## 3 Results and Discussion
In the reported figure, a positive heat flux refers to power entering the pool from the FPS, a negative one is instead connected with power exiting the pool towards the FPS region. This is due to the large differences in the thermal inertia of the two systems. While the fluid in the FPS rapidly becomes colder due to the lack of heating provided by the rods, the one in the pool, consisting in several tons of LBE, cools down following a relevantly slower transient. For a more accurate analysis of the observed phenomena the reader is also be referred to [27]. The impact of this heat transfer on the observed phenomena is relevant. In fact, during the steady-state conditions preceding the PLOFA test the observed heat contributes to the occurring of the observed thermal stratification phenomena; during the postulated PLOFA instead, it contributes in relevantly increasing the temperature inside the FPS, resulting in a total increase of about 20 \({}^{\circ}\)C with respect to the one that would be obtained in case of adiabatic FPS walls. In fact, in front of the 20 kW provided by the FPS at the end of the transient, the power provided through the considered walls also reaches 5 kW, thus relevantly contributing to the global energy balance of the FPS component. Though no comparison between the predicted and measured heat flux distributions is possible, the observed good matching between the measured and calculated outlet temperature trends suggests that the adopted coupling application managed to suitably reproduce the involved phenomena. Figure 6: Comparison of the measured and calculated LBE temperatures at the FPS inlet and outlet sections. Thanks to the presence of a CFD domain, the coupled application also allows reproducing interesting distributions for some of the relevant properties. In particular, it is possible to reproduce the path lines followed by the fluid exiting the HERO steam generator and entering the FPS. Figures 8 and 9 report these trends for two selected time levels; the former refers to the conditions at the beginning of the PLOFA transient (1800 s) while the latter reproduces the situation after 4 min from the beginning of the transient (2040 s). It is worth noticing that, because of the relevant change in the calculated LBE mass flow rates inside the pool, also the flow conditions in the bottom part of the pool changed consistently. Figure 8: Pathlines for the fluid exiting the HERO steam generator and entering the FPS for t = 1800 s.  
Figure 7: Calculated heat flux trend for the walls in correspondence of the top section of the FPS. In particular, at the beginning of the transient, the large inertia forces resulting from the higher LBE mass flow rate at the HERO-SGBT implies a large recirculation region at the bottom of the pool inducing a relevant temperature mixing. As a consequence, only limited temperature variations should be experienced at the FPS inlet section (see also Figure 6). On the other hand, after the decrease of the calculated mass flow rate occurring in the first minutes of the transient, the recirculation region becomes definitively smaller. This implies a sort of shortcut for the fluid exiting from the HERO-SGBT, which enters the FPS. In fact, the flow no more undergoes a strong mixing with the fluid in the pool, maintaining a temperature closer to the one predicted at the outlet of the steam generator. This phenomenon may be responsible for the predicted temperature decrease at the inlet of the FPS occurring in correspondence of the time interval in which the lower mass flow rates are predicted (1960-2060 s in Figure 6). Figure 10 reports the temperature distribution calculated on a selected longitudinal section of the pool; the thermal stratification phenomenon also observed experimentally can be clearly noted, suggesting that the adopted coupled application is suitable for predicting the addressed experimental conditions.