## 5 Results and Discussions
The temperature decreases in both cases, however, a difference of about 10% is found at the end of the simulation between experimental and numerical results. On the contrary, the third numerical analysis, named 'ImpInt', generates a temperature increase after the PLOFA, converging to the FPS temperature of the experiment,  
Figure 6: LBE mass flow rate. Comparison between experimental data and numerical results obtained under different conditions. Figure 7: FPS inlet and outlet temperatures. Comparison between experimental data and numerical results obtained under different conditions. with a difference of about 0.1% at the end of the simulation. This behavior is determined by the increase in the temperature at the HERO-loop outlet (see Figure 7) due to the combination \"LBE mass flow rate--FPS power\" occurring soon after the PLOFA. Indeed, LBE mass flow rate decreases up to 6 Kg/s according to the experiments, while in the other two simulations the value stays around 10 Kg/s. Under this condition, the FPS power (20 kW) easily warms up the LBE increasing the temperature at the HERO-loop outlet and as a consequence at the FPS inlet. In all cases, the differences occurring before the PLOFA starting time could be due to the continuous reduction of the FPS inlet temperature, which should be instead almost constant. This reduction is generated by the same temperature reduction of the LBE at the outlet of HERO-SGBT (see Figure 8). This entails that when LBE comes out the SGBT it enters almost directly into the FPS maintaining the same temperature. Looking at the CFD (Computational Fluid Dynamics) results reported in the literature [17], a longer and more complex path is made by LBE from the outlet of HERO-SGBT to the inlet of the FPS. Indeed, according to [17], soon after entering in the pool, the LBE hits the lower plenum rising the pool in the opposite direction up to the outlet zone of the SGBT to then conclude the path flowing down to the inlet of the FPS. During this route, the LBE has more time to warm up, keeping its FPS inlet temperature almost constant at 420 \({}^{\circ}\)C. Therefore, as a further study, the volume of the lower plenum was intentionally increased by one order of magnitude to let the LBE warm sufficiently. After the PLOFA, the low LBE mass flow could generate a different flow path, avoiding LBE hitting the lower plenum after leaving the HERO-loop and flowing directly to the inlet of the FPS. This behavior joined to the applied combination \"LBE mass flow rate- FPS power\", which determines the same temperature at the end of the simulation. Figure 9 shows the comparison between the experimental FPS temperatures and the related numerical values obtained by increasing the volume of the pool lower plenum. Here, the case with LBE mass flow rate imposed as input parameter was considered. A good agreement between numerical and experimental results during the entire analysis is obtained for both quantitative and qualitative study. This reveals the importance of a coupled CFD-STH analysis to include the exact path made by the LBE and evaluate the resulting temperature. While FPS experimental temperature decreases after PLOFA, numerical results remain almost constant, generating a temperature difference of about 3% at 3600 s. This behavior is generated by the approximation introduced by increasing the volume of the lower plenum. Figure 10 shows the variation of the temperature trend generated in the lower plenum by increasing its volume by one order of magnitude starting from about 1 m\({}^{3}\). When the volume increases the temperature remains almost constant generating a different outlet temperature in the FPS. Figure 8: HERO- Steam Generator Bayonet Tube (SGBT) outlet temperatures. Comparison between experimental values and numerical results obtained under different conditions. The facility represents a complicated thermal-hydraulic system which involves two-phase flow, non-condensable gas and liquid metal flow conditions.