## 3 Results
### Pre-Calibrations  
As a matter of fact, the calibration of the FWMU is not sufficient for the correct reproduction of the LOFA transients because the complete model consists of more pipes than only the FWMU ones. To overcome this burden, two sets of pre-calibration were performed to calibrate the pressure drops, the surface roughness, and the parameters involved in the heat transfer:  
* the first one considering the thermal-hydraulic conditions characterizing the steady-state at both 300 and 330 kW/m\({}^{2}\), without applying any heat load;  
\begin{table}
\begin{tabular}{c|c c|c c|c c|c} \hline \multirow{2}{*}{**Ch.**} & \multicolumn{4}{c|}{**Horizontal Inlet**} & \multicolumn{4}{c|}{**Vertical Inlet**} & \multicolumn{1}{c|}{} \\ \cline{2-7}  & \(K_{v}\) & \(K_{h}\) & \(K_{b}\) & \(K_{v}\) & \(K_{h}\) & \(K_{b}\) & \(\varepsilon(\mu\)**m)** \\ \hline
1 & \(1.45\pm 0.24\) & \(0.71\pm 0.16\) & \(1.60  
the second one considering the thermal-hydraulic conditions characterizing the steady-state at both 300 and 330 kW/m\({}^{2}\), with the application of the heat load hitting the portion of the top plate.  
The first set of calibration addressed the parameters related to the pressure drops (pressure drop coefficients and surface roughness), whereas the second one was mainly devoted to obtaining the best-estimated values for the parameters relevant for the heat exchange between coolant and heat structures, and between the heat structures themselves. Since different aspects are involved in the thermal behavior of the system, the following parameters were selected for the second set of calibrations:  
Figure 5: Results of the pressure drop characterization tests: Channels 3 and 4 (horizontal inlet), and Channels 7 and 8 (vertical inlet).  
* Material properties: conductivity, specific heat, and density of the P92 steel;
* Conduction-related parameters: thickness of the heat structures under heat load, exchange areas among HSs, and characteristic length of all the HSs composing the top plate;
* Convection-related parameters: sensitivity coefficients linked to the Dittus-Boelter formulation embedded.  
As responses, the outlet pressures of Channels 2-4 and 7-9 were selected for the calibrations without heat load (the pressure in the other channels was not taken into consideration since no experimental data are available), while the outlet temperatures of Channels 2-4 and 8-9 were adopted for the calibrations with heat load.  
A marked improvement was found in the pressure values obtained by MELCOR 1.8.6 after the calibrations with the best-estimate pressures within the uncertainties range of the experimental results for almost all the considered channels. As for the temperature, the results are acceptable considering the nature of the MELCOR code. However, temperatures calculated by MELCOR are lower than the experimental data for all the channels under evaluation (max \(\Delta\)T = 2-5 K), showing that the code is not able to reproduce the heating coming from the friction. Table 2 shows the comparison between the experimental data and the results achieved from best-estimate code run at the steady-states at 330 kW/m\({}^{2}\) (with and without heat load). In total, 103 parameters (more than 1500 terms modified in the input decks) have been calibrated in these pre-calibrations.