# Team-Communication Modeling Framework for Multi-Unit Nuclear Power Plant Accident Analysis
## 1 Introduction  
In the nuclear industry, an accident is called a severe accident when the damage degrades the nuclear core [1], which is in the reactor building. The reactor building is also called a reactor unit, and one nuclear power plant site can contain multiple reactor units. When severe accidents happen simultaneously in more than one unit, it is called a multi-unit accident. The risk of a multi-unit accident was mentioned as early as the 1980s; however, it has not received enough attention because of its extremely low occurrence probability [2, 3]. Following each occurrence of a severe accident (the Three Mile Island accident and the Chernobyl accident), more safety measures are taken. After the Chernobyl accident, severe accidents in large nuclear power plants (NPPs) did not occur for almost 25 years, i.e., theFukushima Daichi accident in 2011. However, the Fukushima accident was not only severe; it was also the first multi-unit accident, where four of the six units encountered a nuclear core meltdown [3]. Multi-unit accidents carry higher consequences in health and economy compared to a single-unit accident, as well as more difficult accident mitigation [4]. The massive consequences of the Fukushima accident caused safety measures to prevent a multi-unit accident to be eagerly revisited since more than 70% of nuclear power plant sites contain more than one unit [5]. Likewise happened in South Korea, where this research was conducted, and which has the highest density of NPPs--up to 8 units per site [2].  
To include the concern of multi-unit accidents in nuclear safety analysis, the International Atomic Energy Agency has developed a multi-unit probabilistic safety assessment (PSA) methodology by building on the single-unit PSA [6]. It has provided the technical assumptions, started with the single-unit PSA, proceeded to the initiating event analysis (the triggers of severe accidents) and the multi-unit analysis, such as the multi-unit accident sequence model, multi-unit system analysis model, etc., and ended with the results integration/interpretation of the quantification of multi-unit core-damage sequences. Shifting the focus away from the quantification of multi-unit risk, the multi-unit risk assessment aimed to provide insights into reducing risk through accident-management programs [3]. It identified many technical issues and challenges of risk assessment of multi-unit accidents. Other methods were also proposed [2; 5; 7]. Despite the various technical solutions proposed to include and reduce the risk of multi-unit accidents, a study analyzing the past three severe accidents found that their root causes lay in the non-technical aspects, namely the poor safety management and poor safety culture [8].  
The importance of non-technical aspects has been acknowledged, especially in organizations dealing with high-risk emergencies, such as NPP emergency-response teams (ERTs) [9; 10]. In the case of multi-unit accidents, the most critical factors were the reactor unit locations and the shared safety features between units, and the most common shared feature between reactor units was the organizational aspects, which comprised 44% of the inter-unit dependencies [3]. A significant element of the organizational aspect is operators' non-technical skills (NTSs). Among the non-technical skills, the importance of communication has been emphasized in the reports of major NPP accidents since the 1980s, and the Fukushima accident study also concluded that the reduced communication capability seriously hindered the accident responses, which amplified the accident consequences [11]. Thus, in this study, we focused on team communication, as it is also the first principle of effective emergency response.