## 5 Post-Test Numerical Analysis
### Results  
As for the pre-test analysis, the FWMU model (only the nozzles and the 180\({}^{\circ}\) bend channels) developed for this validation study was initially calibrated against the pressure drop characterization tests discussed in [11]. Then, the full model (Figure 20) was calibrated against the steady-state conditions at 300 kW/m\({}^{2}\) and 330 kW/m\({}^{2}\). The model obtained through these preliminary calibration (called \"Default run\" in the following) was adopted as a starting point for the analysis of the experimental campaign. The six valve opening scenarios characterizing the experimental campaign can be categorized as:  
* Scenarios in which the surface under heat load reaches 550 \({}^{\circ}\)C and backflows are established in some channels (0% LOFA scenarios);
* Scenarios in which the surface under heat load reaches 550 \({}^{\circ}\)C but no backflows are established (5%, 10%, and 20% LOFA scenarios); \begin{table}
\begin{tabular}{l c} \hline \hline
**Parameter** & **Uncertainty (\%)** \\ \hline Concentrated pressure drops at geometrical discontinuities: & \\ \(\bullet\)  From vertical nozzle to square channel; & \(\sim\)10\% \\ \(\bullet\)  Square channel 180\({}^{\circ}\) turn; & \(\sim\)5\% \\ \(\bullet\)  From horizontal nozzle to square channel; & \(\sim\)10\% \\ \(\bullet\)  Other discontinuities. & \(\sim\)10\% \\ Roughness of the pipes walls & \(\sim\)15\% \\ Thickness of the FWMU face under heat load & \(\sim\)3\% \\ Material’s properties & \(\sim\)15\% \\ Inlet pressure of channels 1 and 5 & see Table 3 \\ Input parameters of the conduction enclosure model & \(\sim\)3\% \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Calculated and assumed uncertainties for the considered parameters. * Scenarios in which the surface under heat load does not reach 550 \({}^{\circ}\)C (40% and 60% LOFA scenarios). Among the three categories, the first one is the most challenging to reproduce with the RELAP5-3D model. The 0% LOFA scenario at 300 kW/m\({}^{2}\) is the sole representative of the first category. The first aspect of interest is the evolution of the pressure in the branch affected by the LOFA (channels 3 and 4 in Figure 21a,b correspondingly). In both channels the outlet pressure increased as the velocity of the coolant decreased (LOFA started at 5 s and the valve as fully open again at 26 s), and then returned to the steady-state values once the full mass flow rate is re-established. Four curves were reproduced for both channels:  
* The \"Exp\" curve refers to the experimental results. * The \"Default run\" curve refers to the results achieved with the preliminary calibrated input deck. * The \"BE meth.\" curve refers to the results achieved applying the best-estimate methodology. These results do not come from a code run, but from the numerical framework of the methodology. * The \"BE run\" refers to the results achieved with an input deck built using the best-estimate parameter values calculated through the methodology. In contrast to the \"BE meth.\" curve, these results are obtained from a calculation. The results of the default and BE runs show no substantial differences, suggesting that the default values were already capable of reproducing the transient. The mathematical framework (BE meth. curve) was also in substantial agreement with these runs, and it showed a peculiar reduction in the standard deviations as the mass flow rate decreased because with no--or small--fluid motion the dynamic contribution to the total pressure was low and the static pressure dominated. The good agreement found between the experimental and the computed results for the outlet pressure drop provided the first impression of the validity of the hydro-dynamic model. Figure 21c,d show the outlet coolant temperature from channels 3 and 4.