithivirdi Atomic Power Project Site in Bhavnagar District, Gujarat, India. Radiat Prot Environ [serial online] 2021 [cited 2021 Jun 24];44:12-8. Available from: https://www.rpe.org.in/text.asp?2021/44/1/12/317948




  Introduction Top


Human beings are exposed to radiation arising from various sources including cosmic rays, naturally occurring radionuclides in water, air, soil, and plants, and artificial radioactivity from fallout of nuclear testing and medical applications. The gamma radiation from natural radionuclides and cosmic rays constitute the external exposure while those derived from inhalation and ingestion through food and drinking water constitute internal exposure to humans.[1] It is estimated that 80% of dose contribution in the environment are derived from the natural radionuclides, while the remaining 20% are from cosmic ray and nuclear processes.[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 insignificant. Natural radioactivity is widely spread in the earth's environment and depends primarily on the geological and geographical conditions, and appears at different levels in the soils of each region in the world.[2] The assessment of radiation dose from natural sources is important as natural radiation is the largest contributor to the external dose of the world population.[2] 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 present are widely distributed in the atmosphere primarily resulting from fallout from atmospheric nuclear weapon tests (e.g., 137Cs).

The primary objective of the present study is to determine the natural (226Ra, 232Th, and 40K) and anthropogenic (137Cs) radioactivity in soil samples collected around the proposed Mithivirdi Atomic Power Project (MAPP) site and its contribution to dose rate in the surrounding environment. In addition, the gamma radiation dose rate at 1 m above the ground in air, coming from cosmic radionuclides, was also investigated. This study was carried out as a part of preoperational environmental monitoring of proposed site.


  Materials and Methods Top


Site description

The proposed MAPP site is located on latitude 21°28'8“ N and longitude 72°13'47” E in the western part of India, as shown in [Figure 1]. The site is about 13.6 m–21.2 m above sea level with the bay of Khambhat on the east. On the north side around 2 km from the site, Mithivirdi river meets the sea. The site is located at a distance of 40 km from district headquarter Bhavnagar city. The climate of the area is humid and hot. July to September/October is the monsoon season. The area around the location is used for agricultural purpose. The soil type is a mixture of sand, gravel, and clay as binder.
Figure 1: Environmental sampling map

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Geographical location and gamma radiation mapping

Latitude and longitude of different locations around MAPP site were measured by using Global Positioning System (12XL). Gamma radiation level was measured at each location by using gamma dose rate tracer.

Soil sample collection and estimation of radioactivity

A total of 30 soil samples were collected around Mithivirdi 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 a 180-μm test sieve. About 300-g soil sample was sealed and stored in a precalibrated geometry of a 300-ml bottle 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 radionuclide in the decay series of 232Th and 238U, and naturally occurring 40K. 137Cs and 40K are measured directly by characteristic gamma-rays of 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 and 226Ra with their daughter products, the samples were subjected to gamma-ray spectrometric analysis. The measurements were carried out by using gamma-ray spectrometer with a P-type coaxial high-purity germanium detector (Baltic Scientific make) of 100% relative efficiency and a resolution of 1.9 keV at 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 (International Atomic Energy Agency [IAEA]-375 soil and IAEA-444). The results agreed within ±5% of the certified values.

Calculation of gamma dose rate in outdoor air 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:[2]

D = 0.462 CRa + 0.604 CTh + 0.0417 CK + 0.030 CCs(1)

where CRa, CTh, CK, and CCs are the activity concentration of 226Ra, 232Th, 40K, and 137Cs (Bq/kg dry wt.), 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,[2],[3] 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 in outdoors (μSv/y) was calculated by the following formula:

Effective dose rate (μSv/y) = Dose rate (nGy/h)

× 24 h × 365d × 0.2 (occupancy factor) ×

0.7 Sv/Gy (conversion coefficient) × 10−3(2)


  Results and Discussions Top


Gamma radiation mapping in and around Mithivirdi site

Gamma radiation levels are measured and the latitude and longitude of the locations are shown in [Table 1]. It is observed that the gamma radiation level was 0.022–0.21 μSv/h, with an average value of 0.093 μSv/h. The gamma radiation levels (μSv/h) around different nuclear power plant (NPP) sites in India are reported as 0.03–0.05 in Chutka, Mandla, Madhya Pradesh;[4] 0.061–0.22 in Hisar, Fatehabad, Haryana;[5] 0.035–0.12 in Kaiga, Karnataka;[6] 0.02–0.20 in Jaitapur, Ratnagiri, Maharashtra;[7] 0.05–0.08 in Kakrapar, Surat, Gujarat;[8] 0.05–0.22 in Tarapur, Thane, Maharashtra;[9] 0.16–0.20 in Narora, Uttar Pradesh;[10] and 0.10–0.30 in Kudankulam, Tamil Nadu,[11] respectively. The gamma radiation level of Mithivirdi site is comparable with other NPP sites in India.
Table 1: Gamma dose rate level in and around Mithivirdi nuclear power plant site

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Distribution of natural and anthropogenic radioactivity in soil

The activity levels of 226Ra, 232Th, 40K, and 137Cs in soil samples collected from different locations are given in [Table 2]. The activity of 226Ra in the soil ranged 3.0–32.9 Bq/kg, 232Th ranged 10.0–72.5 Bq/kg, 40K ranged 25.6–331.3 Bq/kg, and 137Cs ranged 0.7–3.6 Bq/kg, respectively. Variation of 226Ra, 232Th, 40K, and 137Cs activity in soil samples is shown in [Figure 2]. It is observed that the mean concentration levels measured in Mithivirdi soil from naturally occurring radioisotopes such as 226Ra, 232Th, and 40K are lower than the corresponding global average values reported in [Table 3]. The 137Cs activity in Kakrapar Gujarat Site soil samples during the preoperational period (1992–1993) was observed to be in the range of 0.5–4.8 Bq/kg dry wt. The availability of this anthropogenic-produced radionuclide (137Cs) in the soil as a fallout radionuclide is mainly due to open atmospheric tests conducted by some countries prior to the ban of open atmospheric tests.[21] Linear regression analysis was carried out for 226Ra versus 232Th and 40K versus 137Cs. A good correlation was observed between 226Ra versus 232Th with a correlation coefficient of 0.93 and no correlation was observed between 40K versus 137Cs shown in [Figure 3].
Table 2: 226Ra, 232Th, 40K, and 137Cs activity levels (Bq/kg dry wt.) in soil samples collected in and around Mithivirdi nuclear power plant site

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Table 3: Comparison of natural and anthropogenic radioactivity in soils of Mithivirdi site with different parts of the world

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Figure 2: Variation of 226Ra, 232Th, 40K, and 137Cs activity in surface soil

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Figure 3: Linear regression analysis of 226Ra versus 232Th and 40K versus 137Cs

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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 2]. The absorbed dose rate in outdoor air was found to be in the range of 13.6–64.8 nGy/h with a median value of 28.0 nGy/h, which is below the corresponding world median value of 51 nGy/h and Indian value of 41.5 nGy/h.[22] The total effective dose rate in air from soils ranged 16.7–79.5 μSv/y with the median value of 34.3 μSv/y. [Figure 4] illustrates the measured relative contributions to total effective dose in air due to 232Th, 226Ra, 40K, and 137Cs content in soil.
Figure 4: Relative contributions to total effective dose in air outdoors due to 226Ra, 232Th, 40K, and 137Cs content in soil

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Tzortzis et al. reported the dose rate and effective dose rate in air outdoors at Cyprus as 0.1–50 nGy/h[23] and 0.1–61.4 μSv/y. Ramli et al. reported the dose rate in air outdoors at Malaysian state of Johore as 9–1262 nGy/h.[24] Mohanty et al., reported the dose rate and effective dose rate in air outdoors at eastern coast of Orissa as 650-3150 nGy.h-1[25] and 0.78-3.86 mSv.y-1, respectively. The reason is due to mainly monazite sands and to a lesser extent, zircons at eastern coast of Orissa. Singh et al. reported the dose rate in air outdoors at some areas of Punjab and Himachal Pradesh as 33–126 nGy/h.[26] Malanca et al. reported the dose rate in air outdoors at Rio Grande do Norte (Brazil) as 15–179 nGy/h.[27] Anagnostakis et al. reported the dose rate in air outdoors in Greece as 40 nGy/h.[28]

The measurement of absorbed dose rate due to cosmic components is tabulated in [Table 4]. It is observed that the absorbed dose rate due to cosmic components around Mithivirdi site was found to be in the range of 44.4–90.6 nGy/h with an average of 62.5 ± 13.2 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),[22] respectively.
Table 4: Measurement of absorbed dose rate due to cosmic components

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  conclusions Top


The activity concentrations of 226Ra, 232Th, 40K, and 137Cs in soil samples collected around Mithivirdi site were studied using gamma spectrometry. The outdoor absorbed dose rate in air was found to be in the range of 13.6–64.8 nGy/h with a median value of 28.0 nGy/h, which is below the corresponding world median value of 51 nGy/h and Indian value of 41.5 nGy/h. The estimated naturally occurring radioactivity in soil 226Ra, 232Th, and 40K is in the range commonly found elsewhere. The estimated total outdoor effective dose rates in air from soils ranged 16.7–79.5 μSv/y with the median value of 34.3 μSv/y. The absorbed dose rate due to cosmic components around Mithivirdi site was estimated to be in the range of 44.4–90.6 nGy/h.

The study indicates that the dose due to naturally occurring radionuclides around MAPP site is marginally less compared to global average value.

Acknowledgments

The authors would like to thank Site Director, Kakrapar Site; Station Director, KAPS-1&2 and ACE (E&US), ACE (Mithivirdi project) for their keen interest and encouragement. The assistance rendered by Smt. Padma Chaudhary, Shri M. K Chaudhary, Shri. J. J. Chaudhary, Late Shri. Mityabhai Chaudhary and all other ESL staff is thankfully acknowledged.

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], [Table 4]



 

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