Assessment of environmental impact from radioactive effluents discharged by the Hanbit nuclear power plants in Korea: analyzing annual reports from nuclear power plants and radioactivity data from environmental monitoring organizations

Abstract

The aim of this comprehensive study was to analyze the annual effective doses to the public due to radioactive effluents discharged from the Hanbit nuclear power plants (NPPs) in Korea and identify potential improvements for the current environmental radiation monitoring system. We meticulously analyzed the amount of radioactive effluents discharged from the Hanbit NPPs and their resulting doses to the public for a decade (2014–23). Carbon-14, which accounted for only 1.90% of the total activity in radioactive effluents, contributed 88.53% of the annual effective doses to the public. This impact was significantly larger than that of tritium, which constituted most of the radioactive effluents. We propose potential improvements for the environmental monitoring system, including an increase in monitoring points and cycles of the seawater and seabed sediments to consider the characteristics of oceanic dispersion and the implementation of a radioactivity analysis of carbon-14 in ingestion and respiration-related environmental samples.

Introduction

Korea currently operates 25 nuclear power plants (NPPs). Among these, the Hanbit NPPs are located in Yeonggwang, on the west coast of South Jeolla Province. This plant features six pressurized water reactors (PWRs), all of which are currently in operation. Units 1 and 2 each have an electric power output capability of 950 MWe, while Units 3–6 have a capacity of 1000 MWe each. The commercial operation years for each unit are as follows: Unit 1 began operations in 1986, Unit 2 in 1987, Unit 3 in 1995, Unit 4 in 1996, and both Units 5 and 6 commenced commercial operations in 2002 [1].

Korea promotes improvements in reliability and transparency regarding the operation of nuclear power plants through Article 10 of the Act on Assistance to Electric Power Plants–Neighboring Areas [2]. This act allows local governments to investigate and confirm the environmental impact on the surrounding areas due to the construction, operation, and decommissioning of NPPs [3, 4]. Five local governments where NPPs are located in Korea operate environmental monitoring organizations (EMOs), which conduct radiological environmental monitoring of the surrounding areas of NPPs [4, 5].

The gaseous and liquid radioactive effluents resulting from NPP operations contain various nuclides, including key nuclides such as ³H, ¹⁴C, ¹³¹I, noble gases, and particulates [6, 7]. These various radionuclides in radioactive effluents can affect humans and the environment through exposure pathways. The discharge of radioactive effluents is influenced by the meteorological and oceanographic characteristics around NPPs, which also have a significant role in the assessment of radiation exposure to the public living around NPPs. Thus, it is fundamental to evaluate the characteristics of radioactive effluent discharges and monitor environmental radiation around the NPPs to achieve radiation exposure to the public as low as reasonably achievable (ALARA). At the Hanbit NPPs, minimal amounts of ³H and ¹⁴C have typically been detected due to their operations. It has been confirmed that the radiation exposure to residents from inhaling or ingesting these radioactive effluents is extremely low, accounting for ~1% of the annual dose limit for the general public.

In this study, we analyzed the characteristics of radioactive effluent discharges and the resulting effective dose to the public living around the Hanbit NPPs, including six PWRs, over the past 10 years (2014–23). Furthermore, we evaluated the results of environmental radioactivities in an emergency planning zone (30 km) of the Hanbit NPPs, focusing on major nuclides in radioactive effluents. We propose improvements in the radiological environmental impact assessment around the Hanbit NPPs based on these analyses and evaluation results.

Materials and methods

Legal basis for radiological environmental impact assessment

In accordance with Article 104 (Environmental Protection) of the Nuclear Safety Act in Korea and Article 136 of its Enforcement Decree, radiation environmental surveys and radiation environmental impact assessments are required for nuclear power facilities [8, 9]. Based on the relevant laws and enforcement decree, the Nuclear Safety and Security Commission (NSSC), the regulatory body in Korea, provides NSSC Notice No. 2017-17 ‘Regulation on the Radiological Environmental Survey and Radiological Environmental Impact Assessment for Nuclear Energy Utilization Facilities’, which specifies the establishment of environmental survey plans, methods, environmental impact assessments, handling and reporting of survey data, etc. [10]. The regulatory body, NPP licensee, and EMOs have established survey plans and procedures and have periodically conducted environmental surveys around NPPs to comply with the regulations.

Legal basis for gaseous and liquid radioactive effluent control

Article 174 of the Enforcement Decree of the Nuclear Safety Act stipulates that the concentration of gaseous and liquid radioactive materials discharged from NPPs at the exclusion area boundary shall comply with the criteria provided by the NSSC to prevent radiological hazards to the environment [9]. The effluent control limits (ECLs) are derived from radionuclide concentrations (Bq m⁻³) corresponding to the annual dose limit to the public (1 mSv y⁻¹), inhaled or ingested continuously over the course of 1 year by a member of the public [11–14]. In addition to the ECLs, the NSSC requires that NPPs comply with the dose standards set forth in NSSC Notice No. 2019-10 [13, 15]. Although these dose standards are the design objectives for licensing a new NPP and environmental radiation protection standards for the operation of multiple NPPs in a single site, these dose standards serve as practical dose constraints to the public living around NPPs [11, 12, 16, 17]. Table 1 presents the annual dose standards for the public resulting from gaseous and liquid radioactive effluents discharged from a single NPP and multiple NPPs in a single site [13, 17]. We analyzed the amounts of radioactive effluents discharged from the Hanbit NPPs and their resulting effective doses to members of the public using the NPP annual reports encompassing a survey of environmental radiation and assessment of radiological impact on the environment in the vicinity of NPPs from 2014 to 2023 [18–27].

Analysis of environmental radioactivity

For a radiological environmental impact assessment, we determined the radioactivity sampling points, including the interior and exterior of the Hanbit NPP site and comparison points within a radius of 30 km centered on the Hanbit NPP Unit 1. The radionuclides to be analyzed are selected considering major nuclides in gaseous and liquid radioactive effluents discharged during NPP operation based on Attached Table 1 in NSSC Notice No. 2017-17, except noble gas nuclides that do not significantly contribute to internal exposure. In addition, samples and monitoring nuclides were selected, as shown in Table 2, to evaluate radiation exposure, determine the accumulation trends of radioactive materials, and monitor changes in radioactivity levels in the environment. The Hanbit NPP EMO has been conducting 300–400 analyses of environmental radioactivity every year since its establishment in 1999. We evaluated the analysis results of environmental radioactivity around the Hanbit NPPs obtained by the Hanbit NPP EMO over the past 10 years, focusing on dominant nuclides for radioactive effluents, as shown in Table 3 [28–37].

In terms of radionuclide analysis, gamma nuclides were analyzed using various pretreatment processes depending on the sample type: bioassay (milk), evaporation concentration (seawater, rainwater, and drinking water), dry ashing (soil, seabed sediment, and agricultural and marine products), and AMP-MnO₂ method (sea water) [38]. ³H was analyzed using the distillation process (seawater, rainwater, and drinking water) [38]. ⁹⁰Sr was analyzed using the fuming nitric acid process (seawater, seabed sediment, soil, and milk) [39]. After the pretreatment processes, the radioactivity of the sample was measured using radiation detectors depending on the radiological characteristics of the nuclide. Its background was also measured under the same conditions (time, measuring container, filter, etc.). Gamma nuclides were measured using high-purity germanium (HPGe) detectors in the energy spectrum of 0 to 8192 channels up to 3 MeV (Mirion, efficiency 30% and 40%). ³H was measured 10 times for 30 min using liquid scintillation counters (LSCs), Quantulus 1220 and Quantulus GCT A622001 (PerkinElmer). ⁹⁰Sr was measured with a Mirion 5XLB low-level alpha-beta counter. To maintain the quality and performance of the radiation detectors, regular calibration was performed every 6 months. Certified reference materials manufactured by the Korea Research Institute of Standards and Science were used as calibration sources for the HPGe detectors and low-level alpha-beta counter [40]. Particularly, the HPGe detector was calibrated using a mixed agar-type source containing 10 mixed gamma nuclides, while the low-level alpha-beta counter used a liquid calibration standard source of ⁹⁰Sr. The PerkinElmer’s ³H Quenching standard source was used for LSC calibration. The experimental and measurement methods were determined such that all analysis results ​met the minimum detectable activity (MDA) in Attached Table 2 in NSSC Notice No. 2017-17. Table 4 presents the MDAs outlined in Attached Table 2 of NSSC Notice No. 2017-17. For instance, the MDA for ³H in air and rainwater samples are 0.1 Bq m⁻³ and 5 Bq L⁻¹, respectively. Additionally, the MDA for ¹⁴C in air samples is 0.25 Bq m⁻³. The radionuclide concentration was measured with a certain uncertainty, and statistical processing of the survey results was conducted in accordance with NSSC Notice No. 2017-17 [10]. The MDA calculation method follows the standards set by the American National Standards Institute (ANSI 13.30), which outlines the performance criteria for radiobioassay [41].

Results and discussion

Analysis of gaseous and liquid radioactive effluents discharged from the Hanbit nuclear power plants

The annual average amount of gaseous radioactive effluents discharged from the Hanbit NPPs from 2014 to 2023 was 17.76 TBq [18–27]. Dominant nuclides in gaseous radioactive effluents include ³H, ¹⁴C, noble gases (⁴¹Ar, ¹³³Xe), and particulates (⁵⁴Mn, ⁵⁸Co, ⁶⁰Co, ⁹⁰Sr). Among them, ³H accounted for 96.90% with 172.1 TBq, ¹⁴C for 1.91% with 3.39 TBq, noble gases for 1.15% with 2.05 TBq, and particulates for <0.01% with 4.65 × 10⁻⁶ TBq [18–27]. The dominant nuclides in the noble gases and particulates were ⁴¹Ar and ⁵⁸Co and ⁶⁰Co, respectively. Table 5 shows the amount of gaseous radioactive effluents discharged from the Hanbit NPPs for the past 10 years. The temporary increase in noble gas effluents in 2014 and 2018, as shown in Table 5, was due to a leak in the steam generator tubes of the Hanbit NPPs.

During the same period, the average annual amount of liquid radioactive effluents was 37.94 TBq, of which ³H accounted for almost 100% of the total discharge [18–27]. Although very small amounts of particulates, such as ⁵⁸Co, ⁶⁰Co, ¹²⁴Sb, ¹²⁵Sb, and ¹³⁷Cs, are included, their contribution to the total activities of liquid radioactive effluents is almost insignificant [12]. Table 6 shows the amount of liquid radioactive effluents discharged from the Hanbit NPPs from 2014 to 2023. In addition, the total amount of gaseous and liquid radioactive effluents discharged from the Hanbit NPPs over 10 years was 557 TBq. The gaseous radioactive effluents (177.6 TBq) accounted for 31.89%, while the liquid radioactive effluents (379.4 TBq) accounted for 68.11%.

Analysis of the effective dose to the public due to radioactive effluents discharged from the Hanbit nuclear power plants

We analyzed the average effective doses to the public over the past 10 years at the exclusion area boundary of the Hanbit NPPs due to gaseous and liquid radioactive effluent discharges. The average of annual effective doses to the public for 10 years due to gaseous radioactive effluents discharged from the Hanbit NPPs was 9.45 × 10⁻³ mSv y⁻¹, while that due to liquid radioactive effluent discharge was 2.48 × 10⁻⁵ mSv y⁻¹. The annual effective doses for the public living around the Hanbit NPPs due to radioactive effluent discharges from 2014 to 2023 are presented in Table 7. The average effective dose to the public for 10 years was 9.47 × 10⁻³ mSv y⁻¹. This is only 0.95% of the annual dose limit for the public. Furthermore, the average dose accounts for 3.79% of the dose standard of 0.25 mSv y⁻¹ when multiple NPPs are operated in a single site.

The contribution of gaseous and liquid radioactive effluents to effective doses to the public was also evaluated using the results of 10-year public doses. As shown in Table 7, the dose contribution by gaseous radioactive effluent was 99.73%, while the dose contribution by liquid radioactive effluent was 0.27% to the total effective dose. The results indicate that the contribution of the annual effective doses to the public is opposite to the contribution of gaseous and liquid radioactive effluents to total effluent discharges, which accounted for 31.89% and 68.11%, respectively, over the past 10 years.

In this study, we analyzed which nuclides among these gaseous radioactive effluents contributed to the annual effective dose and to which extent. The contribution of major nuclides in gaseous radioactive effluents to the public dose from 2014 to 2023 is shown in Table 8. The annual effective dose to the public due to ³H was 9.89 × 10⁻⁴ mSv, accounting for 11.38%, while the public dose due to ¹⁴C was 8.45 × 10⁻³ mSv, accounting for 88.53%. ⁴¹Ar causes an insignificant dose of 3.26 × 10⁻⁶ mSv, accounting for only 0.04%. This result contrasts the findings in Table 5, in which ³H accounts for the majority of radioactive effluents discharged from NPPs. In general, it can be assumed that the types (gaseous and liquid) of radioactive effluents and nuclides with high discharge amounts (high activity) have a significant impact on the public dose [7]. However, according to the findings of this study, gaseous radioactive effluents comprising 31.89% of the total amount (activity) of discharge contribute to 99.73% of the total annual effective dose to the public. Among the major radionuclides constituting the entire gaseous radioactive effluents, ¹⁴C, accounting for ~1.90% of the total discharge as shown in Table 8, contributes significantly with 88.53% to the public dose from radionuclides, compared to ³H, which accounts for 11.38% of the total public dose.

Analysis of environmental radioactivity at the Hanbit nuclear power plants and proposal for improvement

The NPPs strive to achieve ALARA radiation exposure to the public during normal operations, considering economic and social factors [14–17]. This commitment ensures that radioactive effluent discharges do not exceed both ECLs and dose standards in NSSC Notice No. 2019-10 ‘Standards for Radiation Protection’ [11]. Radioactive effluents that spread outside the NPP site boundary through the air and ocean cause direct and indirect radiation exposures to humans, animals, and plants around the NPP site [12]. We evaluated the results of a radioactivity analysis of environmental samples around the Hanbit NPPs, focusing on major radionuclides in gaseous and liquid radioactive effluents, considering both external and internal exposure pathways. The exposure pathways for gaseous radioactive effluents include external exposure by noble gas and contaminated soil and internal exposure by respiration [12]. For gaseous exposure pathways, ³H, ⁷Be, ⁶⁰Co, and ⁹⁰Sr in rainwater, air, and soil samples were evaluated among the environmental analysis results around the Hanbit NPPs over the past 10 years. Exposure pathways for liquid radioactive effluents include external exposure due to swimming or water activities and effects of beach accumulation and internal exposure due to the consumption of marine products [12]. For liquid exposure pathways, ³H, ⁶⁰Co, ⁹⁰Sr, and ¹³⁷Cs in seawater and seabed sediment samples were evaluated among the environmental analysis results around the Hanbit NPPs over the past 10 years.

³H, ⁷Be, ⁹⁰Sr, and ¹³⁷Cs were detected in the surrounding areas of the Hanbit NPPs owing to the discharge of gaseous radioactive effluents. Particularly, the concentration of ³H, the most abundant radionuclide in gaseous radioactive effluents, varied in rainwater samples depending on the direction of the wind from the Hanbit NPPs, atmospheric diffusion factors, and distance. Figure 1 shows the radioactivity analysis points by direction of the Hanbit NPPs. Figure 2 shows the results of average radionuclide concentrations of ³H in rainwater samples from 2014 to 2023. In the assessment of radiation exposure through the exposure pathways of gaseous radioactive effluents discharged from the Hanbit NPPs, the point with the maximum atmospheric diffusion factor outside the NPPs is the exclusion area boundary in the Southwest (SW) direction. However, as this point is in a marine area and cannot be inhabited by humans, the radiation dose assessment for the public is conducted in inland areas where habitation is possible. Dose assessment for the public is carried out assuming that individuals representing a critical group reside at the exclusion area boundary of the Hanbit NPPs in the South-southwest (SSW) direction, where air diffusion factors and deposition factors are maximized [27]. Over the past 10 years, the radioactivity analysis of ³H in rainwater samples revealed that the maximum radioactivities were detected at both Gamami Beach and the back gate of the Hanbit NPPs, which are positioned nearly in the same direction as the SSW (considering that the observatory is near the stack of Hanbit Unit 1). Furthermore, as shown in Table 9, a decreasing trend of ³H was confirmed depending on the distance from the Hanbit NPPs. On the other hand, ⁷Be did not exhibit clear differences in distance and direction, unlike the results of ³H. This is attributed to the short half-life of ⁷Be compared to other nuclides and its insignificant amount of discharge as a gas, which has a monthly sampling period. In addition, ⁷Be is a common environmental radionuclide produced by cosmic radiation.

Similar to the discharge of gaseous radioactive effluents, ³H accounted for almost 100% of liquid radioactive effluents every year over the past 10 years. ³H, ⁹⁰Sr, and ¹³⁷Cs were detected in the surrounding areas of the Hanbit NPPs owing to the discharge of liquid radioactive effluents. ³H in seawater samples is evaluated based on monthly analysis results. The Hanbit NPPs and Hanbit NPP EMO jointly collect samples on a weekly basis at the final discharge point of liquid radioactive effluents, mixing them to create composite samples, which are then analyzed monthly. This is the result of collecting samples four to five times a month regardless of the date and time of liquid radioactive effluent discharge, which reduces the concentration due to dilution, which hinders the evaluation of the radiological impact of actual discharge. For example, in 2020, ³H was detected at levels of up to 129 Bq L⁻¹ at a drain point at the site, although the 10-year average of ³H was 4.33 Bq L⁻¹, as shown in Table 10. According to Article 10 (Reporting) of NSSC Notice 2017-17, this incident must be reported to the regulatory body, which specifies cases where radioactivity levels exceed five times the average of the data from the past 3 years [10]. In addition, the Hanbit NPP EMO independently collected and immediately analyzed ³H of seawater samples from the Goripo sea area in Sangha-myeon, Gochang-gun near the drain, which is located just outside the exclusion area boundary of the Hanbit NPPs, without mixing them into weekly samples. As a result, 20 to 35 Bq L⁻¹ of ³H was detected three times over 3 years, resulting in a case subject to reporting to the regulatory body. The investigation into the cause of these abnormal events found that the sampling occurred around the time of liquid radioactive effluent discharge from the Hanbit NPPs, indicating minimal influence from dilution or dispersion in the seawater. Although the Hanbit NPPs comply with the ECLs by conducting a radioactivity analysis before discharging radioactive effluents, based on this study, it is necessary to improve the sample collection, collection method, and sampling cycle in the future to assess the actual environmental impact due to the discharge of radioactive effluents more accurately. For example, if sampling is conducted when liquid radioactive effluent is discharged from the Hanbit NPPs, it can help minimize the impact of seawater dilution and the time lag in reducing radionuclide concentrations. Additionally, if the current monthly analysis cycle for radionuclide concentrations is changed to a weekly schedule, the accuracy of these analyses will be improved.

Another improvement involves the evaluation of the marine dispersion of liquid radioactive effluents. The distributions of marine dispersion and dilution factors vary significantly depending on seasonal conditions. During summer, the dispersion tends to be stronger in the northward direction, while in autumn and winter, it tends to be stronger in the southward direction [27]. Understanding these seasonal variations is essential for a more accurate assessment of the marine dispersion of radioactive effluents. According to the current environmental investigation plan of the Hanbit NPPs, the sampling points include water intake (0.7 km West-southwest (WSW) direction), drain (2.3 km North-northeast (NNE) direction), Mokmaek area (3.6 km South (S) direction), and Hampyeong area (34.5 km S direction) [18–27]. Considering the marine diffusion and dilution factors around the Hanbit NPPs, radioactivity monitoring is required to be further expanded from the drain points to the northern sea areas, such as Gochang Goripo, Gusipo Port, and Dongho Beach. Furthermore, for ⁹⁰Sr, sampling is conducted only at one drain point and one reference point in the Hampyeong area for seawater and seabed soil samples [18–27]. The sampling points for ⁹⁰Sr in the north and south of the Hanbit NPP site boundary also need to be expanded to ensure a comprehensive assessment. Finally, regarding the addition of sampling nuclides, the Hanbit NPP EMO currently does not conduct a radioactivity analysis for ¹⁴C in environmental samples around NPPs. However, as shown in Table 8, ¹⁴C among gaseous radioactive effluents significantly impacts the effective dose to the public living around the Hanbit NPPs [6]. Thus, we propose that a radioactivity analysis for ¹⁴C should be conducted by the Hanbit NPP EMO for air and ingestion-related samples, such as agricultural products and milk.

Conclusion

The assessment of the radiation exposure to the public due to the operation of an NPP involves a comprehensive evaluation of various factors, including radioactive effluents discharged into the environment, the radionuclide concentration of environmental samples, and radiation exposure through ingestion and respiration by exposure pathways. Thus, it is important to accurately analyze the amount of radioactive effluents discharged from NPPs and establish an environmental survey plan considering the impact of radioactive effluents on the environment to achieve ALARA radiation exposure to the public.

The analysis of the amount of radioactive effluents discharged from the Hanbit NPPs from 2014 to 2023 showed that ³H accounted for 97.03% of the total discharges, while ¹⁴C and noble gases constituted 1.90% and 1.07% of the gaseous radioactive effluents, respectively. Liquid radioactive effluents consisted of ~100% of ³H. In addition, based on the total activity, gaseous and liquid radioactive effluents accounted for 31.89% and 68.11%, respectively, which indicates that a large amount of radioactive effluents is discharged in the form of liquid.

In terms of resulting effective doses to the public due to radioactive effluent discharges, the average of the public doses over the past 10 years at the exclusion area boundary of the Hanbit NPPs was 9.47 × 10⁻³ mSv y⁻¹, which includes 9.45 × 10⁻³ mSv y⁻¹ due to gaseous discharges and 2.48 × 10⁻⁵ mSv y⁻¹ due to liquid discharges. This 10-year average effective dose due to both gaseous and liquid radioactive effluents accounts for only 0.95% of the annual dose limit for members of the public. Although liquid radioactive effluents accounted for 68.11% of the total discharges, the effective dose received by the public living around the Hanbit NPPs is primarily attributed to gaseous radioactive effluents, which constitute 99.73% of the total public dose. ³H, accounting for the majority of total discharges (97.03%), contributed 11.38% to the public dose, while ¹⁴C, accounting for insignificant discharges (1.90%), contributed 88.53% to the public dose, which indicates that ¹⁴C has a larger impact on the radiation exposure to the public.

We evaluated analysis results from environmental surveys around the Hanbit NPPs over the past 10 years to determine the radiological environmental impact of gaseous and liquid radioactive effluents. The radioactivities in most samples in the environment were below the minimum detectable activity of NSSC Notice No. 2017-17, except for ³H, ⁷Be, ⁹⁰Sr, and ¹³⁷Cs. Particularly, in terms of the trends of radionuclide concentration in rainwater samples, the concentration of ³H varies depending on the downwind direction from the Hanbit NPPs, atmospheric diffusion factors, and distance.

The Hanbit NPPs comply with the ECLs and dose standards for the public set forth in NSSC Notice No. 2019-10. However, to assess the actual environmental impact due to the discharge of radioactive effluents more accurately, the sample collection, collection method, and sampling cycle should be improved in the future. Furthermore, the Hanbit NPPs need to expand the sampling points in the north and south directions for the seawater and seabed sediment samples in the current environmental survey plan, considering the marine diffusion and dilution factors around the Hanbit NPPs. We propose that the Hanbit NPP EMO should conduct a radioactivity analysis for ¹⁴C for air and ingestion-related samples, such as agricultural products and milk, considering its dominant impact on radiation exposure to the public.

Conflict of interest

None declared.

Funding

This study was supported by a research fund from Chosun University, 2023.

References