On the 30th anniversary of the Chernobyl Nuclear Power Plant Accident, assessment of the activity concentrations and the radiological hazard parameters of soil samples collected from Rize province and districts

Highlights

• The average ²²⁶Ra, ²³²Th and ⁴ K activity concentrations were lower than the world median values (UNSCEAR).

• The average absorbed dose rate in air values were below the world median values (UNSCEAR).

• Internal and external hazard indexes were lower than the recommended limit value of 1.

• The average ¹³⁷Cs activity concentration was lower than ¹³⁷Cs value obtained by TAEA in 1990.

• Calculated excess lifetime cancer risk values did not exceed 0.29 × 10⁻³.

Abstract

Uranium, thorium and potassium are the main terrestrial sources of radiation that are located at different levels in the soil depending on the geographical and geological conditions and contribute to increase of the natural radiation levels of the regions. In addition to the natural radiation sources, Cesium-137 which can spread from nuclear tests and accidents such as Chernobyl Nuclear Power Plant Accident is also an important artificial radiation source of that contributes to the increase of radioactivity in the soil. Determination of both natural and artificial radioactivity levels of soil is important in terms of assessing people's possible exposure to the internal radiation and ensuring that people can be protected from the harmful effects of the external radiation. In this study, on the 30th anniversary of the Chernobyl Nuclear Power Plant Accident, soil samples were collected from Rize province which was affected by the Chernobyl Nuclear Power Plant Accident in 1986 and the activity concentrations of natural and artificial radionuclides were measured and radiological hazard parameters were calculated. Radioactivity measurements were caried out with gamma spectrometry system with a high purity germanium detector. The activity concentrations were measured in range of 4.45 ± 0.39 to 32.19 ± 0.78 Bq kg⁻¹ with an average of 17.68 ± 0.68 Bq kg⁻¹ for ²²⁶Ra, 5.58 ± 0.65 to 43.61 ± 1.67 Bq kg⁻¹ with an average of 22.53 ± 1.26 Bq kg⁻¹ for ²³²Th, 28.82 ± 1.37 to 773.19 ± 12.14 Bq kg⁻¹ with an average of 277.14 ± 6.24 Bq kg⁻¹ for ⁴⁰K and 1.99 ± 0.17 to 376.85 ± 2.52 Bq kg⁻¹ with an average of 110.11 ± 0.98 Bq kg⁻¹. The average activity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K were found below the world median activity concentrations of 35, 30 and 400 Bq kg⁻¹, respectively. Despite the high ¹³⁷Cs activity concentrations were determined in present study compared to similar works being done in Turkey and other countries, the average activity concentration of ¹³⁷Cs in investigated samples was significantly found lower than the ¹³⁷Cs values in soil samples from Rize in 1990 carried out by TAEA. In present study, the average radium equivalent activity (Ra_eq), external hazard index (H_ex) and internal hazard index (H_in) values were below the recommended limits by UNSCEAR. The average absorbed dose rate in air (D) and annual effective dose equivalent (AEDE) values were found lower than the world median values. The calculated excess lifetime cancer risk (ELCR) values for soil samples were obtained less than the precautionary limit of 0.29 × 10⁻³.

Introduction

Soil formed as a result of physical, chemical and biological degradation processes of rocks and organic materials, covers the top surface of the earth's crust, helps many organisms live in and on it, and is a vital natural resource for the sustainability of the ecological systems (Algan and Bilen, 2005; Brantley et al., 2006; Chapin et al., 2002; Pepper and Brusseau, 2019). The soil contains air, water, minerals and nutrients that enable plants that make up a significant part of people's food to grow and develop and radioactive elements of natural and artificial origin that can cause harmful effects of gamma rays through people's respiratory and digestive systems (Ergene, 1993; Kladsomboon et al., 2020; Nortcliff et al., 2011; UNSCEAR, 2000).

The natural radioactive elements in the soil are largely terrestrial origin and the most important of them are uranium and thorium series product nuclides and ⁴⁰K. Uranium, thorium and potassium concentrations in soil are relatively high in volcanic, phosphatic and granitic rocks and closely related to the breakdown of these rocks by various degradation processes and their mixing into the soil, magmatic formation of the earth crust and the mineralogical properties of radionuclides. (Ivanovich ve Harmon, 1982; Karahan, 1997; Tzortis and Tsertos, 2004).

The uranium, thorium and potassium atoms are generally not found in free form in nature but they combine with various elements and are involved in the structure of minerals and rocks (Eroğlu and Şahiner, 2017). Major uranium minerals are apatite, coffinite, pitchblende, torbernite, uraninite; thorium minerals are allanite, thorite, thorutite, thorianit and potassium minerals are potassium feldspar, kalsilite and muscovite (Abhilash and Pandey, 2015; Alekseev and Marin, 2015; Guillen et al., 2014; Manning, 2010; Orlandi et al., 2017). Although monazite, allanite, uraninite and thorite are highly common in nature, they are very minor constituents of rocks (Anjos et al., 2005).

The main rock types including uranium, thorium and potassium are volcanic rocks (andesite, dacite, rhyodacite, rhyolite, granite, basalt), metamorphic rocks (gneiss, slate, marble, schist, quartzite, hornblende, phyllite) and sedimentary rocks (limestone, sandstone, siltstone, shale, marl) (Reis et al., 2008; Tatar et al., 2007; Zhang, 2016). Granite formed as a result of the partial melting of magma and the combination of uranium and thorium in the liquid phase with silica crystals has a high content of uranium and thorium. Besides, the structure of granite also includes quartz, potassium feldspar and sodium feldspar (Merdanoğlu, 2004).

Rhyolitic rocks such as granite have a high content of uranium, thorium and potassium. Besides, andesite, dacite and basalt have a lower amount of uranium, thorium and potassium content according to the granite and rhyolite rocks. Some sedimentary rocks such as sandstone and limestone are not very rich in uranium, thorium and potassium content (Schön, 2015). Potassium feldspar is a common source of potassium in sedimentary rocks. In sedimentary rocks, other minerals with potassium content are mica and illite (Cowan and Myers, 1988; Fabricius et al., 2003).

Potassium in the soil is the second most absorbed nutrient by plants after nitrogen and has a vital role in maintaining water and mineral balance for human body (Sarıkaya, 2016; Tosun, 2009). The potassium has three isotopes to be ³⁹K, ⁴⁰K and ⁴¹K. Their abundance in nature is %93.26, %0.0118 and %6.73 respectively (Belgin et al., 2009). Of the potassium isotopes, only ⁴⁰K is naturally radioactive and its half-life is 1.28 × 10⁹ years. The radioisotope of ⁴⁰K becomes stable after making a radioactive beta decay ß⁻ at 1.3 MeV and emits a gamma radiation γ at 1460 keV (Santos et al., 2005; Erdtmann and Soyka, 1979).

The uranium has three natural radioactive isotopes such as ²³⁸U, ²³⁵U and ²³⁴U and the thorium has more than five natural radioactive isotopes such as ²²⁷Th, ²²⁸Th, ²²⁹Th, ²³⁰Th, ²³¹Th, ²³²Th and ²³⁴Th (Choppin et al., 2013; Colle et al., 2017). The abundance of ²³⁸U and ²³²Th, whose half-lives are 4.47 × 10⁹ years and 1.41 × 10¹⁰ years, respectively, is >99% in nature and they are the main elements of the natural radioactive series of which they belong (Taqi et al., 2017). ²³⁸U and ²³²Th undergo a series of decay processes until they transform to ²⁰⁶Pb and ²⁰⁸Pb, respecively, they also emit γ-radiation to the environment in addition to α and ß⁻ particles (Missimer et al., 2019).

Since gamma radiation interacts with human tissue is more penetrant when compared to α and ß⁻ particles and it creates a health risk in tissue and DNA, the term of activity concentration (Bq kg⁻¹) is used to determine the health risk caused by gamma radiation emitted from environmental samples such as soil and to calculate the dose and radiological risk parameters (Donya et al., 2014; Günoğlu, 2018).

As the half-life of daughter radionuclides in ²³²Th natural series are usually short, this series is assumed to be in radioactive equilibrium. To calculate the activity concentration of ²³²Th, the activity concentration of ²²⁸Ac radionuclide, one of the daughter products of ²³²Th, is generally used. In contrast to ²³²Th natural series, the half-life of radionuclides in the ²³⁸U decay series are so long that this series requires more time to equilibrate and is not assumed to be in stable equilibrium. In calculating the activity concentration of the ²³⁸U series, the activity concentration of ²²⁶Ra, one of the decay products of the ²³⁸U series is used (Eke, 2017).

The half-life of ²²⁶Ra is 1620 years and turns into ²²²Rn with a half-life of 3.8 days by emitting 1 α and 1 γ radiation. ²²²Rn transforms to ²¹⁴Pb by making 2 α decay first and then transforms to ²¹⁴Bi by making 1 ß⁻ and 2 γ decay. After being 2 α, 3 ß⁻ and 4γ decay in the chain, ²⁰⁶Pb occurs and the ²³⁸U natural radioactive series reach the stable equilibrium (Malling et al., 2013).

²²²Rn is a colorless, odorless and the most heaviest gas in the world (Martin and Sutton, 2014). Since ²²²Rn does not react easily with other elements, it can accumulate in the soil and reach the atmosphere and respiratory system, thereby increasing the risk of lung cancer (Gundersen and Wanty, 1993; Özkorucuklu, 2006). It is estimated that about 52% of the annual average amount of human exposure effective dose from all natural radioactive sources cames from the inhalation of radon gas (Yarahmadi et al., 2016).

The main sources of artificial radioactive elements in the soil are the radioactive contaminants that spread to the atmosphere as a result of nuclear weapons tests and nuclear power plant accidents. Between 1945 and 1980, 543 atmospheric nuclear weapon tests were attempted (UNSCEAR, 2000). Between 1957 and 2011, four major nuclear power plant accidents occurred such as Windscale (UK- October 1957), Three Mile Island (USA- March 1979), Chernobyl (Ukraine- April 1986) and Fukushima (Japan- March 2011) (Pedraza, 2013; UNSCEAR, 2000). Chernobyl Nuclear Power Plant Accident was the worst nuclear power plant accident in the world in terms of the contaminated area, the number of countries it covers, and the number of people affected (Pedraza, 2012, 2013). At the same time, the Chernobyl is a nuclear power plant accident that the artificial radioactive element of ¹³⁷Cs had spread high amount into the atmosphere according to Windscale, Three Mile Island and Fukushima accidents (Colbeck and Lazaridis, 2014). With a half-life of 30.07 years, ¹³⁷Cs tends to be strongly absorbed by soil particles and penetrated deep into the soil, thanks to its geochemical properties similar to natural radioactive elements (Alzubaidi et al., 2016; Robison et al., 2003; UNSCEAR, 2000).

Although the upper part of the soil acts as a conservative shield against to the radioactive elements in the deeper layers of the soil, the radioactive elements in the soil continue to radiate, causing people to be exposed to the radiation by three ways (Ahmad et al., 2019; Ashraf et al., 2014; Belivermiş, 2012; Franic and Petrinec, 2006; Tang et al., 2000): 1. Radioactive elements due to soil particles breaking off from the soil surface by air and water can move from one place to another and they can be inhaled by people when suspended in the air and caused internal irradiation. 2. Radioactive elements reaching to a sufficient depth in the soil can be taken through the roots of the plants and they can join the food chain and they can be transferred to the human body through the digestive system and cause internal irradiation. 3. Radioactive elements in the soil can continue to irradiate even where they are located and causing people to be constantly exposed to radiation through external irradiation.

²²⁶Ra, ²³²Th, ⁴⁰K and ¹³⁷Cs are not homogeneously dispersed in the soil. The activity concentrations of the radionuclides in the soil and the dose values may vary according to the geographical and geological conditions (Veiga et al., 2006; UNSCEAR 2000). In addition, mining and mineraling activities, coal combustion, nuclear energy facilities and usage of phosphorus fertilizers contribute to increasing the level of the radionuclides in the soil (Sowole, 2014). According to the United Nations Scientific Committe on the Effects of Atomic Radiation report (UNSCEAR, 2000), the world median values of ²³⁸U, ²³²Th and ⁴⁰K in soil are 35, 30 and 400 Bq kg⁻¹, respectively and the resulting worldwide median of the annual effective dose is 0.48 mSv, with the results for individual countries being generally within the 0.3 and 0.6 mSv range. According to the same report, worldwide annual per caput effective dose are 2.4, 0.4, 0.005, 0.002 and 0.0002 mSv for natural background, diagnostic medical examinations, atmospheric nuclear testing, Chernobyl accident and nuclear power production, respectively (UNSCEAR, 2000).

The biological effects of radiation dose on human health are two types, deterministic and stochastic effects (Hamada and Fujimichi, 2014; Zeyrek, 2013). Deterministic effects develop in a short time due to the high radiation dose, can cause major damage to the skin, including burns, infertility and cataracts. Stochastic effects can also occur even at very low doses, but their effects can be delayed and occurrence time is not predictable (Manisalıgil and Yurt, 2018; Yeyin, 2015). It is reported that the deterministic effects caused by the radiation dose up to 5 Gy can be improved by medical treatment and that the deterministic effects caused by the radiation dose of 50 Gy and above can not be treated and can cause suddenly death. Besides, somatic effects that can arise on exposure to non-severe radiation dose between 0.01 Sv (1 rem) and 1 Sv (100 rem) are widely investigated (Yeyin, 2015).

One of the most important health problems caused by stochastic effects is cancer. Cancer is an uncontrolled division and proliferation of cells, and can develop due to many factors (Baykara, 2016). Therefore, when exposed to low-dose radiation, the relationship between dose and cancer is not sufficiently known (Mavragani et al., 2017). In order to avoid stochastic effects of radiation, it is recommended that the radiation doses determined as a certain precaution limit should not be exceeded and people are considered not at risk within the safety limits (Daşdağ, 2010).

Since the amount of natural and artificial radionuclides in the soil are at different levels all over the world, the radiation values that people are exposed to are different in every part of the world depending on the radionuclides in the soil. For this reason, researchers in many parts of the regions in the world have conducted studies to determine the radiation activity and dose levels that people are exposed to from soil samples such as Ademola et al. (2014), Alaamer (2008), Alzubaidi et al. (2016), Dolhanczuk-Srodka (2012), Kapanadze et al. (2019), Khan et al. (2011), Mitrovic et al. (2016), Najam et al. (2015), Zubair et al. (2013), Bilgici Cengiz and Öztanrıöver (2018), Kam and Bozkurt (2006), Taşkın et al. (2009), Karataşlı et al. (2016), Bozkurt et al. (2007), Kayakökü and Doğru (2017) and Kapdan et al. (2011). In addition, it is investigated whether gamma radiation released from radioactive elements in the soil poses a radiological risk using radium equvalent activity (Ra_eq), internal hazard index (H_in), external hazard index (H_ex), absorbed dose rate in air (D), annual effective dose equavalent (AEDE) and in order to evaluate the stochastic effects that radiation may have on human health excess lifetime cancer risk (ELCR) calculations are performed.

Rize is a province where the most tea production has done in Turkey (ÇAYKUR, 2015). In addition to tea production many other agricultural products are also grown in the Rize city, such as citrus fruits, kiwi, corn, nuts. Rize was affected by the Chernobyl Nuclear Power Plant Accident that occurred in 1986. After the accident, analyzes were carried out by Turkish Atomic Energy Authority to determine the activity concentration of ¹³⁷Cs in soil samples collected from Rize province (TAEA, 2007). ¹³⁷Cs, which completed its first half-life in 2016, is still present in some soil samples and will continue to be an important source of artificial radioactive elements in the soil in the coming years due to its half-life of 30.07 years. In this study, it was aimed to determine ¹³⁷Cs activity concentration in soil samples on the 30th anniversary of the Chernobyl Nuclear Power Plant Accident, besides natural radionuclides ²³⁸U (²²⁶Ra), ²³²Th, ⁴⁰ K. In addition, radium equvalent activity (Ra_eq), internal hazard index (H_in), external hazard index (H_ex), absorbed dose rate in air (D), annual effective dose equivalent (AEDE) and excess lifetime cancer risk (ELCR) values were calculated for soil samples and it was assessed whether they pose a risk for public health.