## 3 Results
### LOFA Tests with a Sensitivity Matrix Coming from the Pre-Calibrations
These differences are probably linked to the reproduction of the heat transfer among neighbor structures: a too-fast heat exchange among Channels 4 and 8 is established, and the temperature increase is faster (compared to the experimental trends) on Channel 8 (not is LOFA conditions) and slower on Channel 4. The outcome of this behavior is a maximum temperature on Channels 4 roughly 10 \({}^{\circ}\)C below than the experimental figure (6-7 \({}^{\circ}\)C linked to the friction heating and 3-4 \({}^{\circ}\)C related to the heat exchange). By contrast, the temperature in Channel 8 reaches the experimental value at about 29 s, but in a steady-state condition, it was below the experimental value of about 5 \({}^{\circ}\)C.  
Finally, the results for the 0% valve opening scenario at 300 kW/m\({}^{2}\) are shown in Figures 10 and 11 (scenario representative of the C3 category), respectively. This scenario is characterized by the establishment of small back-flows through the channels under LOFA conditions. The effects of this phenomenon are shown on the coolant temperature trend between 16 and 23 s: the temperature increase suddenly stops because the cold coolant is transported back from the outlet header into the mock-up. The coolant in the outlet header is colder than the coolant exiting from Channels 3 and 4, being a mixture of the coolant that crossed also the channels not under heat load (Channels 1, 2, and 5). The establishment of back-flows also turns the outlet temperature sensors into inlet sensors because they start to measure the temperature of the coolant before crossing the mock-up. Figure 7: Outlet coolant temperature—300 kW/m\({}^{2}\), 40% valve opening scenario. Figure 8: Outlet coolant pressure—300 kW/m\({}^{2}\), 10% valve opening scenario. Figure 9: Outlet coolant temperature—300 kW/m\({}^{2}\), 10% valve opening scenario. Figure 11: Outlet coolant temperature—300 kW/m\({}^{2}\), 0% valve opening scenario. Figure 10: Outlet coolant pressure—300 kW/m\({}^{2}\), 0% valve opening scenario. The results for this last scenario are again in quite good agreement with the experimental trends, especially concerning the outlet coolant pressure (Figure 10). For the temperature, the same discrepancy found for the 10% valve opening scenario is also present in this scenario (temperature trends different from the experimental ones), but the most important difference is the lack of back-flow in Channel 4 (Figure 11). The same issue was also found in the analysis with the RELAP5-3D code [4]. In this analysis, this behavior is linked primarily to the assumption made regarding the coolant inlet pressure in Channels 1 and 5: since no measurements are performed on these channels, a mean value coming from the reading of Channels 2-4 is used. Other factors playing a role on this issue are also:  
* The pressure drop coefficients in reverse direction were not re-calibrated while analyzing the steady-state at 300 and 330 kW/m\({}^{2}\) (both with and without heat load) since no experimental data are available;
* The inlet coolant pressure and temperature are available only for few channels. Overall, the results of each scenario agree sufficiently well with the experimental data, thus suggesting the validity of the models adopted in the MELCOR code. The capabilities of the hydrodynamic model are ensured by the very good agreement achieved concerning the pressure evolution in the channels. The capability to reproduce heat transfer phenomena among adjacent heat structures is demonstrated by the evolution of the coolant temperature in Channels 8 and 9 when the LOFA occurs in Channels 1-5. At the same time, the evolution of the coolant temperature in Channels 3 and 4 shows the capabilities of the fluid-wall heat transfer model (Dittus-Boelter correlation).