THE AREA of South Korea is 100,210 km2, and 24 nuclear power plants are in operation within this area. This is the highest density of nuclear power plants in the world. In addition, nuclear power plants in South Korea are located at complex sites where multiple types and numbers of nuclear power plants are located. It is expected that large-scale damage would occur in the event of a nuclear accident. Therefore, it is essential to ensure radiological safety in case of an emergency through radiation monitoring of these complex nuclear power plant sites (Kim et al. 2007).
Environmental radiation monitoring (ERM) is necessary for maintaining public radiation safety during the operation of nuclear installations or a radiation emergency (Ware and Fern 1988). ERM has been implemented based on technical and social factors, including the estimation of public radiation exposure, public nuclear reliability, and radioactivity measurements in a nuclear emergency (Hwang et al. 2011). In particular, after the Fukushima nuclear accident it has been emphasized that radiation and nonradiation factors, including natural disasters, significantly affect the living environment (Yang 2014).
Fast political decision making regarding countermeasures has become one of the important issues for protecting the public and environment. A radiation monitoring system based on a fixed monitoring station can predict and evaluate nuclear accidents (Urso et al. 2012). Rapid, real-time measurement of radioactivity and comprehensive understanding of radiation level distribution is required in areas of concern, where effective decision making is essential to the public under conditions of radionuclide release into the environment owing to the operation of nuclear power plants and unexpected radiation accidents at industrial facilities, including nondestructive tests (Jiao et al. 2008). Rapid and comprehensive understanding of radiation level distribution in regions of concern is required for exact measurement of dose rates in emergencies, despite uncertainty in measurements (Buehling et al. 1988). It is important to measure the distribution of radiation and take appropriate measures according to the radiation dose, rather than depend on accurate and precise radioactivity analysis, in the case of a radiation accident. This can protect the environment and residents from personal and material loss. Recently, the June 2015 decision to decommission the Kori nuclear power plant unit 1 led to the requirement for rapid, on-site, environmental radiation-monitoring technology for environmental restoration of the large decontamination and decommissioning (D&D) site. At nuclear power plants, rapid distribution monitoring of artificial radionuclides, including radiocesium (137Cs) and radioiodine (131I), which are fission products from nuclear reactions of neutrons, is being emphasized. Because information on the radiation dose rate can be confirmed in real time in the field without bringing samples to laboratory, it is possible to check the emission of radionuclides from a nuclear facility or decontamination site. This is a way to ensure radiation safety.
A technology for rapid, real-time measurement of radiation level distribution was developed by employing a combination of radiation technology (RT) and information technology (IT) using a radiation detector and global positioning system (GPS)-based contour-mapping programming (Kim et al. 2008). This allowed for comprehensive understanding of the radiation level distribution in a measurement area represented by two-dimensional (2D) or three-dimensional (3D) radiation contours on a corresponding map. However, the limitation of this technology was that it showed only the total radiation level, including natural and artificial radioactivity, and it could not distinguish between radionuclides. Thus, it could not provide an estimation of the radiation from each radionuclide in a nuclear reaction. Estimation of dose using external and internal exposure, which requires an understanding of released radionuclides, is essential for nuclear D&D, emergencies, and accidents, in addition to the normal operation of nuclear power plants (Faw and Shultis 1999). Hence, a portable detection system with nuclide-recognition capability is required for measuring on-site environmental radiation level distribution. Prompt analysis and display of the type of radiation produced should immediately follow measurement in an arbitrary area. Information about measured data should be shared between on-site and main offices for effective radiation-environment control and management, which includes the function of simultaneously displaying the measured radiation level distribution at both locations (Park et al. 2006).
This study aims to develop a countermeasure system for nuclear emergencies in Korea and to ensure the safety of residents at accident sites using an on-site, rapid, radiation-distribution monitoring system. Therefore, this monitoring system is focused on displaying radiation level distribution using a single mouse click immediately after measurement, and simultaneously identifying nuclides and visualizing measurements and dose estimates according to the given environmental radiation model at on-site and main offices in a nuclear accident environment. A portable, rapid, nuclide-recognition system is developed by integrating the hardware of a commercially used detector and a MATLAB-based (MathWorks, Natick, MA, U.S.) contour-mapping program for displaying radiation level on a GPS-connected digital map. It is possible to check the radiation dose distribution of a desired region in real time, along with geographic information through a portable gamma detector. With this information, it is possible to make quick decisions and evacuate residents in the accident area safely and quickly.
In Korea, it is specified that the radiation emergency site should be divided according to the radiation dose rate, and emergency response should occur within a short time. Therefore, the system proposed in this study can be used directly in the case of a radiation accident because the distribution of the radiation dose can be confirmed quickly.
COUNTERMEASURE FOR A RADIATION EMERGENCY IN KOREA
Headquarters and zoning in a radiation emergency
To manage a radiation emergency, it is necessary to define national response procedures and matters of management. In South Korea, the Nuclear Safety and Security Commission is a government agency belonging to the Prime Minister (NSSC 2015). It has responsibility for protecting nuclear facilities from internal and external threats, such as terrorism; strengthening the radiation emergency planning and preparedness system; and acting as a general countermeasure during radiation accidents or terrorist attacks (NSSC 2016). In the event of a radiation accident, the command center at the accident site is composed of a joint investigation team, a primary action team, a situation team, a field action team, and a support team. To provide technical support to the radiation emergency countermeasure headquarters, the physical protection technical support headquarters (head of the Korea Institute of Nuclear Nonproliferation and Control), the radiation protection technical support headquarters [head of the Korea Institute of Nuclear Safety (KINS)], and the radiation emergency medical service headquarters (head of the Korea Institute of Radiological and Medical Science) conduct on-site response activities with an on-site response team.
The organization for responding to radiation emergencies is not established legally; however, it is structured by applying the nuclear accident response system. In the event of a large-scale radiation accident involving radiation exposure or environmental pollution caused by emissions of radioactive materials, the central countermeasure headquarters for radiation accidents is established based on the Nuclear Safety Act.
In the event of a radiation accident, the radiation protection technical support headquarters and the radiation emergency medical service headquarters are established at the KINS and the Korea Institute of Radiological and Medical Science, respectively. The two headquarters support the radiation accident central countermeasure headquarters. The radiation accident field command headquarters, which is supported by the first action team led by the head of the fire department, operates the situation team, support team, and field technical support team responding to provide first aid treatment. The field technical support team shares primary action, surveillance, and correspondence activities with the radiation protection technical support headquarters and the radiation emergency medical service headquarters.
The roles in the organization are as follows:
- Radiation accident central countermeasure headquarters: overall national countermeasures for radiation accidents.
- Radiation accident field command headquarters: command at the radiation accident field and situation management.
- Radiation protection technical headquarters: technical support for radioactive accidents and technical dispatch support team.
- Radiation emergency medical service headquarters: medical care for radiation injuries and management of on-site dispatch medical support team.
- Situation team of each local government: emergency protection countermeasures for local residents
In the event of a radiation accident, the accident area should be controlled to rapidly evacuate personnel and prevent secondary pollution or additional victims. It is required that the police establish a control line rapidly in these zones to manage anyone involved in the accident and to manage the area. Rapid zoning is crucial to prevent additional exposure to humans and to prevent increasing the costs of recovery due to damage from the accident. The area is divided into the following three zones: hot zone, warm zone, and cold zone. Management criteria for each zone are as follows.
The countermeasure for this area is to prevent radiation damage (radiation management area). The spatial radiation dose rate in this area is more than 20 μSv h−1. KINS or the ubiquitous regional radiation emergency supporting team, which is an expert volunteer organization with knowledge of initial radiation protection measures, is involved if the spatial radiation dose rate is more than 100 μSv h−1.
This is the area between the radiation management area and the police control line that is established to restrict access to the public and vehicles, which can be accessed by emergency response personnel only. This area is outside the radiation management area. The radiation dose rate in this area is between 0.1 and 0.2 μSv h−1 (natural radiation level). This is the region between the radiation management area and the police control line, which is required to carry out emergency countermeasures.
This area is outside the police control line.
Evacuation of residents
A large amount of radioactive material may be emitted into the air in the event of a nuclear emergency. Suitable measures are required to protect residents living near the accident area when radiation levels above a certain amount are expected. The evacuation of residents should be completed before the radioactive plume reaches the residential area. Therefore, it is crucial to perform a rapid analysis and predict the various scenarios expected in the event of an actual emergency. This facilitates speedy decisions regarding issues such as the scope and route of evacuation so that harm to residents is minimized.
The assumptions and procedures for estimating evacuation time in Korea are currently based on NUREG‐0654‐FEMA-Rev. 1, Appendix 4 (hereinafter referred to as NUREG‐0654) (Kantor et al. 1996). However, factors such as the analysis of actual traffic flow including the geographical, environmental, and social characteristics around each nuclear power plant during an emergency should be taken into account. Therefore, it is essential to quickly assess the distribution of radiation based on geographical information such as traffic conditions in the accident area to ensure the radiation safety of the residents during a nuclear emergency.
RADIONUCLIDE MONITORING AND IDENTIFICATION SYSTEM
Figs. 1 and 2 depict a schematic of the radionuclide monitoring system and its various components, respectively (NeosisKorea 2013). The system consists of a portable radiation detector equipped with a GPS, a digital map, and a laptop and desktop computer on which the radiation-mapping software is installed to display the distribution of radiation levels. A portable scintillation-type gamma-radiation detector was designed and fabricated for on-site radiation measurement of various radionuclides (Cember 1969). The contour-mapping program was written using MATLAB. It is capable of displaying the radiation level distribution in 2D and 3D immediately after scanning of the target area by the portable gamma-radiation detector is completed. Simultaneous display of the radiation distribution on both the on-site laptop computer as well as the desktop computer at the main office is facilitated by data transmission using code division multiple access (CDMA).
Exposure dose assessment
The dose from radiation exposure, which includes external and internal exposure, is calculated based on certain assumptions using actual data from the detector. The external dose rate is calculated using the specific gamma constant. In cases where the radioactive source is on the surface, the distance between the radioactive source and the human body is conservatively estimated as 3 cm. The parts of the human body closest to the ground are the feet; the average thickness of the shoe soles is assumed to be 3 cm. Radiation is assumed to pass through the material of the shoes without attenuation. For a conservative evaluation, the exposure assessment was conducted assuming that the radioactive source was on the ground surface. The radiation detector used in the system provided gamma dose rates. The external dose could be checked by comparing the exposure time to the measured external dose rate value.
Radiation-distribution mapping program
Communication with the instrument using an RS‐232 serial connection was performed to obtain spectral data of the radiation source, and the spectrum was analyzed by the Mariscotti method. We identified the peak according to the nuclide and evaluated the effects of artificial radionuclides (Mariscotti 1967).
When the measured radiation dose rate is high, a nuclide analysis is conducted. The average of the measured values before the nuclide analysis is defined as the natural dose. The analysis time is 1 min or longer at a fixed point. The value obtained through spectrum analysis is defined as the value of the artificial radionuclide.
Through this procedure, data relating to radiation levels is prepared. MATLAB is used to show the distribution of radiation in the form of 2D and 3D contour maps. Two gamma-emitting radionuclides are used together to test the system. The two radionuclides have different gamma energies (60Co: 1.33 MeV, 1.17 MeV; 137Cs: 1.176 MeV) and can be measured together. The concept of the MATLAB code is shown in Fig. 3. The data are composed of geographical (longitude and latitude), radiation (natural, 137Cs, 60Co, artificial, total), and map image data. These data are imported into MATLAB and converted to MATLAB variables. Finally, functions such as plot, contour, and quadratic interpolation are used to write the radiation distribution program.
It is important to eliminate noise, which leads to distortion of contours, during radiation measurement. Fig. 4 shows an example of actual detected peak data with high and low points. In Fig. 4, detected data increase or decrease based on the highest activity value, i.e., continuity exists between each data point, leading to a consistent tendency in the contours, because the activity increases as the detector approaches the radioactive source and decreases as the detector moves away from the source. Fig. 5 depicts an example of the detection of two kinds of spurious data. Radioactivity values with a negative sign constitute the first type of spurious data; they can be simply removed. The second is high-value points without continuity, which are unlike data in Fig. 4 where only one value is abnormally and considerably higher than the surrounding values. These values represent noise. Actual data sets contain not only radiological data from radiation sources (meaningful data) but also electrical noise or statistical fluctuation (spurious data). The spurious data distort the distribution of radiation levels. Therefore, elimination of noise is essential.
In general, when the values of one or two data points are particularly high without continuity, as compared to the surrounding data points, they could represent noise. However, when a radiation accident occurs, the radiation levels in a specific region are detected to be abnormally high. Therefore, it is important to distinguish between radiological data and electrical noise or statistical fluctuation. The fundamental algorithm is as follows:
- The random point is selected and the mean of its immediately adjacent data points is calculated.
- If the value of the random point lies within a range of mean ± 1.96 × standard deviation (m ± σ), it could be considered radiological data. (A radiation counter statistically follows a Poisson distribution. If the number of samples is larger than 30, it would follow a normal distribution. A significance level of 5% is used, which corresponds to a confidence interval of 95%, that is, k = 1.96.).
- If the data do not satisfy this condition, they are classified as spurious data, such as noise or fluctuations.
The radiological data is used to plot contours, which are overlaid on a digital map of the target area (MathWorks 2005). The contour map and the digital map image of the target area are matched using the geographical information of latitude and longitude as follows:
- A digital map of the target area is obtained.
- The map image file and the text file containing geographical data and radiation data can be imported into the program.
- The text file contains the following four important values: the maximum and minimum values of latitude and longitude.
- These values are used to match the map image with the contours.
- The four points correspond to each corner of the map image, and contour lines are created based on the corresponding points.
- Finally, the radiation-distribution contour-mapping program is configured in an executable file format so that it can be operated in the computer environment without MATLAB.
Simultaneous display of the radiation distribution
Simultaneous display of the radiation distribution on the on-site laptop and the desktop at the control office was carried out using CDMA. This is a type of mobile-communication technology used throughout the country; it offers high-quality connectivity and few data losses. The concept of the simultaneous display system is as follows:
- The detector and the laptop, which are located at the measurement site, communicate with each other via Bluetooth (Bluetooth SIG, Inc., Kirkland, WA, U.S.). If the detector and the laptop are connected, the current location and the radiation dose rate are displayed on the laptop.
- The laptop at the site and the desktop in the control office communicate with each other using CDMA. Data such as radiation dose rate and information on latitude and longitude, which are acquired at the site, are transmitted to the desktop as soon as the detector measures radiation so that the laptop and desktop can display the distribution of radiation levels. The CDMA connection algorithm consists of two steps. First a CDMA device connected to the laptop at the measurement site, which requires the internet protocol (IP) address of the desktop [server personal computer (PC)], sends a short message service (SMS) message to another CDMA device connected to the desktop in the control office. Next the server PC returns an SMS message that includes its IP address, and then the CDMA device at the site accesses the server PC.
- Finally, the contour map of the radiation level distribution is superimposed on the digital map of the target area using a MATLAB-based contour-mapping program on the on-site and control office computer screens.
RESULTS AND DISCUSSION
Quick contour mapping for radiation distribution
The contour maps of the radiation distribution with geographic information are generated in one operation. Fig. 6 is an example of the monitoring system’s output display. Real-time detection was carried out for the site surrounding the main facility using a vehicle monitoring point with a speed of about 30 km h−1. The laptop in the field, which has a 3.40 GHz Intel core i5‐3570 (Intel Corp., Santa Clara, CA, U.S.) central processing unit (CPU) and 8 GB of memory, was used to check the operating time of the radiation-distribution monitoring system code. The operating time to generate a radiation distribution contour immediately after measurement was about 2 s for each data set. Detailed results are described in Table 1. It was confirmed that the monitoring system can be used to obtain a rapid understanding of the circumstances involved in an emergency situation.
The desktop in headquarters, which has a 2.00 GHz Intel core i7‐3537U CPU and 8 GB of memory, was used to calculate the operating time of the radiation-distribution monitoring system code per the number of data sets. The “Profiler” function in MATLAB was used. Fig. 7 shows the results of the radiation-distribution monitoring system code. The time taken to generate the radiation distribution contours immediately after measurement was about 11 s for 10,000 data points, as seen in Table 2. The existing technique for radiation distribution monitoring requires more than 1 h for the same results. This means that the developed monitoring system can be used in radiation emergency scenarios that require quick measurement or location of hotspots on large sites. It is expected that this system, which can quickly monitor the radiation dose distribution over a large area, can be used directly in the event of radiation accidents. In order to evacuate residents in the event of radiation accidents, it is essential to identify a safe evacuation route by mapping the radiation distribution and geographic information, such as roads and traffic.
The system operation was tested at the Ulsan National Institute of Science Technology (UNIST) through a CDMA connection between a laptop at the site and a desktop in the control office. A man walked around specific target areas with a portable detector to obtain data. The screens at the site and in the control office displayed the radiation distribution level simultaneously, as seen in Figs. 8 and 9, respectively. The screens in both locations showed the same map image and dose-rate value, thus confirming the function of the simultaneous display of radiation distribution measurements. Through a vehicle or drone, it is possible to remotely check the radiation distribution of the desired area, together with the geographical information. Based on the information, the residents of the radiation accident area can be safely evacuated through a low-level radiation area.
Quick measures to ensure public safety were demonstrated to be possible by the development of a system that can rapidly visualize environmental radiation level distribution in the event of a nuclear emergency. An on-site, rapid, radiation-distribution monitoring system based on real-time measurements for appropriate decision making was established by combining detector-based hardware and MATLAB software. The 2D and 3D radiation level distributions were simultaneously displayed on the field laptop and office desktop screens a few seconds after the target area was scanned. In conclusion, we expect the developed on-site, rapid, environmental radiation-distribution monitoring system, which visualizes the geographical information-based radiation distribution, to be directly applicable for performing quick measurements during a nuclear emergency to ensure radiation safety.
This work was supported by a Nuclear Safety and Security Commission grant funded by the Korean government (Ministry of Science, ICT, and Future Planning) under contract number 12030250314SB110 and NRF‐22A20153413555.
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Keywords:© 2018 by the Health Physics Society
accidents, nuclear; emergency planning; monitors, radiation; radiation protection