## 3 Structure and Salient Characteristics of the BeSYC Software and Its GUI  
BeSYC is developed as a MATLAB App, i.e., an interactive application written to perform technical computing tasks also capable to run as a standalone application or a web app. The BeSYC software can be used in conjunction with both the RELAP5-3D, the MELCOR 1.8.6, and the MELCOR 2.2 system codes [11; 12; 13]. The data required to run BeSYC are contained in an _input file_, but two additional files are always required: an input deck of the system code in use to reproduce the investigated experimental setup and a _csv_ file containing the experimental data. BeSYC follows a quite linear structure (Figure 1) subdivided into eight phases:  
1. A preliminary check of the license to see if the computer is allowed to run the BeSYC software;
2. A rearrangement phase to process the input data stored in the input file and to assess the code input deck and the experimental csv file. Error messages are thrown if errors are found in the data provided. 3. A phase in which the required _sensitivity input decks_ are created. An output file is also created during this phase to list the investigated parameters and the values used in the _sensitivity_ code runs. 4. An optional fourth phase in which the _sensitivity_ code runs are executed. The code runs are performed through the use of bash/batch files depending on the adopted platform. BeSYC starts a new calculation when a previous one is over following the maximum number of parallel code runs (RELAP5-3D, MELCOR 1.8.6, and MELCOR 2.2 codes are _single thread applications_) defined by the user in the input file. Before starting the execution of the code runs, a check is also performed to ensure that number of parallel runs is smaller than the number of CPU threads to avoid system unresponsiveness. The status of the different code runs is not performed through MATLAB commands but through the _pgrep_ (Linux) and the _tasklist_ (Windows(r) system commands. This phase is also optional because the eventual modification of the BEPU methodology input data do not usually require the re-execution of such code runs. 5. A fifth optional phase devoted to the application of BEPU methodology and the creation of the output files summarizing the main findings. In this phase, the Pearson's, Spearman's and Kendall's coefficient are also calculated. If the BEPU methodology is not applied, the results of a previous run are loaded (if available) to safely prosecute through the next phases. A sixth phase in charge of writing a _best-estimate input deck_ (if requested). * An optional seventh phase to run the _best-estimate input deck_ (if requested). * A final eighth phase to plot the comparison between the responses and their standard deviation in the experimental file, the default run, the BEPU methodology, and the BE run (if available). The data required to run the BeSYC app must follow a quite strict organization, so a GUI was created to ease this process (Figure 2). The GUI was developed with _App Designer_[14], and it works seamlessly with the scripts composing the BeSYC software. The four buttons at the top of the GUI can be used to load and save BeSYC input files as well as to run the BeSYC software. Below these buttons, the interface is divided into three main  
Figure 1: Flowchart of the BeSYC software. panels: the _main panel_ (Figure 2a) in which most of the options are set, the _responses panel_ (Figure 2b) dealing with the aspects concerning the responses, and the _parameters panel_ (Figure 2c) hosting the data related to the parameters.