Introduction
South Korea is recognized internationally for its proficiency in nuclear energy. It currently operates 25 nuclear power plants (NPPs), placing it among the leading nations in terms of the number of operational reactors. As of May 2022, South Korea ranked third globally in terms of the number of nuclear reactors under construction, following China and India. Furthermore, South Korea ranked fifth worldwide in terms of the distribution of nuclear energy generation, highlighting its significant contribution to global nuclear power production [1]. One of the main factors driving South Korea’s adoption of nuclear energy for power generation is the scarcity of natural resources. Consequently, South Korea has dedicated substantial efforts to the development of its nuclear industry, primarily to generate electricity to sustain its industrial growth [2]. Despite the expansion of the nuclear industry in South Korea, both the previous ruling government from 2018 to 2022 and antinuclear activists have expressed reservations and concerns regarding nuclear energy. Their apprehensions are often rooted in misconceptions surrounding the risks associated with nuclear energy projects within the country, particularly regarding the potential consequences of an NPP accident and its impact on public health [3]. Therefore, conducting comprehensive studies on the potential health effects of a nuclear accident in South Korea and raising awareness among the population are essential. However, there is limited research on the long-term cancer risk resulting from radiation exposure due to NPP accidents, considering factors such as inhalation, external exposure, and ingestion of contaminated food and animal products. Therefore, this study calculated the incidence of cancer associated with a nuclear accident and compared it with the existing cancer incidence rates in South Korea and the established safety goal for NPPs of 0.1% in terms of cancer risk.
This study focused on the assessment of the long-term radiological impacts of the release of radionuclides into the atmosphere caused by a nuclear accident, particularly on the Advanced Power Reactor 1400 (APR1400)-Saeul NPP. The APR1400-Saeul NPP represents the latest advanced pressurized water reactor (PWR) developed by South Korea. Radionuclides released from NPPs can enter the human body through various pathways, including inhalation, external exposure, and ingestion of contaminated food and animal products [4]. NPP accidents can arise from various causes, such as natural disasters, plant system failures, and human operator errors. In this study, the focus was on the analysis of the loss of coolant accident (LOCA) scenario. LOCA events are particularly significant because they typically require an extended period to restore normal operational status. Furthermore, LOCA incidents tend to occur more frequently than long-term station blackout scenarios, which could result in larger quantities of radionuclides being released into the atmosphere [5, 6]. LOCA occurs when there is a loss of coolant from the reactor coolant system in an NPP. This can be caused by various factors, such as piping breaches and failures of components, including pumps and valves. Therefore, the water level within the reactor core rapidly decreases, which can lead to fuel rod overheating and potential melting. To prevent or mitigate the effects of an LOCA event, PWRs are equipped with safety features, such as emergency core cooling systems (ECCSs). ECCSs rapidly inject coolant into the core following an LOCA event. Furthermore, PWRs are designed to have containment structures to prevent the release of radioactive materials into the atmosphere in the event of a breach in the primary containment component. These safety measures are crucial for ensuring the safe operation of NPPs during LOCA events. However, in the case of a severe accident, all these mitigation measures usually fail, and radioactive materials are released into the environment, posing significant health risks to workers and the public [7]. Studies have examined the potential consequences of a hypothetical nuclear accident at the Saeul NPP. However, these studies have primarily focused on acute exposure scenarios. The results indicated that the total effective dose equivalent doses and thyroid doses measured within the precautionary action zone exceeded the Korean regulatory limit for indoor sheltering, which is set at 2 mSv in 2 days, and the thyroid protection level of 100 mSv [8]. The objectives of this study were determined by identifying the limitations of previous studies, as detailed in Table 1 [9–13]. This study aimed to determine the long-term radiological effects of all three radiation exposure pathways: inhalation, external exposure, and food ingestion.
Indeed, no radiological assessment has been performed to evaluate the chronic effects of radiation exposure from various pathways on the population residing within the emergency planning zone (EPZ) of the Saeul NPP. The EPZ comprises 4,067,470 individuals according to the Design Control Document of the Saeul NPP. Currently, no comprehensive studies have been conducted on the long-term radiological impacts of radiation exposure on the individuals living within this zone.
The Radiological Assessment System for Consequence AnaLysis (RASCAL) code [14] was used in this study to obtain relevant source terms that comprised 131I, 134Cs, 137Cs, and 90Sr, which are known radionuclides with significant risks to human health and the environment [15]. The released radionuclides, such as 133Xe and Kr isotopes, have very short half-lives; thus, these gasses decay relatively quickly and pose a temporary risk rather than a long-term hazard to public health. The release rates of radionuclides, food consumption rates, inhalation rates, and hourly meteorological data for the Ulsan area for 5 years were input into the GENII code to compute the annual total individual effective doses to maximally exposed persons in different compass directions at selected distances from the NPP site. The GENII code was then used to compute cancer incidence among children and adults for a chronic exposure period of 5–60 years. These factors were considered because the greatest health fears of nuclear accidents are the possibilities of causing cancer and cancer is the leading cause of death in South Korea currently [16]. The results of this study were compared with those of the prevailing cancer cases in South Korea to ascertain whether the magnitude of the impact caused by a nuclear accident was significant enough to cause public fear in the country.
Materials and Methods
1. Study Area
The Saeul NPP is located in Seosaeng-myeon, Ulju-gun, Ulsan, on latitude 35020′06.57″N, longitude 129018′41.84″E, and altitude of 1,010 m. As shown in Fig. 1, the NPP is bordered by the sea on the eastern side, and most settlements are located within the bearing of 200° to 360° from the NPP site.
The hourly meteorological data for the Ulsan area, where the Saeul NPP is situated, were obtained from the Meteoblue Organization, a provider of weather and climate data services certified with ISO 9001:2015. The data included information on wind speed and direction over a period of 5 years. The wind data were processed using Origin 2023b to generate a wind rose diagram, as shown in Fig. 2. The wind rose diagram reveals that the predominant wind direction during the 5-year period was from the south to the east, which corresponds to the direction toward the East Sea. Furthermore, significant wind patterns were observed blowing toward the residential areas of Wollae and Seosaeng, situated in the south-southwest and southwest directions, respectively.
2. RASCAL
For the simulation of LOCA scenarios, RASCAL 4.3.4 was used to obtain the source terms of the APR1400 type reactor. The reference model used to simulate this accident was the Barakah Unit 1 NPP, which is embedded in the RASCAL code. The detailed reactor parameters of the NPP that were input into the code are indicated in Table 2.
During an LOCA event in a PWR NPP, such as APR1400, the sequence of events is as follows: It begins with a breach or rupture in the primary system, causing rapid coolant loss. As the coolant level drops, the ECCS is automatically activated to provide additional cooling and maintain core integrity. The high-pressure safety injection system injects coolant to maintain pressure and prevent fuel rod damage, whereas the low-pressure safety injection system ensures long-term cooling. ECCSs work together to flood the core with coolant, removing decay heat. The containment spray system reduces the temperature and pressure in the containment building.
APR1400 is a third-generation PWR designed by Korea Electric Power Corporation (KEPCO) and incorporates advanced safety features to prevent and mitigate accidents, including LOCA events. In some instances, despite the activation of safety features, there may be a failure and the accident progress, and decay heat builds up, leading to breaches in containment and eventually radioactive materials released into the environment.
The LOCA event in this simulation was assumed to have occurred on January 11, 2015, because of a major rupture in the primary coolant system. The accident sequence involved the shunt down of the reactor at 4:00 AM due to a significant loss of coolant due to the failure of inherent safety systems, such as the ECCS and decay heat removal systems. The core was uncovered at 5:00 AM, and radioactive materials were released into the atmosphere. The release rate was 3% volume per day until 11:00 AM when the operators recovered the reactor core. The containment pressure remained high, which kept the release ongoing, and the pressure was reduced along with the release rate at the design leak rate of 0.1% at 3:00 PM, as summarized in Table 3.
3. GENII Code
GENII version 2 was created by the Pacific Northwest National Laboratory for the Environmental Protection Agency to integrate the recommended internal dosimetry models from the International Commission on Radiological Protection (ICRP) and the radiological risk estimation procedures outlined in Federal Guidance Report 13. This code works by selecting the appropriate modules as shown in Fig. 3 and offers advanced, peer-reviewed, and well-documented programs for calculating the radiation dose and risk resulting from the release of radionuclides into the environment [17]. The GENII code employs the straight-line crosswind averaged Gaussian plume model for the dispersion computation expressed as in Equation (1):
The amount of atmospheric dilution and dispersion is expressed as χ/Q, where χ is the concentration at the receptor (Bq/m3), Q is the release rate (Bq/s), σy and σz are the horizontal and vertical deviations of the plume concentration distribution (m), respectively, U is the average wind speed (m/s), F(y) is the horizontal off-axis correction term, and FyFz are the lateral and vertical exponential terms. Seven modules were required for the simulation of this type of accident scenario, each with specific input values.
The average consumption rates for the Korean population reported in the National Health and Nutritional report of 2022 was taken as the average food consumption rates for children and adults living close to the Saeul NPP [18]. The other input parameters into the GENII code, such as the inhalation rates and time spent indoor and outdoor, were obtained from relevant literature [19–21], as indicated in Table 4.
The annual individual effective doses in various directions and distances from the NPP were obtained after executing the code. The code enabled the calculation of the incidence of cancer at various time intervals following initial exposure from accidental release.
Results and Discussion
1. Determination of the Source Terms
Using RASCAL 4.3.4, the accident scenarios were simulated, and the total activity of all radionuclides released into the environment was 1.8×1017 Bq and the composition values of noble gasses, iodine, and particulates are indicated in Fig. 4.
For this study, the representative radionuclides for the source terms were 131I, 134Cs, 137Cs, and 90Sr with their individual total activity releases, as shown in Fig. 5. These radionuclides have half-lives of 8.02 days, 2.01 years, 30.17 years, and 29 years, respectively. Only iodine considered because of its significant impact on the thyroid glands; the rest have longer half-lives that contribute to chronic exposure.
2. Exposure Contribution of Each Radionuclide
The individual effective doses in the exposed individuals were attributable to radionuclide concentrations and exposure pathways. Table 5 indicates the mechanism by which the four radionuclides contributed to the total effective doses over a 5-year period. The results indicate that only long-lived radionuclides have a significant contribution to effective doses in humans. Iodine-131 (131I) with a half-life of 8.05 days had a mere contribution of 0.5% in the first year and had zero contribution in the subsequent years. The contribution of 134Cs to the total individual effective dose gradually decreased over time because of its short effective half-life of 2.06 years, whereas the contributions of 137Cs and 90Sr increased with time because of their long half-lives of 30.17 years and 28.9 years, respectively. In general, the magnitude of the contribution was proportional to the half-life of the three radionuclides of 134Cs, 137Cs, and 90Sr starting from the third year of exposure.
3. Contribution of Each Exposure Pathway
Humans experience chronic radiation exposure through various pathways, including external exposure from the ground, inhalation of contaminated soil particles, and ingestion of contaminated food and animal products. Over a 1-year period, the contribution to the annual effective doses was highest from the ingestion of food and animal products, followed by external exposure from the ground and the ingestion of soil particles. This information is shown in Fig. 6. Consequently, food ingestion emerged as the primary pathway for long-term exposure because the population continued to consume contaminated food throughout the year. Although ground exposure and soil ingestion contributed to radiation exposure, their overall impact on the total effective dose for humans was typically lower than that of food ingestion. This is because the concentration of radionuclides in the soil and ground was relatively low. Furthermore, individuals had less frequent contact via these pathways than via the daily consumption of contaminated foods. Therefore, the primary pathway for long-term exposure was through the ingestion of contaminated food because individuals continued to consume food throughout the year, leading to a more significant contribution to the total effective dose. In this study, the contribution to the annual effective doses of the persons living close to the NPP through the inhalation pathway was not considered because of mainly environmental dispersion and dilution of the radionuclides. When a nuclear accident occurs, radionuclides discharged into the environment can scatter across a broader area and become diluted, lowering the concentration and risk of inhalation.
4. Contribution of Each Food Type
Regarding the contribution of the various foods and animal products to the effective doses, as shown in Fig. 7, vegetables contributed the most, whereas milk contributed the least to the annual effective doses.
The total effective doses resulting from eating various food types in an area after a nuclear accident are influenced by several factors, such as the nonuniform distribution and deposition of radionuclides in the environment, varying contamination levels in different food types, consumption habits, radionuclide half-lives, and regulatory measures imposed by the relevant government authorities. These factors are explained on how each influences effective doses to the public.
1) Food characteristics
Different food types have varying characteristics that influence their susceptibility to radioactive contamination. For example, leafy vegetables, fruits, and grains are more likely to accumulate radionuclides because they grow directly in contact with soil and absorb nutrients from it. Milk and meat become contaminated when the animals consume contaminated pastures, drink contaminated water, and are directly exposed to external radiation.
2) Consumption habits
The amount of food consumed daily also contributes to the effective dose. Food items consumed in larger quantities, such as grains and vegetables, can contribute more to the overall effective dose than smaller portions of fruits or milk.
3) Radionuclide half-life
The radioactive decay of specific radionuclides present in the environment can affect the effective dose. Radionuclides with shorter half-lives, such as 131I, may have a more immediate impact on the effective dose than those with longer half-lives, such as 137Cs or 90Sr. Therefore, for the purpose of radiation protection of individuals, measures should be put in place to regulate the eating of foods, which greatly contribute to radiation exposure to humans.
The individual effective doses in children and adults expected to be living in the area with dominant radionuclide deposition throughout the year are shown in Fig. 8. Effective doses are the summation of the doses from the inhalation, external exposure, and ingestion of food and animal products.
The graph illustrates a pattern in which the individual effective doses decrease as the distance from the NPP site increases for both children and adults. This decrease can be attributed to the dispersion and dilution of radioactive materials as they travel away from the NPP site. Consequently, individuals residing closer to the NPP site received higher radiation doses than those living farther away. Furthermore, note that adult doses were consistently higher than those for children across all distances. This disparity can be attributed to factors such as higher average inhalation rates, spending more time outdoors, and consumption of larger quantities of contaminated foods, which are more prevalent in adults than in children. In case of a nuclear accident, the Korean government has established a food intake restriction standard for some foods to help mitigate the consequences of radiation exposure. The regulatory limits are presented in Table 6.
5. Cancer Incidence in the Exposed Individuals
The incidence of cancer was determined in two specific age groups of the radiation-exposed population following the accidental release of radionuclides into the environment. The age groups chosen were children with a representative age of 10 years and adults with a representative age of 20 years. The incidence of cancer represents the likelihood of developing cancer due to radiation exposure in a sample population of 100,000 individuals. Fig. 9 presents the results depicting the behavior of the calculated cancer incidence at three distances from the NPP site: 3, 16, and 30 km. The analysis considered all types of cancer that can occur in the human body.
Assessment began 5 years after the radiation exposure accident because certain cancers develop within that timeframe. The evaluation period concluded at 60 years, considering the average life expectancy of approximately 80 years for the Korean population. This duration accounts for an adult initially exposed at the age of 20 years. The incidence of cancer exhibited a gradual decline as the distance from the NPP increased. This decrease was attributable to the lower radiation doses received by individuals residing farther away from the NPP site. Considering the worst-case scenario of individuals exposed at a distance of 3 km from the NPP site, the incidence of cancer after 5 years in children and adults was 4.917×10−3 and 7.256×10−3, respectively, whereas, after 60 years, it was 9.448×10−4 and 1.393×10−3, respectively. Following the nuclear accident, the occurrence of cancer decreased because of the gradual decay of the radioactive materials dispersed into the environment. In general, adults experienced higher incidence rates of cancer than children, primarily because of their higher annual effective radiation doses. These findings align with the information presented in the ICRP publication 146, which asserts that the risks of developing cancer due to radiation exposure are directly proportional to the level of radiation doses received by individuals, although the severity may vary depending on factors such as age and sex.
6. Comparison of National (South Korean) Cancer Prevalence Rates with Computed Values from This Study
The number of newly diagnosed cancer cases in South Korea increased every year, except in 2018, as shown in Fig. 10. In this comparative analysis, published statistical information on cancer incidences in the South Korean population for the past 8 years (2015 to 2022) [16, 22–28] was compared with those expected to arise due to radiation exposure to persons residing 3 km away from the Saeul NPP site as shown in Fig. 11.
The findings of this particular study showed that the average contribution of radiation exposure as a risk factor for cancer development from a single NPP was 0.000491%. Assuming a conservative scenario in which all 25 operating NPPs in South Korea were of the APR1400 type with the largest thermal power and experienced a LOCA simultaneously, the total contribution of 0.012% was obtained from Equation (2):
where TC is the total contribution to cancer development from all NPPs. N is the total number of NPPs that experienced LOCA events, and IC is the contribution from an individual NPP. This value remains below the safety goal of NPPs, which is set at 0.1%. Therefore, these results suggest that radiation exposure from nuclear accidents had limited significance in causing the observed increase in newly diagnosed cancer cases in South Korea. The results of this study correlate with those of other studies that have shown that the number of cancer cases in South Korea has been increasing during the past several decades because of the aging Korean population, smoking, obesity, and westernized dietary habits [29, 30].
Conclusion
This study investigated the long-term radiological impacts of the release of radioactive materials following a severe nuclear accident at the Saeul NPP. The analysis focused on areas located within a radius of 3–30 km from the plant. The highest annual individual effective doses were observed at a distance of 3 km from the plant. Among the different exposure pathways, including external exposure, ingestion of soil particles, and consumption of contaminated food and animals, the latter significantly contributed to the annual total effective doses. In particular, the majority of these doses originated from the intake of vegetables and meat. The incidence of cancer after a 5-year exposure period was 4.917×10−3 and 7.256×10−3 in children and adults, respectively. Furthermore, this study revealed that radiation exposure resulting from this type of accident accounted for only 0.012% of newly diagnosed cancer cases in South Korea. This finding falls well within the safety objective of NPPs, which aims to keep the contribution to cancer cases below 0.1%. Regarding these results, it is recommended that in the aftermath of such an accident, the public should refrain from consuming large quantities of vegetables and meat sourced from within the affected area during the initial years. Moreover, decontamination measures should be implemented in residential areas located closer to the NPP.
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