While the probability of a severe accident at a nuclear power plant resulting in the leakage of radioactive materials is exceedingly low, the potential health hazards to residents and the economic consequences of such an accident are substantial. The Fukushima nuclear disaster in 2011, triggered by an unexpected natural disaster, led to the evacuation of more than 70,000 local residents from the vicinity. Research is underway to remediate the land contaminated with radionuclides [1], and decontamination efforts are in progress in the contaminated areas. However, the recovery has not yet been completed [2], and significant costs are being incurred. Despite various challenges encountered during the evacuation process, the early evacuation efforts ensured that there were no reported cases of radiation exposure among the evacuees [3]. As shown by the Fukushima nuclear accident, early but well-organized evacuation in case of a radiation emergency is an important measure to protect the safety of residents.
In South Korea, radiological emergencies are classified into three levels based on their severity and the anticipated degree of damage: a Facility Emergency (as white alert) where impacts are confined within the building, a Site Area Emergency (as blue alert) where impacts are confined to the facility site, and a General Emergency (red alert) where impacts extend beyond the site.
In the event of a red alert, residents within a 5 km radius of the nuclear power plant, located in the Precautionary Action Zone (PAZ), must evacuate immediately to shelters. Additionally, those residing within a 30 km radius, in the Urgent Protective Action Planning Zone (UPZ), are directed to either take shelter indoors or evacuate to shelters, depending on the circumstances. Typically, evacuations are initiated prior to any external leakage of radioactive materials. Nonetheless, the outcome of evacuations in actual accident scenarios may not always be positive. To minimize the risk of radiation exposure for evacuees during the evacuation process, swift evacuation is necessary. The reduction in evacuation time decreases the duration of exposure to radiation, allowing evacuees to arrive at shelters more quickly and receive necessary assistance.
Staged evacuation has been proposed as a method to shorten evacuation times. Unlike simultaneous evacuation, where all residents in the impacted area evacuate concurrently, staged evacuation involves the sequential evacuation of residents from different zones according to a predetermined order. Many researchers have conducted studies on staged evacuation. Chen and Zhan [4] examined staged evacuation strategies in three different road network structures with varying population densities. Yang [5] assessed staged evacuation strategies through simulation, combining pedestrian choice models with agent-based pedestrian behavior simulation models. Park and Jae [6] identified the optimal staged evacuation strategy by segmenting evacuation zones based on level 3 probabilistic safety assessment (PSA) outcomes and employing a mathematical approach to compute evacuation and delay times. Zhao [7] introduced a comprehensive risk assessment method to quantify potential risks in nuclear emergency evacuations and proposed partitioned evacuation organizational strategies and cell transmission models to minimize exposure risks based on the assessment outcomes.
Some studies on staged evacuation have conducted evacuation simulations on theoretical road layouts, such as radial or grid patterns. However, urban environments in reality are comprised of a complex network of roads that vary in form and connectivity. It cannot be assumed that results obtained from simulations on theoretical roads would directly translate to actual road networks [4], highlighting the necessity to model real-world road networks in evacuation simulations. Most research in this area has presumed a uniform distribution of populations or has delineated evacuation groups by dividing the evacuation area with hypothetical boundaries, such as a specified kilometer radius from a nuclear facility, to facilitate staged evacuations. However, in reality, population distribution is not uniform, and the division of evacuation areas cannot be neatly segmented in practical scenarios. The division of evacuation areas in reality cannot be neatly accomplished, and issuing evacuation orders based on distance may lead to residents being unsure if they are subject to evacuation, potentially resulting in shadow evacuations [8, 9]. Such factors, not considered in previous research, could significantly change the outcomes when applied in real-world scenarios. To overcome these limitations, this study incorporates more realistic elements. It utilizes GIS data of an arbitrary city to model the city's road network and buildings, and sets the initial locations of evacuees at buildings to create a more realistic population distribution. Additionally, it divides evacuation areas based on administrative district GIS data of the arbitrary city. The results of the evacuation simulations are analyzed to examine the impact of staged evacuation on evacuation times and speeds, thereby evaluating the effectiveness of each staged evacuation strategy.