Tritium contents in drinking and surface seawaters before the nuclear power plant planned in Sinop (Türkiye) and their radiological risks on human population

Tritium, a radioactive isotope of hydrogen, emits a beta particle with a maximum energy of 18.6 keV as it decays to helium and its half-life is 12.3 years (Lucas & Unterweger 2000). Tritium isotopes are both naturally occurred and artificially produced. High-energy cosmic rays reacting with nuclei of oxygen and nitrogen in the upper layers of the atmosphere continually produce natural tritium (Masarik & Beer 2009). Many anthropogenic nuclear operations, such as the testing of nuclear bombs and the operation of nuclear reactors, create artificial tritium. Furthermore, ³H can be artificially created via neutron capture in reactors (Okada & Momoshima 1993). The atmosphere's main sources of tritium come from atmospheric thermonuclear tests conducted between 1953 and 1963 (Harms et al. 2016). The atmosphere's fast oxidation of formed tritium results in the formation of element tritium (HT), tritiated water (HTO) and tritiated methane (CH₃T).

At present, the Akkuyu nuclear power plant with a power of 4,800 MW is under construction in Mersin province of Türkiye and the first unit of this power plant is planned to be put into operation in 2023 (DNEPI 2012; Dizman et al. 2015). Tritium's chemical forms are closely related to types of the nuclear power plant. In pressurized water (PWR) and boiling water (BWR) type reactors, tritium releases mostly occur as HTO and only 1% is in gaseous HT form (Hou 2018). Tritium's health hazard depends on its chemical form and HTO is one of the tritium types that are dangerous for people to consume. HTO can interact directly with the metabolic process after entering the body through the skin, water and food. This can have negative health implications such as an increased risk of cancer and mutation (Matsumoto et al. 2021). Because of this, determining the tritium activity levels in waters meant for human consumption is crucial. Many researches on determining the tritium contents in water samples from diverse sources, including seawater samples, have been conducted in various nations (Damianova et al. 2016; Osman et al. 2016; Deng et al. 2017; Ansari et al. 2018; Dizman et al. 2018; Jean-Baptiste et al. 2018; Dizman & Mukhtarli 2021). Although there are many studies on determining tritium activity in drinking water in Türkiye, no research has been conducted on seawater in recent years (Görür & Genç 2012; Gören et al. 2014; Karataşlı et al. 2017; Dizman & Korkmaz Görür 2019).

The Black Sea receives large amounts of water via the discharges of the central- and eastern-European rivers. There are nuclear power plants on some of these rivers. Therefore, the surface tritium concentration is higher than those of other seas at comparable latitude band. A systematic monitoring of tritium in the Black Sea is not available. However, some studies have been carried out on the determination of tritium in the Black Sea (Nikitin & Vakulovsky 1996; Rank et al. 1999; Top 1999).

The aim of this work is to determine both tritium concentrations and radiological risks in surface seawaters and drinking water before the nuclear power plant to be established in Sinop province starts to operate. In order to achieve this, the annual effective dose rates in three different groups (infants, children and adults) of the Sinop population were assessed, along with the lifelong cancer risks associated with tritium exposure by gender. In addition, determined radioactivity values have been compared with the limit values suggested by the international organizations (Turkish Standards Institution, European Commission and World Health Organization). The results of this study can contribute to the monitoring of tritium radioisotope in the Black Sea and can be utilized as a guide for future research. Moreover, the results will be very valuable, especially for studies to be carried out after the nuclear power plant comes into operation.

An intergovernmental agreement was signed in 2013 with France and Japan for Türkiye's second nuclear power plant in Sinop. In accordance with the signed agreement, it is planned to establish a French–Japanese joint design ATMEA-1 type nuclear power plant with a total power of 4,480 MW, consisting of four units of 1,120 MW each (NADA 2015). It is stated that the nuclear power plant will be established in İnceburun locality (NADA 2015).

Firstly, a total of 165 seawater samples were taken at 1 km intervals along the coastline of Sinop province (from the Samsun provincial border to the Kastamonu provincial border). Later, a total of nine drinking water samples were taken, one from each of Sinop center and districts. Both seawater and drinking water samples were taken with 500 mL plastic bottles. The drinking water samples were taken from the city water supply consumed by the people in the relevant district. The coordinates of each sampled point were recorded with the GPS device (Magellan Explorist 510). Then, the water samples were transferred to the laboratory for the tritium measurement. The water samples were first filtered to remove physical particles. The samples were then distilled. The distillation procedure described in detail by Tjahaja and Sukmabuana was carried out by adding 6 M NaOH and 0.05 N KMnO₄ into a 250 mL water sample (Tjahaja & Sukmabuana 2012). The plastic vials used in measurements (Zinsser Analytics, 20 mL) were first filled with 10 mL of the distilled samples, and the remaining vials were filled with Ultima Gold LLT scintillation (Perkin Elmer Inc.). The background sample (BKG) was created using groundwater that had undergone two distillations and had little tritium present.

A common unit of measurement used to express tritium concentrations in measurements of environmental samples is the tritium unit (TU), which is equal to one tritium atom per 1 × 10¹⁸ hydrogen atoms (1 TU = 0.118 Bq/L). The results obtained in this study are given in both Bq/L and TU units.

Average cpm (count per minute) values of DWS and background sample in LSC were found as 2,377.67 cpm for DWS and 2.44 cpm for BKG. Using these values, the efficiency and MDA values were calculated as 26% and 1.48 Bq/L (12.54 TU), respectively.

As a result of this work, the tritium concentration values obtained in seawater samples could only be compared with the study conducted by Top in 1999 in Türkiye. The tritium concentrations found in this study were lower than the determined values in the study done by Top (1999). On the other hand, there are studies on the determination of tritium activity concentrations in seawater samples in different countries of the world. The study in which tritium concentrations were determined in seawater samples and the average tritium concentration values found in studies conducted in different countries are given in Table S1 of the Supplementary Materials.

The average tritium concentration of seawater samples taken along the coast of Sinop province located on the Black Sea coast is lower than from the tritium concentration of the seawater samples by Russian side of the Black Sea, from the tritium concentration in the seawater samples from different points of the Turkish side of the Black Sea, from the tritium concentration of the Pacific ocean water samples from the Misava port in Japan, from the tritium concentration of the water sample from the Bohai sea in China and from the tritium concentration of the Mediterranean water sample from the French coast. However, it was found to be higher than the tritium level of water samples from various locations in the Black Sea and the average tritium level of Irish Seawater samples. As seen in Table S1, it can be said that this tritium concentration difference between seas depends on the mobility of the water in the sea (current, circulation, etc.), whether it is in the gulf or whether there are nuclear power plants on the sea coast. In addition, tritium can exhibit considerable environmental variability and behavior in different ecosystems.

In our country and around the world, investigations have been done to determine the tritium activity concentrations in drinking water. This study in which tritium concentrations were determined in drinking water samples and the average tritium concentration values found in studies conducted in different countries are given in Table S2 of the Supplementary Material. As seen in Table S2, the average tritium concentration determined in the examined drinking waters was found lower than the tritium concentrations in the drinking waters of Italy, Portugal and Türkiye (Mersin), but higher than the tritium concentrations in the drinking waters of Spain, Bulgaria and Türkiye (Rize). It can be said that these concentration differences between and within countries may be due to the fact that tritium varies greatly in the environment.

Table 3 lists the estimated lifetime cancer risk values for people (males and females) who drank water samples collected from Sinop Province's center and districts. The lifetime cancer risk values determined for males and females varied from 1.27 × 10⁻⁷ to 1.64 × 10⁻⁷ and 1.36 × 10⁻⁷ to 1.76 × 10⁻⁷, respectively. Whole LCR values of adults are found to be well below the permissible value (1.6 × 10⁻³) for cancer risk as radiological (UNSCEAR 2000; Patra et al. 2013). Hereof, these findings demonstrate that there are no radiological risks associated with consuming the investigated waters. In addition, the findings of this study will be probably used as a reference in further studies.

The measurement outcomes of the reference sample are found as 710.81 ± 18.62 Bq/L. Experimentally determined tritium activity agrees with the spiked activity. Additionally, given that the Z value (0.19) ranges from −1 to 1, these results demonstrate that the current analytical procedure produced satisfactory results.

In this study, the tritium contents of drinking and sea water samples taken from Sinop province were examined before Turkey's second nuclear power plant, which is planned to be established in Sinop province, went into operation. The tritium contents in drinking and seawater samples were measured using Liquid Scintillation Counter (LSC). The average tritium contents of examined drinking and sea water samples were determined to be 22.81 and 21.78 TU, respectively. In addition, the annual effective dose rates for people consuming the examined drinking waters were estimated and these values were found to be 12.12, 16.96 and 35.38 nSv for infants, children and adults, respectively. The Turkish Standards Institution (TSI), the World Health Organization (WHO) and the European Commission (EC) control the maximum allowable level of tritium and radioactivity levels in drinking water. This work confirmed that the tritium levels in the drinking and seawater samples analyzed were low and much below the tritium exposure limits advised by international organizations. As a result, there is no tritium radiation risk associated with the tested water samples. In addition, the findings of this study can serve as a guide for future investigation.

This research was done as part of a master's thesis that was written in 2020 at Recep Tayyip Erdogan University.

S. D. conceptualized the whole article, conducted the formal analysis, developed the methodology, wrote the original draft, wrote the review and edited the article. F. Z. A. conducted the formal analysis and investigated the work, A. E. O. conducted the formal analysis, investigated the work, and visualized the project. R. K. conducted the formal analysis and investigated the work. F. K. G. conducted the formal analysis and investigated the work and rendered support in funding acquisition.

The Scientific and Technological Research Council of Türkiye (TUBITAK) funded this work under the project number ‘117Y285’.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.