## 3 Results and Discussion  
In this section, a comparison between the numerical results and the experimental data is presented, for a comparison with results provided by a STH code stand-alone calculation the reader is referred to [24]. Figure 5 shows the comparison between the measured and the calculated mass flow rate at the inlet section of the hot leg; a qualitative trend of the injected Argon mass flow rate is also reported as reference. As it can be observed, the calculated trend predicts very well the measured data and provides a good matching also for the final part of the considered time-frame. Only small variations in the predicted trends are expected for later times since no changes in the imposed operating conditions occur anymore. The calculated trends do not reproduce the oscillating behaviour observed experimentally, predicting a smoother trend. The authors speculate that could be due to instantaneous fluctuations of the injected Argon mass flow rate, not sampled by the measurement system which may impact relevantly on the resulting LBE mass flow rate. Argon is in fact injected in order to enhance the natural circulation capabilities, affecting the pressure head at the bottom end of the hot leg and, as observed during the performed calculations, even small fluctuations of pressure at the two ends of the internal loop may induce relevant changes in the predicted LBE mass flow rate. Another possible cause of the observed mismatching may be related to the presence of entrapped air pockets (see, e.g., Ref. [25]) in the vicinity of the nozzle. This way, Argon injection may undergo cycles in which it is first accumulated, thus resulting in a decrease in the amount provided to the loop and a consequent decrease of the mass flow rate, and subsequently released in mass resulting in the experimentally observed peaks. In this case, the investigation of this phenomenon would require a detailed analysis of the injection system, also requesting the adoption of dedicated correlations, that cannot simply be implemented in the STH code. Eventually, as also discussed in Ref. [26] the observed mismatching could also be due to unavoidable approximations introduced by correlations of the adopted models resulting unsuitable to perfectly reproduce two-phase flows characterized by relevant amounts of transported incondensable gases. Given the number of uncertainties, both on the experimental and numerical side, the obtained results can be considered a good prediction of the observed phenomenon. Figure 6 reports instead the comparison between the measured and calculated temperature trend at the inlet and outlet sections of the FPS. As it can be observed, a good prediction is again obtained. In particular, especially during the first part of the reported time interval the predicted outlet temperature matches fairly well the measured trend. Again, for later times, a smoother trend is predicted, in accordance with the predicted mass flow rate. In particular it must be observed that the outlet temperature underestimation occurring between time levels 1920 and 2000 s is due to the overprediction of the LBE mass flow rate in the corresponding leg during the same interval. This can be clearly observed comparing the trends reported in Figures 5 and 6. Figure 5: Comparison of the measured and calculated Lead Bismuth Eutectic (LBE) mass flow rates at the Fuel Pin Simulator (FPS) inlet section. It is also worth highlighting the thermal interactions occurring between the LBE flowing inside the FPS and the one inside the pool. In fact, during the time interval preceding the beginning of the PLOFA test, heat transfer is predicted to occur from the FPS to the pool; after a few seconds, instead, heat transfer occurs from the pool to the FPS. This is clearly shown in Figure 7, which reports the trend of the predicted heat flux in correspondence of the wall in the top section of the FPS.