Soil Sampling Procedure for Management and Analysis of Legacy Sites in the Korean Peninsula
Article information
Abstract
Background
The Korean Peninsula is split between the South Korea and Democratic People’s Republic of Korea (DPRK, North Korea), with North Korea possessing nuclear weapons. There are rising concerns over legacy sites, areas contaminated primarily by nuclear activities. In North Korea, these sites lack regulatory oversight and are likely neglected even if inactive. Hence, it is vital to devise strategies to manage and assess these sites. Yongbyon, a prominent nuclear site in North Korea, has been proposed for environmental sampling to gauge contamination levels. The goal is to determine contamination by suggesting sample collection points in Yongbyon.
Materials and Methods
Using recent data and satellite imagery from the International Atomic Energy Agency’s periodic Safety Measures Report, we selected legacy sites to assess their contamination levels with recent data and satellite imagery. We identified sampling locations using the Visual Sample Plan (VSP) to check nuclear contamination. From these results, we devised a scenario evaluating accessibility and the local environment.
Results and Discussion
For Yongbyon in North Korea, this study created two sampling scenarios based on interior access feasibility. Given North Korea’s constraints, we finalized the scenario without interior access. The sampling areas include two steam lines, four vehicle paths, and five green zones outside the facility.
Conclusion
Nuclear facility operations worldwide are concerning. When halted, these become legacy sites requiring contamination management. Despite North Korea’s limited record-keeping and challenges in determining contamination, we devised a sampling scenario using VSP software, factoring in accessibility and movement paths.
Introduction
The Korean Peninsula remains divided between the Republic of Korea (South Korea) and the Democratic People’s Republic of Korea (DPRK, North Korea), with North Korea being a nuclear-armed state. Periodic reports of nuclear facility activities in North Korea continue to be published, raising concerns about legacy sites within the Korean Peninsula. South Korea is a signatory to international treaties, such as the ‘Treaty on the Non-Proliferation of Nuclear Weapons,’ aimed at preventing the proliferation of weapons of mass destruction [1]. Through the International Atomic Energy Agency (IAEA)’s safety measures, it is possible to confirm contamination at legacy sites or nuclear activity facilities. However, North Korea has not signed international treaties and refuses inspections of nuclear activities by the IAEA. This makes it difficult to identify the status at legacy sites. Sites involved in nuclear and related activities, leading to contamination with artificial or natural radioactive materials, are referred to as legacy sites [2]. These sites include land, water, buildings, structures, and materials that can potentially be reused or require dismantling. Legacy sites are primarily associated with facilities involved in nuclear weapon production, nuclear test sites, or facilities related to the handling and disposal of nuclear weapons. Legacy sites on the Korean Peninsula include South Korea’s TRIGA MARK-II (General Atomic) and North Korea’s Yongbyon (nuclear activity facility), Pyongsan (uranium mine and uranium refining facility), Pakcheon (uranium mine), and Punggye-ri (nuclear test site) [3]. Facilities related to legacy sites in South Korea are currently operating under the existing regulatory framework, and their decommissioning will be managed according to current regulatory standards. This approach aims to minimize potential issues and ensure the safe handling of these sites [3]. However, in the case of North Korea, facilities related to legacy sites lack operational information within an established regulatory framework. There is a higher likelihood that these facilities may be abandoned or left unattended even if they are discontinued [2]. So, it is necessary to manage and analyze places where facilities exist on the Korean Peninsula or are likely to be converted to legacy sites, so it is expected that operation information on the site and the resulting pollution level will be identified. However, it is expected that there will be a lack of information and temporal-spatial limitations in North Korea. Therefore, soil samples from the environment should be utilized for nuclear activity analysis at legacy sites. Among North Korea’s legacy sites, Yongbyon is a significantly developed nuclear facility, and it continues to actively conduct nuclear activities to this day. Since using official samples is expected to be challenging, it is necessary to collect environmental samples to assess contamination at legacy sites. Environmental samples provide relatively less conclusive information about nuclear activities compared to official samples. In order to collect environmental samples, it is necessary to determine the target facility and site to identify contamination and then select an environmental sampling point. For this purpose, trends in the legacy site were checked, and collection points were determined using Visual Sample Plan (VSP) software (Pacific Northwest National Laboratory [PNNL]).
Materials and Methods
1. North Korea’s Main Legacy Sites and Nuclear Activities
In May 1992, North Korea officially reported to the IAEA for the first time in accordance with the Full Scope Safeguards Agreement, declaring two uranium mines and two refining facilities located in Pyongsan of Hwanghae-bukdo and Pakchon of Pyongan-namdo [4]. The uranium mine and refining facility in Pakchon seem to be for minimal pilot operations, and particularly, the lack of changes in satellite images of this facility suggests that the area might not have been operational for several decades [4]. Conversely, the Pyongsan mine and refinery have undergone significant improvements in recent years. According to observations made through artificial satellites, there appears to be an increase in uranium tailings piles, and thermal imagery indicates heightened thermal output, suggesting increased activity at the Pyongsan mine [4]. However, the grade of uranium ore extracted from Pyongsan remains elusive as a non-declared matter, making it challenging to ascertain the precise refining processes used for ore extraction [4].
In 2010, North Korea invited American nuclear experts to visit and publicly disclosed its uranium enrichment facility in Yongbyon [5,6], as shown in Fig. 1A and Fig. 2. During this visit, it was confirmed that North Korea had the capability to produce approximately 30–40 kg of highly enriched uranium, enough to make one to two nuclear weapons per year [7]. Uranium hexafluoride is one of the primary minerals of uranium, chemically represented as UO2. It is used in nuclear fuel production and nuclear weapon manufacturing, playing a critical role in nuclear energy and nuclear weapon production [7].
Therefore, if continuous signs of nuclear activity are detected in Yongbyon, meaningful analytical data would be obtained during the process of assessing the extent of contamination at legacy sites.
2. Visual Sample Plan
To assess the contamination of legacy sites, we established a soil sampling plan using the VSP program and recent activity records of the legacy sites. The VSP program, developed by the PNNL, allows for the selection of the optimized number and location of environmental sample collection points to achieve the required level of confidence in decision-making based on statistical tests performed on the data collected through sampling plans [8]. The program was developed in collaboration with various organizations, including PNNL, the U.S. Department of Energy, the U.S. Department of Defense, the U.S. Environmental Protection Agency, the U.S. Department of Homeland Security, the National Institute for Occupational Safety and Health, the U.K. Government Decontamination Services, and the U.K. Atomic Weapons Establishment [8]. The VSP program operates based on the Data Quality Objectives (DQO) process, where criteria are established and decisions are made. As an example of using VSP software, the U.S. Department of Defense transfers military land for public use by removing weapons and explosives that could harm the land and sea. After weapons and explosives are removed, a quality assurance/quality control process must be completed to manage land and sea risks. In this case, there are instances where VSP software, a statistical and analysis tool, is used to verify on-site calibration. In this paper, we propose using VSP software to determine sampling locations to confirm contamination rather than for correction purposes [9]. Before using the built-in sampling methods in the program, a decision through the DQO process is necessary [8]. This process is recommended when activities such as determining average concentration levels of contaminants and estimation are to be carried out [10].
Results and Discussion
1. North Korea’s Nuclear Activities
In order to analyze facilities related to nuclear activities, it is necessary to obtain official samples with North Korea’s cooperation and conduct an analysis with the IAEA. However, since this is expected to be challenging, it is necessary to analyze environmental samples based on predictions about North Korea’s nuclear activities [11]. Radioactive materials are released into the soil from facilities after nuclear activities. Uranium and plutonium are often mentioned as indicator nuclides to verify contamination due to nuclear activities. Both uranium and plutonium are classified as spent nuclear fuel, and this spent fuel has a high possibility of being utilized for nuclear weapons through reprocessing activities. Additionally, uranium is used as a raw material for nuclear weapons. If there is nuclear activity, it is possible to detect it by analyzing the specific isotope patterns of uranium. As plutonium is associated with reprocessing activities for nuclear weapon production, its presence increases the likelihood of detecting plutonium if such activities exist [4]. Plutonium belongs to the heavy nuclides, so its behavior slows down when released into the soil, while uranium either settles deep underground following the groundwater flow or deposits profoundly depending on the type of soil [12]. Uranium, being a volatile chemical species, is widely distributed in areas within the soil, such as created piles. Considering the environmental behavior of plutonium and uranium, it is necessary to use soil samples to verify contamination. Furthermore, since the majority of nuclear activity facilities are located close to the soil and plutonium released during the reprocessing process belongs to the heavy nuclides, there is a high probability of its presence within the soil. Therefore, soil samples from the environment should be utilized for nuclear activity analysis at legacy sites.
Based on publications such as “Application of safeguards in the Democratic People’s Republic of Korea” from the IAEA and reports from North 38, nuclear activities could be identified in various facilities within Yongbyon until recently [13]. In particular, since signs of activity have been detected at the steam plant within the radiochemistry laboratory in Yongbyon, it has been decided to establish a plan for sample collection in order to detect contamination within the Yongbyon radiochemistry laboratory.
2. Sampling Method Performance
Before conducting sampling at the legacy sites, it has been decided to select the sampling method integrated into the VSP program, which utilizes equations used for calculating nuclear materials. Since the objective is to determine the presence or absence of contamination at the sites, the sampling methods “Show that at least some high % of the sampling area is acceptable” and “Estimate the mean” have been chosen among the available options. The sampling process began with “Show that at least some high % of the sampling area is acceptable” to initially determine the number of samples required to assess the presence or absence of contamination. The results from this initial sampling were then used to estimate the mean contamination level at the sites using the “Estimate the mean” method, which helped determine the number of samples needed for estimating the average contamination level across the sites. The “Show that at least some high % of the sampling area is acceptable” method is used to determine the minimum number of samples required to assess the presence or absence of contamination sites. It calculates the number of samples needed to confirm whether contamination is above or below a specified threshold (action level) in order to determine the contamination status [8].
This method is segmented into several categories: (1) using quantitative measurements that follow a normal distribution; (2) using quantitative measurements from an unknown distribution; (3) using presence/absence measurements for different strata; and (4) using presence/absence measurements.
Since there is no existing contamination data for the selected sites, it was not possible to assess the contamination level. Therefore, the “Using presence/absence measurements-nonparametric upper tolerance limit” method was employed, and the input values of P and 1-P were adjusted to determine the locations and number of samples to be collected. Table 1 provides explanations for each input value [8].
Input Values of “Show that at Least Some High % of the Sampling Area Is Acceptable” Sampling Method
For the sampling, input values, as described in Table 1, were used. Considering the lack of sample values for contamination, the sampling was carried out. Required input values include the probability of not rejecting an accurate result in the sampled area and the acceptable decision ratio at a confidence level of 100.
Detailed descriptions of each input parameter are as follows.
(1) α represents the type I error that can occur in the VSP program, and it is used to derive the confidence level. The confidence level represents the probability that the action level will not be exceeded.
(2) P is the proportion of the population values, representing the percentage of contaminated values that must be below the action level.
(3) Accordingly, if the confidence level is set at 0.95 (95%) and the proportion of the population that must be below the action level is specified as 0.80 (80%), it must be ensured with 95% probability that at least 80% of the population is below the action level. Conversely, the probability that less than 80% of the population is below the action level is at most 0.05 (5%).
(4) By collecting 14 samples, if 80% or more are below the action level, the area can be considered uncontaminated. Conversely, if 20% of the samples exceed the action level, the area can be considered contaminated.
(5) The value of 1-P can be interpreted as a threshold for determining contamination, meaning it represents the upper limit of the proportion where contamination may occur.
In situations where contamination levels are unknown and distributions cannot be identified, commonly accepted confidence values were adopted. The adopted values for confidence level were 95%, while 1-P was 20%, and P was 80%. So, the number of samples taken to check for contamination is 14. The sampling location selection results are shown in Fig. 3.
Selected sampling locations of “Show that at least some high % of the sampling area is acceptable” sampling method. The red dots in the figure indicate the selected sampling locations.
Before selecting a sampling method to measure the average contamination level of a site, it is essential to determine if the data is similar. The research assumed non-uniform data due to the unknown contamination status. The distribution of facilities within the Yongbyon radiochemical laboratory is not uniform, and specific nuclear activities are observed in certain facilities. It is estimated that the contamination distribution of nuclear activity within the areas is not uniform. Under the assumption of non-uniform data, three measurement methods are utilized. In a situation where the extent of contamination was unknown, “stratified sampling” was applied, and the software was operated. “Stratified sampling,” also known as layer sampling, involves comparing the sampling result values of each layer. The main purpose is to determine the number of samples to be collected to estimate the overall strata average for estimating the average across the entire site where sampling is conducted [8]. Upon conducting sampling using the stratified method, the values obtained from “Show that at least some high % of the sampling area is acceptable” were entered. To establish a scenario for soil sample collection, minimizing errors and selecting overlapping sampling locations as much as possible, the sampling values from the “Show that at least some high % of the sampling area is acceptable” method were incorporated into the input parameters of the “stratified sampling” method. This approach helps ensure that the soil sample collection scenario is well-founded and minimizes errors by utilizing data from the initial sampling method. If common input values existed, those from the “Show that at least some high % of the sampling area is acceptable” method were used. If the desired sampling input values were present in the results of the previous method, those values were utilized. Shared inputs between the sampling methods, like costs, were addressed by applying the results from the “Show that at least some high % of the sampling area is acceptable” method to the “stratified sampling” values, minimizing error rates. It needs to estimate the mean of the sampling site through “stratified sampling,” to choose one of the three sub-methods based on the specific situation and then conduct the sampling accordingly.
In this study, the minimize variance of sample mean for fixed cost method was applied because it allows for the maximization of precision in estimating the population mean within a fixed total cost, as defined in this context [8]. There are other methods, such as minimize cost for the required variance of sample mean and predetermined number. Minimize cost for required variance of sample mean is an approach where there are no cost limitations, and it is based on the understanding that the site to be sampled is located within North Korea, making access both time-consuming and costly. Predetermined number provides the user with a specific total sample size, but it does not evaluate suitability, so it was excluded from consideration [8]. While the actual cost is not expected to be a major factor, we evaluated it by cost factors in light of time constraints and other considerations.
The input value for sampling using the first method was $8,000 for the fixed cost for 14 sample counts resulting from the “Show that at least some high % of the sampling area is acceptable.” The sampling result was a cost of $100 for collecting a sample and an analysis cost of $400. As it is a value for one sample, the value for a total of 15 samples is $7,500. Including the starting cost of $1, and the final cost is $7,501. This demonstrates that it is possible to collect samples at a cost lower than the total fixed cost, showing the appropriateness of collecting samples in a time and space-constrained environment using “stratified sampling.” The sampling results are shown in Fig. 4.
Selected sampling locations of “Stratified sampling” sampling method. The blue dots in the figure indicate the selected sampling locations.
3. Sampling Result
1) Sample collection facility selection
Based on the results of each program, the sampling locations were divided into inside and outside the radiochemical laboratory in this study. Collecting and analyzing soil samples from facilities inside the laboratory is advantageous for determining nuclear activity. However, in situations with time and space constraints, access to required facilities might be limited. Based on the characteristics of the facilities inside and outside, the final sample collection locations were selected, considering whether access to the internal facilities is possible or not.
(1) If access to internal facilities is possible
Inside the radiochemical laboratory, facilities exist that can extract plutonium required for nuclear weapon production. The internal facilities and their purposes include:
(1) Uranium recovery facility: Extracts and processes uranium from raw materials in ore form. The extraction process typically involves mining, excavation, leaching, and refining.
(2) Plutonium extraction facility: Separates and processes plutonium from spent nuclear fuel, making nuclear weapon production possible through reprocessing.
(3) Steam lines from the thermal plant: Convert heat generated in the reactor core into high-pressure, high-temperature steam.
(4) Spent fuel receipt building: Temporarily stores nuclear fuel produced and used in nuclear activity facilities.
(5) Ventilation: An internal air system of nuclear activity facilities that can help determine activity status.
(6) Maintenance workshops: Workshops for maintenance of various equipment, machines, and systems within nuclear activity facilities.
(7) Motor pool area: Stores vehicles and machine equipment inside nuclear activity facilities and manages various transport means and equipment necessary for operation and maintenance.
Based on the program’s results and the facility’s purpose, the facility had been determined when internal access was possible—The facility was determined based on the program’s output value and the purpose of the facility, allowing access from the inside. The sampling locations and quantities based on the results of the two sampling methods are the same as in Table 2.
Internal Facility Collection Locations and Quantities Based on Program Results
If access to the facility is possible, the first facilities for soil sample collection would be the uranium recovery facility and the plutonium extraction facility. The uranium recovery facility has a high probability of uranium being released into the external environment during the uranium extraction process, and the plutonium extraction facility has a high probability of plutonium being released into the external environment due to reprocessing activities. Therefore, it is anticipated that the analytical value of collected samples will be sufficient, and it should be possible to determine the contamination status.
Next is the radioactive waste treatment facility. Given the possibility of contamination with various nuclear isotopes, it should be capable of analyzing nuclear isotopes other than uranium and plutonium. If prioritizing between these two facilities, the first priority would be the uranium recovery facility and the plutonium extraction facility, which allow for the analysis of the use of nuclear materials. Following that, the radioactive waste treatment facility, capable of analyzing nuclear isotopes other than uranium and plutonium, would be considered.
(2) If access to internal facilities is not possible
Given the current state of relations between South Korea and North Korea, as well as the closed nature of North Korean society, the probability of accessing the facilities and collecting soil samples from within is low. Therefore, external sampling locations and quantities were determined based on the program’s results, as outlined in Table 3.
External Facility Collection Locations and Quantities Based on Program Results
The considered locations outside the facilities are the steam lines from the thermal plant, roads above the facility, and green areas around the radiochemical laboratory. The uranium recovery and plutonium extraction areas are close to the road, so samples from this area would be valuable for analysis. The surrounding green area might have a lower analytical value than the other two locations, but if sampling from the two sites is impossible, analyzing samples from the green area can still help determine nuclear activity.
2) Final sample collection location decision
Considering the relationship between North Korea and South Korea, the closed-off nature, and the temporal and spatial limitations, the final sample collection site was decided for cases where access to indoor facilities is not possible. The final sampling locations and quantities are shown in Table 4 and Fig. 5. In Fig. 5, steam lines from the thermal plant are indicated with a blue circle, the vehicle route is indicated with a yellow circle, and the green area beyond the facility boundary is indicated with a green circle.
Final Selected Locations for Sample Collection Considering Data Accessibility
Final sample collection location. The blue circles in the figure represent steam lines from the thermal plant, the yellow circles represent vehicle routes, and the green circles represent the green area beyond the facility boundary.
The VSP program method applied in this paper is summarized in the Fig. 6.
Visual Sample Plan (VSP) method flowchart.
Conclusion
A legacy site can give rise to various issues such as environmental contamination and the spread of contamination, and particularly if activities like nuclear weapon development have taken place, it can pose international security and safety concerns, leading to increased risks associated with illegal trafficking of nuclear materials and similar related dangers. The operation of nuclear facilities raises international concerns, and even when these facilities are discontinued, they can transition into legacy sites, posing additional challenges in terms of management and contamination. To implement effective legacy site management, it is crucial to gather information regarding the operation of nuclear facilities and the associated contamination. A plan to collect contamination information for the management of legacy sites on the Korean Peninsula was presented. An environmental sampling method was selected to collect pollution information, and the VSP program was used to select sampling points.
Among the legacy sites within the Korean Peninsula, contamination cannot be determined in North Korea. Legacy sites in North Korea are difficult to access, and the probability of preserving records related to nuclear activities is low. This makes it challenging to manage the sites using activity records as a source of information. In anticipation of the future need to manage legacy sites within North Korea, this study established a soil sampling approach using the probability-based program VSP to detect contamination. Under the assumption of not knowing the exact contamination status of the sites, the program’s built-in sampling method was utilized to determine the location and quantity based on facility accessibility, travel routes, and the environmental behavior of uranium and plutonium. This soil sampling approach can be used not only to assess contamination levels at legacy sites in the future but also as a procedural guideline for inspecting undisclosed nuclear facilities within the Korean Peninsula.
Notes
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.
Author Contribution
Conceptualization: Park S. Methodology: Park S. Formal analysis: Park S. Funding acquisition: Kim Y. Project administration: Kim Y. Visualization: Park S, Han J, Park J. Writing - original draft: Park S. Writing - review & editing: all authors. Approval of final manuscript: all authors.
Acknowledgements
This work was supported by the Korea Foundation of Nuclear Safety (KoFONS) of South Korea (Grant No. 2020008 10004).