## 1 Introduction and Backgrounds
### Functional Resonance Analysis Method (FRAM)  
Applied as a system modeling method, FRAM serves as a means to achieve a thorough understanding of a system of interest/work domain, comparable to general-purpose system modeling methods [21]. FRAM's properties allow the system analysts to capture the relations between functions without the limitations from the physical elements' boundaries and work sequences, enabling performance variability analysis that is restricted in other modeling approaches. FRAM accommodates modeling in a dynamic manner, which is needed to model and analyze complex socio-technical systems covering numerous interactions between humans, technology, and organizations, such as the severe accident management in NPP. Since its first publication in 2004, FRAM has become well-known with more than 200 publications worldwide in 2020 [13].  
FRAM is developed based on system theory that focuses on functions and their relations in a complex socio-technical system/work domain [12; 22]. The most notable characteristic of FRAM is that the relations between functions are not restricted by either the physical elements' boundaries or the sequences of works/processes [23]. This characteristic leads to FRAM's key analysis, that is, the ability to identify the performance variability inherent to certain functions and, through the connections with other functions, can unfold into daily normal (successful) operations or unexpected failures/incidents. The performance variability from a function can spread to other connected functions, where the connection determines either to maintain, amplify, or dampen the variability, hence the'resonance' of the function [24]. FRAM is especially useful to analyze unanticipated critical events, such as accidents, in complex socio-technical systems [21].  
The FRAM model, i.e., the modeling result using FRAM, consists of functions, the functions aspects that connect functions, and the contents of the aspects; together, they compose and define the work domain's activities and behavior attributes [24]. The FRAM function has six aspects: Input, Output, Precondition, Resource, Time, and Control. The Input starts the function or the function processes the Input to create the Output. When Precondition exists, it must be satisfied before the function can be carried out. The function needs or consumes Resource while it is being carried out. The temporal constraints are included in the Time aspect (starting time, finishing time, or duration) and the monitoring or controlling terms are included in the Control aspect. The allowed connection is between the Output aspects and the five other aspects: Input, Time, Control, Precondition, Resource [24]. The FRAM function is drawn as a hexagon and the six aspects are placed in each of the outer corners, as shown in Figure 1a. The connected group of functions forms a model, as shown in Figure 1b.  
Figure 1: FRAM model. (**a**) The function representation in the FRAM model with the 6 aspects: Input (I), Output (O), Precondition (P), Resource (R), Time (T), and Control (C). (**b**) An example of the FRAM model from [25], reprinted with the authors’ permission.