Assessment of Public Dose due to Gaseous Radioactive Effluent in Decommissioning Nuclear Power Plant
Article information
Abstract
Background:
In order to gain approval of decommissioning of nuclear power plants (NPPs), the operator has to submit a final decommissioning plan (FDP) to the regulatory body. The safety assessment is an essential part to be described in the FDP. In safety assessment, dose of the general public near the plant site due to decommissioning activities should be evaluated and described. As gaseous radioactive effluents are expected to be released in the process of decommissioning, it is necessary to assess the public dose. In this study, we assessed the public dose due to gaseous radioactive effluents released in NPP decommissioning activities.
Materials and Methods:
The source term of gaseous radioactive effluent was set based on the decommissioning scenarios suggested in the NUREG-0130. The critical group assumed the maximum individual for adults on the exclusion area boundary. Public exposure pathway was set in accordance with the guideline 2.2 of the Korea Institute of Nuclear Safety (KINS). In addition, the public dose was assessed through the GASPAR computer code with the factors suggested by the KINS.
Results and Discussion:
In the decommissioning scenario, the source term for gaseous radioactive effluent was set by using the nuclide fraction according to the contamination type of structures, systems, components in the reference reactor. As a result of the public dose assessment, total annual public dose in the overall decommissioning scenario was 1.95×10–2 μSv/yr. Segmentation on non-activated stainless steel task was the highest at 1.43×10–2 μSv/yr. In the overall scenario, the nuclide with the highest contribution to the public dose was Co-60. As a result of the assessment, it was found that the public dose due to the gaseous radioactive effluent during the decommissioning of the NPP was insignificant compared to the legal dose limit.
Conclusion:
The results of this study can be used as a basis for radiological safety assessment and the management of radioactive effluents in the decommissioning of NPP.
Introduction
Kori Unit 1 and Wolseong Unit 1, two commercial nuclear power plants (NPPs) in Korea, have now reached the end of their design lifetime and have been shut down permanently. In addition, 10 commercial plants in Korea are expected to reach the end of their design lifetime by 2030, which is almost half of the domestic operating NPPs [1]. Due to this situation, the decommissioning industry for NPPs is expected to grow drastically. The Korean Nuclear Safety and Security Commission’s Notification No. 2021-10 states that when a NPP is decommissioned, the operator must prepare a final decommissioning plan (FDP) that assesses and describes the expected radiation dose to the general public during decommissioning activity [2]. It is anticipated that gaseous radioactive effluent will be released into the atmosphere during the decommissioning of NPPs as highly activated and/or contaminated structures, systems, components (SSCs) are decontaminated and decommissioned [3]. Therefore, it is necessary to assess the dose to the general public due to the release of gaseous radioactive effluents.
The International Atomic Energy Agency (IAEA) has identified possible exposure pathways for the public living near the site to derive a methodology to assess the safety of decommissioning a NPP [3]. In the United States, companies such as Southern California Edison (SCE), Entergy, and Holtec have compared the expected dose to the general public from decommissioning of NPPs with the dose levels from the existing operation of NPPs, and concluded that the doses are comparatively low on decommissioning [4–6]. Shimada et al. [7, 8] analyzed the exposure pathways of the public to gaseous radioactive effluents during decommissioning based on the experience of decommissioning research reactors and developed a computer code to evaluate them. To assess the safety of NPP decommissioning, Seo et al. [9] investigated the exposure pathways of the public living near the site due to the operation and decommissioning of NPPs and analyzed the differences between each pathway. Lee et al. [10] analyzed radionuclides in gaseous effluents released during decommissioning of NPPs and derived annual release objectives per nuclide to meet the dose limit for the public.
Chapter 6 of the FDP for the decommissioning of a NPP should describe the safety assessment of decommissioning activities. The dose assessment is one of the sections of the safety assessment and should assess the dose to the public according to the exposure scenario and describe the source term, major assumptions, models, and inputs used in the assessment and the results of the assessment [2]. To authorize the decommissioning of a NPP, it is necessary to assess the dose to the public in the vicinity of the site due to decommissioning activities. In the case of the United States, the decommissioning process focuses on assessing the dose to workers; however, there are some differences in site characteristics and data on local residents’ habits compared to Korea. In Korea, a number of dose assessments have been conducted at operating NPPs. However, there are few studies that evaluate the radiation impact of specific decommissioning scenarios on residents near the site. Therefore, studies are needed to assess the dose to the public resulting from the release of radioactive effluents during decommissioning activities.
The objective of the present study is to assess the dose to the public from gaseous radioactive effluents generated during decommissioning activities at a NPP. To this end, we first analyzed the amount of radioactive material released into the environment. Then, we set the radiation exposure pathway of the public due to decommissioning activities. Finally, the dose to the public from the gaseous radioactive effluents was assessed based on the activities released into the environment and the exposure pathway.
Materials and Methods
Gaseous radioactive effluents from the decommissioning of NPPs include radioactive particles scattered at segmentation and decontamination of surface contaminated SSCs. In this study, the dose to the public from gaseous radioactive effluents during the decommissioning of a NPP was assessed.
1. Selection of Decommissioning Activities Inducing Gaseous Radioactive Effluents
To assess the dose to the public from gaseous radioactive effluents, it is necessary to select the tasks where the release of radioactive effluents is to be expected. NUREG-0130 categorized the tasks that may have radiological effects on the public during normal decommissioning activities for the reference plant to assess the cost and safety of decommissioning light water reactor [11]. Table 1 shows the decommissioning scenarios presented in the literature. The decommissioning scenario for the reference NPP involved nine steps, including the cutting of non-activated stainless steel and the cutting of activated reactor vessels. The work scenario primarily consists of decontamination and cutting work, during which the generation of large amounts of gaseous radioactive effluents is typically expected. Radioactive particles generated during the segmentation operation are released into the environment as gaseous effluent through a building filter such as a high efficiency particulate air (HEPA) filter. In the reference plant, only one HEPA filter with a 99.95% filtration rate was considered [11]. Therefore, in this study, only one HEPA filter was considered when assessing the amount of radioactivity released into the environment. There are other tasks that have the potential to generate gaseous radioactive material. However, the IAEA recommends a graded approach to assessing the safety of decommissioning NPPs, in which only the major tasks are assessed. On the other hand, NUREG-0130 presented the types of operation scenarios for generating gaseous effluent during the decommissioning activities of the reference plant, as well as the amount of radionuclides and radioactivity emitted in those scenarios. Therefore, this study considered only the operation scenarios presented in the NUREG-0130 when assessing the dose to the public and utilized the types of radionuclides and the amount of radioactivity presented in it [3, 11].
Decommissioning Activity Scenarios Which Gaseous Effluent Occur
2. Radiation Dose Assessment for the Public by Exposure Pathway
1) Setting exposure pathways for the public due to the release of gaseous radioactive effluents
Fig. 1 shows the exposure pathway for the public due to gaseous radioactive effluents during the decommissioning of a NPP. The IAEA has launched the Evaluation and Demonstration of Safety for Decommissioning of Facilities Using Radioactive Material (DeSa) project to establish a safety assessment methodology for the decommissioning of NPPs [3]. In the DeSa project, the DecDose computer code was used to assess the dose to the public from gaseous radioactive effluents to be expected during the decommissioning of a reference NPP. This code presented the same exposure pathways as those assessed in operating NPPs [7, 12]. Therefore, the exposure pathways used in this study were those specified in the Regulatory Guideline 2.2 of the Korea Institute of Nuclear Safety (KINS), a guideline for assessing the radiation dose to the general public in Korea [12].
Public exposure pathways by gaseous radioactive effluent.
2) Determination of assessment factors for the radiation dose to the public
To assess the radiation dose to the public from radioactive material released into the environment, the behavior of the radioactive material in the atmosphere must be analyzed. The atmospheric behavior of gaseous radioactive effluents is expressed by an atmospheric diffusion factor, which is evaluated using a model appropriate to the climatic and topographic characteristics of the site [13]. In this study, the maximum atmospheric diffusion factor at the exclusion area boundary proposed by Site A in Korea and its bearing were used to evaluate the dose to the public from gaseous radioactive effluents.
Table 2 shows key input values used to assess the dose to the public in this study. Key inputs consist of pathway factors and consumption values. To assess the radiation dose to the public, characteristic factors reflecting the environment in Korea, such as site-specific factors and maximum individual intake, are required. In this study, we used the Korean characteristic factors proposed by KINS [12]. The decommissioning period used for dose assessment was set to the period generally considered in the decommissioning plan for NPPs in Korea.
Key Input Values Used in Assessment of Public Dose
3) Assessment of the radiation dose to the public due to the release of gaseous radioactive effluents
In Korea, when assessing the impact of radiation on the environment of a NPP, the dose is calculated for the maximum individuals located in the exclusion area boundary [14]. The assessment of the maximum individual exposure dose from gaseous radioactive effluents from an operating NPP is based on the methodology of United States Regulatory guide 1.109 and KINS Regulatory Guideline 2.2. The IAEA’s methodology for the safety assessment of decommissioning suggests that the assessment of doses from normal decommissioning activities can be performed using the same methods as for operating NPPs [3]. Thus, in this study, the GASPAR computer code based on the assessment model presented in the above methodology was used to assess the dose to the public from gaseous radioactive effluents. GASPAR code is an assessment tool developed for Nuclear Regulatory Commission (NRC) by Pacific Northwest National Laboratory. GASPAR code can assess the radiation dose to individuals and groups from emissions when radioactive particles emitted from light water reactor NPP leak into the environment as gaseous radioactive effluent [15].
Results and Discussion
In this study, we assessed the dose to the public from gaseous radioactive effluents produced during the decommissioning of a NPP. First, we selected the tasks where gaseous radioactive effluents are expected during the normal decommissioning activities of the NPP to set the source term. We analyzed the exposure pathways of the public in the vicinity of the site due to decommissioning activities. Finally, the dose to the public from gaseous radioactive effluents was assessed using the dose assessment methodology for the operation of NPPs according to the applicable exposure pathways.
1. Results of Assessing the Radioactive Amount from Gaseous Radioactive Effluents
Table 3 shows the nuclide fraction and nuclide-specific activity released to the environment by type of radioactive contamination during decommissioning as proposed by the NRC, and Table 4 shows the contamination type and radioactive amount released to the environment by decommissioning scenario. The source term for gaseous radioactive effluents is based on the nuclide fraction derived by the NRC for reference light water reactor. In the literature, nuclide fractions for the five types of contamination at the reference reactor and a total of 26 nuclides were categorized by contamination type. In addition, we presented the nuclide-specific activities released to the environment considering the radioactivity, scatter ratio, and filtration rate according to the normal decommissioning scenario. In this study, the radiation dose by nuclide was calculated using the nuclide fraction according to the decommissioning scenario of the reference reactor and the contamination type to calculate the dose by nuclide, which was set as the source term for the gaseous radioactive effluents [11].
Nuclide Vector in Non-accidental Nuclear Power Plant Decommissioning Activity
Nuclide Vector and Activity according to Decommissioning Activity Scenarios
2. Results of Assessing the Radiation Dose to Local Residents by Exposure Pathway
1) Results of setting an exposure pathway for the public from gaseous radioactive effluents
In this study, the following exposure pathways from gaseous radioactive effluents were assessed: (1) external exposure from the cloud shine (inert gas); (2) external exposure from ground deposition; (3) internal exposure from inhalation; and (4) internal exposure from the intake of agricultural and livestock products. For the internal exposure pathway through the intake of agricultural and livestock products, grain, kimchi, leafy vegetables, and fruit were considered as agricultural products, and meat and milk as livestock products.
The International Commission on Radiological Protection (ICRP) proposes to create an exposure scenario that considers the lifestyle, food intake, and place of residence of people in the vicinity of the site and to calculate the radiation dose accordingly [16]. Therefore, the calculation of the dose is usually evaluated in two parts: the maximum individual dose and the collective dose for the entire public within a radius of 80 km. For the collective dose, the public are usually assessed by categorizing them as infants, 1-, 5-, 10-, 15-year-olds, and adults. In the dose assessments for the decommissioning of foreign NPPs, the concept of collective dose was excluded and only the maximum individual dose for an adult at the site boundary was assessed [3, 11]. Therefore, only the maximum individual dose for an adult at the exclusion area boundary was assessed in this study.
2) Results of setting a dose assessment factor for local residents
In this study, the critical group, the target group for assessing exposure due to NPP decommissioning, was assumed to be the adult maximum individual living within the exclusion area boundary. The exclusion area boundary is located within a radius of 700 m around the center of the plant containment building. For the atmospheric diffusion factor, we set a value of 3.04×10−6 s/m3 in the NNW direction, which corresponds to the maximum value at the exclusion area boundary of site A in 2019–2020 in Korea.
For the maximum individual intake of an adult in the critical group, the site-specific factors, and other inputs for dose assessment, the values provided by KINS were used [12]. The dose conversion factors are based on the values of the ICRP 72 [15]. The assessment of the radiological safety of the public is presented as part of the FDP. As FDP is prepared after the permanent shutdown of the plant, the decommissioning period for the public dose assessment is set at 15 years, which corresponds to the typical period for an immediately decommissioned plant from the date of permanent shutdown to the planned end of decommissioning [17]. In general, for operating NPPs, a period corresponding to half the lifetime of the NPP is used to assess radiation doses to the public. Thus, a period of 7.5 years, corresponding to half of the decommissioning period, was used as the input factor for the decommissioning period in this study.
3) Results of assessing the radiation dose to local residents from gaseous radioactive effluents
Table 5 shows the results of the radiation dose assessment for the public by exposure pathway under the normal decommissioning scenario. Accordingly, the task of segmenting non-activated stainless steel had the highest radiation impact. The next highest doses were measured during surface cleaning operations, followed by in situ chemical decontamination. Generally, the segmentation of the activated reactor is expected to scatter most of the radioactive material during cutting. However, it is suspected that this is due to the fact that the reference nuclear facility that performed this operation installed additional filters during cutting to prevent the scattering of radioactive material in advance [11, 18].
Segmentation of non-activated stainless steel resulted in a total annual dose of 1.43×10−2 μSv/yr to local residents, while surface cleaning operations and in situ chemical decontamination resulted in 4.27×10−3 μSv/yr and 5.12×10−4 μSv/yr, respectively. As the underwater cutting method and one additional HEPA filter were applied in segmentation of activated reactor operation, radiation dose to the public of this scenario only showed 1.29×10−4 μSv/yr. The total annual public dose for the normal decommissioning scenarios was found to be 1.95×10−2 μSv/yr. This is about 10−4 degree of the Korean legal dose limit of 0.25 mSv/yr for the public during normal operation for multiple units on the same site [19].
The dose assessment for the public showed that external exposure from ground deposition was the dominant pathway for the total dose to the public. In the guidelines for the operation of NPPs, inert gases, particulate nuclides, radioactive iodine, tritium, and gross alpha nuclides are specified as targets for analyzing the samples of gaseous radioactive effluents [20]. The source term nuclides in the gaseous radioactive effluents determined in this study are mostly particulate nuclides, i.e., nuclides that are continuously deposited on the ground in the course of decommissioning activities. As the dose conversion factor for cloud shine is insignificant for radionuclides other than inert gas, ground shine caused by radionuclides deposited on the plant site ground becomes the dominant external exposure pathway [21]. Hence, it is assumed that the external exposure pathway through ground deposition had the greatest impact on the public dose.
It was also found that the external exposure from the cloud shine was very low. For the external exposure pathway through the cloud shine in a NPP during operation, only the radiation effects of inert gases are considered. In addition, during the operational phase of a NPP, the inert gas produced is stored in a decay tank for about 60 days to attenuate the radioactivity and release it into the atmosphere. In the case of the reference plant used in this study, the sole inert gas nuclide was Ar-39. Krypton-85 (Kr-85) and Xe-131, typical inert gaseous nuclides that occur during the operational phase of a NPP, are fission products and are rarely detected in the reference nuclear facility, as decommissioning activities were initiated after the spent fuel had been removed. The technical specifications for NPPs in Korea specify a total of eight nuclides for analyzing the samples of gaseous radioactive effluents, and all except Kr-85 have half-lives of less than 5 days. Accordingly, it is likely that the inert gaseous nuclides will have decayed more than seven half-lives after 7.5 years, i.e., half of the decommissioning period determined in this study. In other words, the exposure of the public from the cloud shine during normal decommissioning activities is considered negligible.
Fig. 2 shows the dose contribution of each major nuclide by exposure pathway for the entire decommissioning scenario. As the public dose from cloud shine is extremely low, external exposure pathway of cloud shine is excluded in Fig. 2. It was found that the external exposure is dominated by the deposition of Co-60 on the ground surface, which accounts for about 87% of the total radiation dose, and Cs-137, which accounts for about 10%. In the case of internal exposure by inhalation, Co-60 contributes to about 82% of the total dose, Co-58 to about 8%, and Zr-95, Cs-137, and Ce-141 to a fraction. In the case of internal exposure through the intake of agricultural products, Cs-137 contributes to about 62% of the total dose, while Co-60 accounts for about 33%. For the exposure pathway through the intake of livestock products, Cs-137 contributes about 57%, followed by Co-60 with about 17%, Cs-134 with about 10%, and some contributions from Nb-95, Ru-103, and Co-58. Cobalt-60 (Co-60) and Cs-137 contributed the most to the public dose in all exposure pathways. This is originated to be due to the release of nuclides from the surface contamination, Chalk River Unidentified Deposit (CRUD), of the SSCs into the environment as a result of cutting operations during a normal decommissioning scenario [22].
Dose contribution by nuclide according to pathways.
Conclusion
In this study, we assessed the public dose from gaseous radioactive effluents released during the decommissioning of a NPP. To this end, we first selected decommissioning tasks that may result in gaseous radioactive effluents and determined the radioactive materials released into the environment during these tasks. Moreover, we analyzed the pathways of exposure from gaseous radioactive effluents for the public. Finally, the public dose from gaseous radioactive effluents was assessed based on the applicable exposure pathways.
First, we selected tasks where gaseous radioactive effluents may occur during decommissioning of NPPs and analyzed the nuclide-specific activities released to the environment during these tasks. In NUREG-0130, a scenario was selected for the cost and safety assessment of the decommissioning of reference pressurized light water reactor that could have a radiological impact on the public during decommissioning activities. The scenario consists of a total of nine tasks. In this study, the source term of gaseous radioactive effluents was set based on the nuclide fractions reported in the literature for the nuclide-specific activity and contamination type.
Second, we analyzed the exposure pathways for the public due to gaseous radioactive effluents. The IAEA has clarified that the methodologies and computer codes used in the existing NPPs can be used to assess the public dose in normal decommissioning. Therefore, these exposure pathways for the public were considered: (1) external exposure from cloud shine; (2) external exposure from ground deposition; (3) internal exposure from inhalation; and (4) internal exposure from the consumption of agricultural and livestock products.
Third, the public dose from gaseous radioactive effluents was assessed. The assessment tool was the GASPAR computer code, atmospheric diffusion factors are based on the assessment data from site A, and the site-specific factors and other input factors are based on the values provided by KINS. The assessment of the public dose from gaseous radioactive effluents resulted in a total annual public dose of 1.95×10−2 μSv/yr for the normal decommissioning scenarios. This is about 10−4 degree of the dose limit for multiple units. The task with the highest radiation impact for the public under the decommissioning scenarios was the segmentation of non-activated stainless steel with a total annual dose of 1.43×10−2 μSv/yr. Regarding the contribution of the individual nuclides to the public dose, it was found that the contribution of Co-60 was most dominant in two of the exposure pathways, and the contribution of Cs-137 was also dominant in two pathways. It was found that this was due to CRUD that generated during the operation of the NPP. The most important exposure pathway for the public was external exposure from ground deposition, while the least important pathway was external exposure from cloud shine. The results of this study can be used as a basis for radiological safety assessment and the management of radioactive effluents in the decommissioning of NPPs.
Notes
Funding
This work was supported through the National Research Foundation of Korea (NRF) using the financial resource granted by the Ministry of Science and ICT (MSIT) (No. RS-2022-00143994).
Conflict of Interest
No potential conflict of interest relevant to this article was reported.
Ethical Statement
This article does not contain any studies with human participants or animals performed by any of the authors.
Data Availability
Data sharing is not applicable to this article as no new data were created or analyzed in this study.
Author Contribution
Conceptualization: Kim KP. Methodology: Kim KP. Data curation: Son GW, Lee SD, Han CH. Formal analysis: Kim KP. Funding acquisition: Kim KP. Investigation: Son GW, Kim KP. Visualization: Son GW, Lee SD. Software: Son GW, Kim KP. Validation: Kim KP. Writing - original draft: Son GW, Kim KP. Writing - review & editing: Kim KP. Approval of final manuscript: all authors.