## 2 Materials and Methods
### Case Study: Transient Overpower Event (TOP) in a Sodium Fast Reactor (SFR)
#### 2.3.1 Case Study Background--Reactor and Accident Description  
For this case study, a Sodium Fast Reactor (SFR) will serve as a typical complex engineering system. As these type of reactors rely on fast-neutron activity, the need for other equipment is minimized, making them useful models for complicated nuclear reactions. In addition to the nuclear core, the system has a balance of plant and an auxiliary cooling process; however, for this demonstration, the focus will be on the reactor core itself. Although there are multiple components to a sodium fast reactor that provide a significant amount of system information through sensors and operational reports, this initial study will focus on a limited number of data sources, namely the primary drivers for the automatic scramming process to shut down reactor power.  
In this case study, the primary accident event described through the DBN model is a transient overpower (TOP) event. Such an event can be caused by external factors, e.g., an earthquake, that result in a sudden surge of power generation in the reactor. When such an event occurs, the reactor's automatic scram mechanism is expected to respond by inserting control rods into the reactor to greatly reduce power generation; common indicators for the automatic scram mechanism include large power-to-flow readings, as well as high values of reactor pressure, inlet, and outlet temperature (KThe accident data used in this case study is from the study by [PERSON] et al. [24]. In their report, a series of accident event scenarios were constructed using a dynamic event tree that addressed potential failure points in response to the TOP. Based on the event scenario specifications, simulation models focusing on different aspects of the nuclear reactor were used to produce different parameters necessary for monitoring overall system health. The models were run to simulate data readings throughout the reactor and balance of plant a day after a transient overpower event (86,400 simulation seconds). The scenario was considered finished when either: the cladding fraction of the core channels reached an average of 90% (representing a clad melting failure); the temperature of the cold pool had reached a significantly high temperature (representing a fuel relocation); or the reactor had survived the day without reaching those other thresholds. In that instance, it is assumed that operators would have enough time to address any problems with the system's processes.  
Three datasets captured different parameters that could be used to provide system operational data. They provided information about the reactor channels, overall reaction values, and information about the balance of plant and auxiliary systems. These datasets were created from the following two models:  
* **SAS4A/SASSYS-1:** this code provides information about the nuclear reactions occurring within the four channels of the reactor. Data provided from this part include inlet and outlet temperature and inlet and outlet flow. Additionally, the model was used to generate any current nuclear core activities, such as power generation and reaction coefficient values. These model data provide insight into the current power generated from the reactor and other information about physical nuclear reaction;
* **PRIMAR4:** this simulation code generates values for the overall piping and thermodynamics of the system, including the balance of plant and other auxiliary systems. This includes information about the temperature and pressures of compressible volumes and pools around the reactor, measurements of the pumps and the different elements of the balance of plant and cooling systems.  
From these models, an operational timeline can be comprised of the different parameters necessary for monitoring overall system health.