Assessment of primordial radionuclide contents in soil samples and of impact of coal-based thermal power plant: A study in Tarn Taran district in Punjab, India

Mansi Dhingra; Manish Kumar; Rohit Mehra; Navjeet Sharma

doi:10.4103/rpe.RPE_11_20

ORIGINAL ARTICLE

Year : 2020 | Volume : 43 | Issue : 1 | Page : 49-54

Assessment of primordial radionuclide contents in soil samples and of impact of coal-based thermal power plant: A study in Tarn Taran district in Punjab, India

Mansi Dhingra¹, Manish Kumar², Rohit Mehra¹, Navjeet Sharma³
¹ Department of Physics, Dr. B. R. Ambedkar NIT, Jalandhar, Punjab, India
² Department of Physics, Kanya Maha Vidyalaya, Jalandhar, Punjab, India
³ Department of Physics, DAV College, Jalandhar, Punjab, India

Date of Submission	27-Mar-2020
Date of Decision	01-Apr-2020
Date of Acceptance	09-Apr-2020
Date of Web Publication	12-May-2020

Correspondence Address:
Mansi Dhingra
Research Scholar, Department of Physics, Dr. B. R. Ambedkar NIT, Jalandhar - 144 011, Punjab
India

Source of Support: None, Conflict of Interest: None

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DOI: 10.4103/rpe.RPE_11_20

Abstract

In the present study, an attempt has been made to measure the radionuclide contents in soil samples from the region around coal-based thermal power plant (TPP) in Tarn Taran district of Punjab state in India and to assess the impact of coal-based TPP on the radionuclide distribution in surrounding areas. For this purpose, soil samples collected from the region, coal samples, and fly-ash samples collected from the plant were analyzed using the gamma spectrometry technique employing thallium-activated sodium iodide detector. The activity concentration of radionuclides was observed to be higher for the samples collected from the locations nearer to the power plant and decreased as the distance from power plant increased indicating an increase in soil radioactivity due to TPP.

Keywords: Gamma spectrometry, radiation exposure, radioactivity, radionuclide

How to cite this article:
Dhingra M, Kumar M, Mehra R, Sharma N. Assessment of primordial radionuclide contents in soil samples and of impact of coal-based thermal power plant: A study in Tarn Taran district in Punjab, India. Radiat Prot Environ 2020;43:49-54

How to cite this URL:
Dhingra M, Kumar M, Mehra R, Sharma N. Assessment of primordial radionuclide contents in soil samples and of impact of coal-based thermal power plant: A study in Tarn Taran district in Punjab, India. Radiat Prot Environ [serial online] 2020 [cited 2021 Feb 26];43:49-54. Available from: https://www.rpe.org.in/text.asp?2020/43/1/49/284224

Introduction

Human beings continued quest for energy, to satisfy its ever-increasing demand, has severe adverse impact on environment. Although an increased emphasis is being placed on the development and use of renewable energy sources, still fossil fuels are a major resource for producing energy worldwide. Coal-based thermal power plants (TPPs) currently generate nearly 27% of total electricity production in the world.^[1] Natural coal contains, along with other minerals, primordial radionuclides radium, thorium, and potassium, in varying concentrations.^[2],[3] The combustion of coal in thermal plant produces bottom ash and fly ash as the main residue products. Some fraction of fly ash produced in the plant gets dispersed in atmospheric air, and finally, gets deposited on the soil in areas surrounding the TPP. The deposition of fly ash-containing radionuclides enhances the radioactivity of soil in areas surrounding the power plant. The contribution of fly ash to radionuclide contents in soils depends on various factors such as radionuclide's concentration in coal, fly-ash content of coal, and mode of dispersal in environment. It has been reported that radionuclides concentration in fly ash is more than its source, i.e., coal and this increment may be up to the first order of magnitude.^[4] Hence, the fly ash can be characterized as technologically enhanced naturally occurring radioactive materials. Since many TPPs, worldwide are located in populated areas, the enhancement of natural radioactivity levels in soil in the region surrounding TPPs can have serious radiation-related health implications for the population in these areas.^[5] The contribution of coal-based TPPs to natural radioactivity in surrounding areas is not fully characterized due to the lack of data in this regard. In recent years, a number of studies have been performed worldwide to assess the impact of coal-based TPP on the radioactivity levels in the surrounding region.^{[6],[7],[8],[9],[10],[11]}

The results from these studies report the impact from nonsignificant to slight significance in some cases. These facts necessitate more studies in this regard to fully characterize the contribution of coal-based TPPs to environmental radioactivity. The present study has been conducted to determine the effect of 540 MW GVK Power Goindwal Sahib Limited gunupati venkata krishna (GVK )TPP located at Goindwal Sahib in Tarn Taran in Punjab, India. The purpose of this study is to measure the activity concentration of radium (²²⁶ Ra), thorium (²³² Th), and potassium (⁴⁰ K) in soil to assess the contribution of this power plant to natural radioactivity levels in its vicinity and estimate the levels of radiation dose for population around this power station.

Materials and Methods

Experimental techniques

Sample collection and analysis

A total of 40 samples, which comprised 36 soil samples, 2 samples of fly ash, and 2 samples of coal, were analyzed for radionuclide's content. The fly-ash samples were collected from the ash pond and the electrostatic precipitator. The coal samples were collected from the inner side of the plant. The soil samples have been collected from different locations within an area of 5 km from the periphery of thermal plant, as shown in [Figure 1]. While collecting the soil samples, standard protocol was followed. First, the top surface of the soil was cleared of grass and other organic matter. Then, about 1 kg of soil was collected by using cylindrical soil probe, after removing top 5 cm of the soil, and samples were transported to the laboratory in air-tight plastic bags. In laboratory, the soil samples were cleared of impurities and dried in the oven at 110°C constant temperature for 24 h. Dried samples have been sieved through 150 μm size mesh, and about 250 g of each sample was stored in air-tight container for 28 days to attain secular equilibrium, before gamma spectrometry analysis. [Figure 1] depicts the region of TPP and sampling. The samples were analyzed for the activity concentration of²²⁶ Ra,²³² Th, and⁴⁰ K using the gamma spectrometry technique with the setup having 63 mm × 63 mm size NaI (Tl) detector having an energy resolution of ≤7.5% for 662 keV gamma ray line. Photopeak of 1460 keV was used for measuring the activity of⁴⁰ K, whereas energy peaks of 1764 keV from²¹⁴ Bi and 2610 keV from²⁰⁸ Tl were used for²²⁶ Ra and²³² Th, respectively.^[12] The secondary standards prepared using International Atomic Energy Agency quality-assurance materials RGU-1 and RGTh-1 were used for efficiency and energy calibration.

Figure 1: Map of GVK thermal power plant Goindwal Sahib, Tarn Taran District, Punjab

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Theoretical calculation of radiation exposure index

Radium-equivalent activity

The radium-equivalent activity (Ra_eq) in Bq/kg for the samples has been calculated using the following equation (1):^[13]

Where C_Ra, C_Th, and C_K are the activity concentrations in Bq/kg of radium (²²⁶ Ra), thorium (²³² Th), and potassium (⁴⁰ K), respectively.

Estimation of air-absorbed dose rate

The air-absorbed dose rate (D) (nGy/h) has been calculated using the following equation (2):^[14]

Where C_K, C_Th, and C_Ra are the activity concentrations in Bq/kg of potassium (⁴⁰ K), thorium (²³² Th), and radium (²²⁶ Ra), respectively.

Indoor annual-effective dose

The estimated indoor annual-effective dose (AED) (mSv/y) can be calculated using the following equation (3):^[15]

Where 0.7 Sv/Gy is conversion coefficient, and 0.8 is the indoor occupancy factor.

Outdoor annual effective dose

The estimated outdoor AED (mSv/y) has been calculated using the following equation (4):

Where 0.7 Sv/Gy is conversion coefficient and 0.2 is the outdoor occupancy factor.

Estimated external exposure index

The external exposure index is defined (H_ex) as

Where C_Ra, C_Th, and C_K are the activity concentrations of²²⁶ Ra,²³² Th, and⁴⁰ K, respectively, in Bq/kg.

Internal exposure index

The internal exposure index is defined (H_in) as

Where C_Ra, C_Th, and C_K are the activity concentrations of²²⁶ Ra,²³² Th, and⁴⁰ K, respectively, in Bq/kg.

Gamma index

The gamma index (

I_γ) was determined by the following equation:

Where C_K, C_Th, and C_Ra are the activities of⁴⁰ K,²³² Th, and²²⁶ Ra in Bq/kg, respectively. The maximum observed value is that is less than the unity (permissible limit).

Alpha index

Alpha index (I_α) was measured by the following equation:^[16]

C_Ra is the activity concentration of²²⁶ Ra (Bq/kg).

Results and Discussion

Activity concentrations of radionuclides

The measured activity concentrations of radium (²²⁶ Ra), thorium (²³² Th), and potassium (⁴⁰ K) in the soil, coal, and fly-ash samples are presented in [Table 1]. Samples codes S-1 to S-36 represent soil samples, FA-1 and FA-2 represent fly-ash samples, while C-1 and C-2 represent coal samples. The soil samples S1 to S4 have been collected from the locations within 1 km of distance from the periphery of coal-based TPP. Samples S5 to S19 have been collected from the locations within 1–3 km of distance, and samples S20 to S36 have been collected from the location within 3–5 km of distance from TPP. The average activity concentration of radium (²²⁶ Ra) in soil samples collected from the locations within 1 km of distance from TPP was found to be 33.6 Bq/kg as compared to the values of 28.2 Bq/kg and 23.9 Bq/kg for the samples collected from the locations at the distance of 1–3 km and 3–5 km, respectively. The observed activity concentration of radium (²²⁶ Ra) and potassium (⁴⁰ K) in all soil samples was found to be less than the world average value of 35 Bq/Kg and 400 Bq/kg, respectively. However, the activity concentration of²³² Th in all soil samples was higher than the world average value of 30 Bq/kg.^[14] These observed values are comparing with the values reported by the researchers from other locations from in India but are significantly lower than then values measured around Mawan TPP in South China.^[3],[17] Analysis of results show that mean values of activity concentration of radionuclides are maximum for the samples collected from the locations nearer to the power plant and goes on decreasing as the distance from power plant increases. This trend points to an increase in radioactivity due to TPP. Amin et al., in 2013, reported similar results from the studies around a TPP in Malaysia. The average activity concentration of radium (²²⁶ Ra) and thorium (²³² Th) in coal samples was 29.2 Bq/kg and 36.4 Bq/kg, respectively, while the activity concentration of potassium (⁴⁰ K) was below detection limit (BDL). The world average activity concentration values of²²⁶ Ra,²³² Th, and⁴⁰ K in coal are 20 Bq/kg, 20 Bq/kg, and 50 Bq/kg, respectively.^[4] This shows that average activity concentration values of²²⁶ Ra and²³² Th are higher than the world average values, whereas the activity concentration of⁴⁰ K is negligible.

Table 1: The activity concentration of radium (²²⁶Ra), thorium (²³²Th), potassium (⁴⁰K), and radium-equivalent (Ra_eq) activity in soil, coal, and fly-ash samples around GVK thermal power plant, Punjab

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Average activity concentrations of radium (²²⁶ Ra) and thorium (²³² Th) in fly-ash samples have been found to be 66.8 Bq/kg and 92.3 Bq/kg, while the activity concentration of potassium (⁴⁰ K) was found BDL of the measurement setup. The average activity concentration values in fly ash for radium (²²⁶ Ra) and potassium (⁴⁰ K) were significantly lower than the world average values of 240 Bq/kg and 265 Bq/kg, whereas the average value for activity concentration for thorium (²³² Th) was higher than the world average value of 70 Bq/kg for thorium (²³² Th). Furthermore, the average activity concentration of²²⁶ Ra and²³² Th in fly-ash samples was 2.3 and 2.5 times higher than the corresponding values in coal samples.

The concept of Ra_eq is used to compare the specific activities of various samples having different concentrations of radium (²²⁶ Ra), thorium (²³² Th), and potassium (⁴⁰ K). The measured values of Ra_eq for different samples analysed are listed in [Table 1]. The Ra_eq values for soil samples varied from a minimum value of 77.7 Bq/kg to a maximum value of 138.1 Bq/kg, with an average value of 102.5 Bq/kg. All the observed values were less than the reference level of 370 Bq/kg.^[18] Calculated values of various radiological exposure indexes such as external and internal exposures indexes, estimated air-absorbed dose rate, internal and external AED, I_γ, and alpha index are presented in [Table 2].

Table 2: Computed values of exposures indexes, absorbed-dose rate, internal and external annual effective dose, gamma index, and alpha index for the population in study area around GVK thermal power plant, Punjab, India

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The absorbed-dose rate varied from 34.9 nGy/h to 94.36 nGy/h, with an average of 48.34 nGy/h. This value is comparable with the world average value of 57 nGy/h. The indoor AED varied from 0.17 mSv/y to 0.46 mSv/y, with an average value 0.23 mSv/y and outdoor AED varied from 0.04 mSv/y to 0.12 mSv/y with an average of 0.05 mSv/y. The values of H_ex varied from 0.21 to 0.58 with an average value 0.28 and H_in varied from 0.27 to 0.78 with an average value 0.36. The observed values are less than unity, indicating that the soil in the study area does not pose significant radiological threat to population. The value of alpha index ranged from 0.10 Bq/kg to 0.36 Bq/kg, with an average value 0.14 Bq/kg. The values of I_γ varied from 0.27 Bq/kg to 0.74 Bq/kg with an average value 0.38 Bq/kg.

Conclusions

The average activity concentration values of radium (²²⁶ Ra) and thorium (²³² Th) in coal have been found to be higher than the world average values, whereas the activity concentration of potassium (⁴⁰ K) is negligible. The average activity concentration values in fly ash for²²⁶ Ra and⁴⁰ K were significantly lower than the world average values of 240 Bq/kg and 265 Bq/kg, whereas the average value for activity concentration for²³² Th was higher than the world average value of 70 Bq/kg. The observed variation of radionuclide's content with distance from the coal-based power plant points to a slight increase in soil radioactivity due to proximity to TPP. The Ra_eq values for all soil samples were less than the reference level of 370 Bq/kg.

Acknowledgments

The authors are grateful to the Staff of GVK coal-based TPP and resident of study area for their cooperation and help during the extensive field work.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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