1. Introduction
Korea has undergone continuous growth in the chemical processing industry, corresponding with its ongoing economic development. Consequently, the quantity of chemical substances being handled is steadily increasing, which also heightens the risk of chemical accidents. According to the "5th Statistical Survey on Chemical Substances" published by the Ministry of Environment[1], the distribution volume of chemical substances surged by 162.80%, increasing from 432.5 million tons in 2010 to 704.1 million tons in 2022, accompanied by an increase in chemical accidents. According to statistical data from the National Institute of Chemical Safety’s Chemicals Information System[2], the number of chemical accidents increased by 1.95 times, from 58 cases in 2019 to 113 cases in 2024. Specifically, among the reported incidents were 77 fires, 69 explosions, 750 leakages, and 77 classified as other types. Leakage accidents accounted for 77.08% of all chemical accidents, representing the largest proportion. These statistics show that the majority of chemical accidents are directly linked to the leakage of flammable or toxic gases, emphasizing the urgent need for effective countermeasures. Additionally, changes in international circumstances further heighten the risk of chemical accidents and terrorism attacks. Since the outbreak of the Russo-Ukraine War in 2022, the risk of terrorist attacks on critical infrastructure has increased, as observed in the Nord Stream gas pipeline explosion, which is suspected to be an act of strategic sabotage[3]. Given that South Korea remains in an armistice with North Korea, a potential risk exists of North Korea conducting chemical terrorism against key facilities in South Korea. The Ministry of National Defense regards these concerns as genuine threats[4,5]. Considering North Korea's strategy of utilizing asymmetric forces, there is a potential risk of mass casualties from chemical warfare, necessitating urgent countermeasures. Currently, Korea addresses chemical accidents and chemical terrorism through the National Fire Agency's standard operating procedure (SOP) and field standard operating guidelines (SOG), which are categorized by specific incidents, including those involving nuclear power plants. These guidelines outline fundamental principles and procedures for accident response; however, they lack detailed strategies for various accident types and specific rescue methods appropriate for the physical characteristics of rescue subjects [6]. The National Fire Agency has established a training testbed for field personnel at the new site of the National Fire Research Institute of Korea to enhance the efficient management of chemical terrorism incidents disguised as chemical accidents, with its operation currently being prepared[7]. Therefore, responses should be adaptable to field conditions, as the types and properties of gases involved in chemical accidents or terrorism can vary. Additionally, further research is required to develop effective solutions for managing chemical incidents and attacks.
To investigate gas flow in a confined space and the inhaled gas volume according to respiratory capacity by body weight during chemical accidents or terrorism, this study conducted a gas discharge experiment in an enclosed space. The aim was to analyze gas flow characteristics in such incidents and measure the inhaled gas volume based on the physical conditions (age, gender, body weight, and height) of rescue subjects. Considering the risks associated with toxic gases, the experiment used CO2 as a substitute owing to its higher specific gravity than air. Rescue dummies representing various physical conditions were positioned to collect and analyze the data. In a previous study, experiments involving the discharge of toxic gases were restricted because of various risks. However, this study aimed to use CO2 gas to simulate different chemical accidents and terrorist incidents that may occur indoors[8]. Consequently, this study sought to develop more effective rescue strategies by considering the varying characteristics of rescue subjects and contribute to enhancing the chemical accident/terrorism response system.
2. Experimental Setup and Method
2.1 CO2 gas discharge experiment
The goal of this experiment was to analyze the diffusion and accumulation properties of CO2 gas and to track its movement in environments where rescue subjects may be exposed to it. Figure 1 displays the experimental equipment used for CO2 discharge experiment conducted in a confined space. Figure 1(a) shows a CO2 meter (MIC 98132) employing a non-dispersive infrared (NDIR) mechanism for measuring CO2 concentration. It was used to measure the threshold limit value-time weighted average (TLV-TWA) of 5,000 ppm set for this experiment. Figure 1(b) depicts a regulator (KX-5B) that was used to maintain a constant pressure and flow rate of CO2 gas discharge. This device has a flow meter adjustable up to 25 L/min and a pressure gauge that provides discharge pressure data, enabling the maintenance of a consistent flow rate (5, 15, and 25 L/min) necessary for the experiment. Figure 1(c) shows the CO2 monitoring program, which processed data collected from the measuring instruments. The collected measurement data from the start to the end of the experiment were processed and converted into graphs for visual representation, facilitating the analysis of the experimental results.
Figure 2(a) presents the chamber modeling that depicts the arrangement of CO2 sensors for the gas diffusion experiment. The experiment was conducted inside a confined chamber with dimensions of 191.5 cm in width, 291.5 cm in height, and 230.0 cm in depth. A total of five CO2 sensors were installed inside the chamber to measure gas diffusion and accumulation over time. CO2 sensors were positioned at varying distances and heights from the wall to capture CO2 concentration data from multiple locations. The CO2 gas discharge experiment was designed to examine the diffusion rate of gas in a confined space and measure CO2 concentration at locations where rescue subjects are likely to be present.
Figure 2(b) displays the arrangement of five CO2 measuring instruments inside the chamber. Each CO2 sensor was strategically positioned to efficiently capture data on gas diffusion and accumulation. Horizontally, sensors no. 1 and no. 5 were placed 109.75 cm from the chamber's center, whereas sensors no. 2 and 4 were positioned 77.75 cm away from the center. Sensor 3 was positioned directly above the outlet to facilitate real-time monitoring of CO2 level fluctuations upon the initial release of the gas. Vertically, sensor no. 1 was positioned 53.75 cm from the wall, whereas sensors no. 2, 3, 4, and 5 were positioned 18.25 cm from the wall.
Figure 2(c) depicts the three distinct positions of measuring instruments corresponding to adult breathing levels. The upper position was set at 172.5 cm, representing the average height of Korean men, whereas the middle and lower positions were set at 115 cm and 57.5 cm, respectively, enabling the measurement of the vertical diffusion speed of the gas.
The experiment started with the initialization of the chamber's internal environment. Before the experiment, the chamber's internal air was ventilated to maintain indoor CO2 levels below 1,000 ppm, the national allowable standard. After the door was sealed, a two-minute waiting period was observed to stabilize gas movement. To comparatively analyze gas diffusion speed based on flow rate, the CO2 discharge experiment utilized a regulator to maintain a pressure of 5 MPa and applied three different flow rates: 5, 15, and 25 L/min. After gas discharge commenced, it continued until the CO2 concentration reached the instrument’s maximum limit of 10,000 ppm, with real-time data being recorded throughout the process. For data analysis, the collected data from CO2 measuring instruments were used to examine the diffusion and accumulation characteristics of the gas. The diffusion and accumulation patterns of gas in a confined space were examined by comparing the changes in CO2 concentration and accumulation speed at each instrument position. After the experiment concluded, the chamber was properly ventilated to reduce the CO2 level below 1,000 ppm. The experiment was repeated three times in the same manner, and the measurement values were averaged to ensure the reliability of the results.
2.2 CO2 inhalation experiment using rescue dummy
This experiment was designed to analyze the variation in the volume of gas inhaled based on the rescue subject's height, weight, and respiratory capacity when exposed to gas in a confined space. Figure 3 shows the details of the custom-designed rescue dummy used to analyze the amount of CO2 inhaled in relation to the physical characteristics of the rescue subject. The inflow and flow rate of gas are controlled using an air blower fan, whereas a gas sensor measured the concentration of CO2 inhaled by the body.
The minute volume varies significantly based on age and physical condition, making it difficult to obtain precise data. Therefore, the following equations were used to calculate these values[9-11]. Eq. (1) calculates the body surface area (BSA) of a subject based on their height and weight, using the Dubois equation. Eqs. (2) and (3) were used to calculate the respiratory capacity per minute (minute volume, VE) of male and female subjects.
Table 1 presents the minute volume calculated based on weight and height of rescue dummies.
Figure 4(a) shows an image of the rescue dummies used in the experiment, which consisted of one male (child), one female (youth), one female (adult), and two male (adult) dummies, as detailed in Table 1. For verifying the accuracy of the experiment measurement data, a gas analyzer (Wohler A550, USA) shown in Figure 4(b) was additionally installed.
As with the previous CO2 discharge experiment, the chamber's internal environment was initially prepared and then sealed before the experiment began. When the chamber's CO2 concentration was reduced to below 1,000 ppm, the rescue dummies and gas sensors were placed at designated positions to start the measurement. A two-minute waiting period was observed to stabilize the gas movement during the establishment of the experimental environment, and a total of 47 L of CO2 gas was discharged. Subsequently, the CO2 concentration inhaled, based on the respiratory capacity of the rescue dummies, was measured, and the results were analyzed through comparison.
3. Experimental Results
3.1 Results of CO2 gas discharge experiment
Figure 5 shows the experimental results of CO2 gas discharge. Zone 3 was excluded from the results because the CO2 discharge outlet and gas sensor were installed vertically, causing the experimental specimen to directly interact with the gas sensor as it was discharged, unlike in other zones. In the experiment conducted at 5 L/min, the CO2 concentration at the upper, middle, and lower positions increased in a similar manner. Specifically, the lower sensor reached 10,000 ppm the quickest, in about 950 s, whereas the upper sensor took the longest, reaching it in 1,850 s. Thus, CO2 accumulated more rapidly in the lower section of the chamber. In the experiment conducted at 15 L/min, the lower sensor reached 10,000 ppm the quickest, in about 450 s. The upper sensor took approximately 900 s, with CO2 accumulation occurring more quickly than at the 5 L/min flow rate. In the experiment conducted at 25 L/min, the lower sensor reached 10,000 ppm the quickest, in about 350 s, whereas the upper sensor took approximately 500 s. In this experiment, the CO2 accumulation speed was higher than the 5 and 15 L/min flow rate experiments, showing that a higher flow rate resulted in quicker CO2 accumulation, particularly in the lower section of the chamber. Additionally, the CO2 concentration increased more quickly from the upper to the lower sections of the chamber. A comprehensive analysis of the graphs revealed that as the flow rate increased, the slope of the graph became steeper, and the time required to reach the target concentration level decreased.
3.2 Results of CO2 inhalation experiment using rescue dummy
CO2 gas was discharged into a sealed chamber, with the inhalation points of all rescue dummies set to the same position within the chamber. For the 10 kg rescue dummies representing a child or youth, their inhalation points were set lower than those of the other dummies to consider the height difference from adults.
Figure 6(a) presents the graphs depicting the CO2 inhalation volume of the rescue dummies. For all rescue dummies with weights of 90, 70, 50, 30, and 10 kg, the oxygen concentration gradually decreased as the CO2 concentration increased. The 10 kg rescue dummy had the lowest respiratory capacity, but the child dummy, with its inhalation point positioned lower than the others, recorded the highest CO2 concentration. This result corresponded with the findings from the previous CO2 gas discharge experiment, where the gas tended to accumulate from the bottom to the top of the chamber. Except for the child dummy, the CO2 concentration was highest in the 90 kg dummy, followed by the 70, 50, and 30 kg dummies, corresponding to their respective respiratory capacities. The gas sensors installed under identical conditions yielded comparable results for oxygen concentration, as well as the lowest and highest CO2 concentrations, as depicted in Figure 6(b). These results highlight the variation in CO2 inhalation volume based on the inhalation point and weight of the rescue dummies.
3.3 Discussion
The CO2 concentration values used for comparing the experimental data on gas discharge are 10,000 ppm, the upper limit of the measuring instrument, and the time required to reach the threshold limit value-time weighted average (TLV-TWA) of 5,000 ppm, as specified by the administrative regulations of the Ministry of Employment and Labor. CO2 concentration was measured to numerically represent the differences in graphs, and the times required to reach 5,000 and 10,000 ppm are given in Tables 2 and 3, respectively. The measurement times at the lower section, where sensors in zones 1, 2, 3, 4, and 5 exhibited the fastest response (used as a baseline multiple of 1), were compared for different flow rates of 5, 15, and 25 L/min and expressed in relative multiples. Excluding zone 3 from the analysis, the time required to reach 5,000 ppm, the CO2 TWA standard, and 10,000 ppm was compared. The results indicated that sensors in the lower section responded clearly faster than those in the upper section. The sensors in the lower section reached 5,000 ppm approximately 2.24 times faster and 10,000 ppm about 2.02 times faster compared with those in the upper section. Additionally, across all flow rate conditions, the sensors in the lower section reached the target concentration more quickly than those in the upper section. These results imply that CO2 is accumulated more toward the bottom as it is heavier than air and is reacted faster in the lower section even at faster flow rates. Thus, response time varied with different flow rates, with sensors in the lower section reacting more quickly.
Table 4 displays the average speed of each zone, determined based on the response time corresponding to the flow rates in the upper, middle, and lower sections. The experiment results demonstrated variations in speed corresponding to different flow rates, offering valuable insights into CO2 accumulation, response time, and evacuation speed. At the highest flow rate of 25 L/min used in this experiment, the gas reached the chamber's maximum height of 2.3 m in 122.99 s. Conversely, the flow rates of 15 and 5 L/min took 283.95 and 575 s, respectively.
In the inhalation experiment using rescue dummies modeled after actual rescue subjects, the highest CO2 concentration was recorded in the 10 kg child dummy, which had the lowest inhalation point, owing to the difference in specific gravity compared with air, regardless of respiratory capacity. For the 30, 50, 70, and 90 kg rescue dummies with the same inhalation point, the CO2 concentration level was highest in descending order of 90, 70, 50, and 30 kg dummies, corresponding to their higher respiratory capacities. Additionally, gas sensors installed under the same conditions as the rescue dummies exhibited similar patterns in oxygen concentration and the highest and lowest CO2 concentration levels, confirming the reliability of the rescue dummy data. These findings confirmed that the characteristics of rescue subjects can differ based on their inhalation position in the event of gas leakage accidents or chemical terrorism.
4. Conclusion
This study analyzed the behavior of gas by releasing CO2 as a substitute for toxic substances, simulating scenarios of chemical accidents and chemical terrorism in a confined space. In addition, the vertical accumulation speed of gas was analyzed, and the inhalation volume was measured using rescue dummies with varying respiratory capacities based on gender and physical conditions. This was performed to assess the state of rescue subjects in relation to their respiratory capacities during chemical accidents and chemical terrorism. The following conclusions were drawn.
1. In the experiment using CO2, which is heavier than air, the response time of the CO2 measuring instrument was quickest at the lower section of the chamber, excluding the direct CO2 gas discharge point. This result indicates that heavier toxic gases begin to accumulate at the bottom and flow upward in areas away from the gas discharge point, making the lower sections relatively more hazardous when gas is discharged from higher positions.
2. When the experiment was repeated under the same conditions but with different flow rates, the properties of CO2 remained unchanged. When the times were compared based on the accumulated CO2 levels in the measuring instruments, higher positions exhibited a faster accumulation at increased flow rates, whereas the vertical accumulation rate from the lower to the middle section was greater than that from the middle to the upper section. Additionally, the time for CO2 to reach the top of the chamber was determined using the gas's vertical accumulation speed data at the lower, middle, and upper sections.
3. In a simulated scenario of chemical accidents and chemical terrorism, CO2 gas was discharged into a chamber, and the inhalation volume was compared among rescue dummies with varying respiratory capacities based on gender and physical characteristics. The results indicated that, in addition to the 10 kg rescue dummy, the 90 kg dummy with the highest respiratory capacity exhibited the greatest inhalation, whereas the 30 kg dummy had the lowest inhalation volume. This suggests that greater respiratory capacity results in higher inhalation volume; males generally have a higher inhalation volume than females, and individuals with greater body weight tend to inhale more. Notably, the 10 kg rescue dummy had the lowest respiratory capacity owing to its lower inhalation point compared with other dummies, yet it exhibited the highest CO2 concentration level. This indicates that individuals positioned at lower levels may experience a high inhalation volume despite having a smaller respiratory capacity.
This study examined gas behavior in a confined space and identified inhalation characteristics based on respiratory capacity using rescue dummies, along with data on the vertical accumulation speed of gas. Applying the findings of this study to a response training system for chemical accidents and chemical terrorism can enhance rescue expertise by enabling relevant personnel to make faster and more accurate decisions in real-world scenarios. Additional experiments should be conducted on toxic gases such as hydrogen cyanide and phosphine, which have a lower specific gravity than air, along with alternative specimens and diverse environmental conditions. A follow-up study incorporating the findings of this research and additional experimental conditions could expand the applicability of rescue dummies and contribute to developing more advanced strategies for effectively responding to the increasing occurrence of chemical accidents and chemical terrorism.
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