## 4 The Experimental Campaign  
The purpose of the experimental campaign was to provide a robust set of data and benchmarks for the validation of numerical tools. This was achieved by exploring different thermal-hydraulic configurations and different flow control strategies. The information gathered from the pre-test analysis was used as a basis for the set-up of the experimental campaign. During normal operation, the total flow through the mock-up was set to 500 g/s or 650 g/s by setting the speed of the HELOKA circulator accordingly, so that each circuit would receive half from the total flow rate. In reality, due to the presence of the valve V-1 and the higher pressure-loss coefficients characterizing channel 1 and 2, the flow does not distribute evenly between the two branches, the flowrate for the channels 1 to 5 representing only 46% of the total flow when the main valve (V-1) was fully (100%) opened. The LOFA event was simulated by closing V-1 to a predefined value, i.e., similarly to what was simulated with the RELAP-3D model (Section 3.1.2). However, to have a quick response at the initiation of LOFA event, the bypass valve (V-2) had to be kept slightly opened (5%). The reason for this was that, due the valve characteristic, below this value no measurable change in the flow distribution can be observed and the flow through the valve can be assumed as zero. On the other hand, operating V-2 from a 5% opening gives an immediate response in the flow distribution, avoiding the dead time that would occur if the valve would be fully closed. A PID controller was initially tested to control the bypass valve, but it led to oscillations within the HELOKA loop when trying to control the fast opening and closing of the bypass and the main valves. The best results were obtained by setting an optimal V-2 opening for each V-1 closing position. For both valves, V-1 and V-2, the limit for the opening and closing speed was set to 10% per second. When the LOFA event was initiated, the two valves changed their opening to the specified value. These optimum values were determined through several trials, while the mock-up surface was not heated. The heating of the mock-up surface was achieved by using the electron beam gun. This was set to generate a homogeneous heat load of 300 kW/m\({}^{2}\) on an area of 580 \(\times\) 160 mm of the mock-up surface (see Figure 14). The area corresponds to a surface that covers only the channels 3, 4, 7, and 8. The heat flux was estimated calorimetrically by calculating the helium power intake using the inlet and outlet mock-up temperatures, the flow rate, and the coolant pressure in the circuit. Adding up all the associated uncertainties, the overall uncertainty of the heat flux measurement can be estimated to be about \(\pm\)15%. Figure 14: View of the mock-up as installed in HELOKA. The IR camera measuring the surface temperature excursion during the LOFA was set up by imposing two parameters: the emissivity and transmission coefficient. The transmission was always set as 0.33 based on previous calibration experiments. The emissivity was set at the beginning of each experimental session and rechecked at the end of the daily campaign using the procedure described in [1]. The fact that the camera was installed outside the vacuum chamber with the image of the FWMU reaching the camera lens after the reflection onto two mirrors has two main drawbacks:  
* The image is inclined and so the thermal shield affects the recordings (Figure 14). The shield hides channels 4 and 8 (the last one only partially). Reflections due to the shield are also shown on top of channel 7 (darker area in Figure 15a), and at the right and left ends of the heated zone (brighter spots in Figure 15a). This is the reason why the temperatures averaged over the rectangular area marked Ch 7_2 in Figure 15a appear to be higher, the actual temperature being in the same range as the one recorded for Ch 8_14. * The mirrors and the associated structures were left uncooled, and they started to move as their temperature increased.