## 3 Experimental Set-Up
### Experimental Matrix and Pre-Test Analysis
#### 3.1.2 Experimental Set-Up RELAP5-3D Model and Transient (LOFA)  
Thermal-Hydraulic Analysis  
The 3D simulations provided a rather detailed view of the temperature fields within the mock-up, but they tend to become very demanding in terms of computational time for simulating transient behavior, making this approach impractical for investigating a wider range of operating conditions. In particular, the investigation of the dynamics of the mock-up included in the thermal-hydraulic response of the experimental loop was of primary interest when setting-up the experimental campaign. Such a model can provide the information needed for thermal-hydraulics parameters, hints for the finalization of the piping design and components, and it can also ease the definition of the test matrix. To address these issues, a RELAP5-3D model simulating the mock-up together with the experimental loop was created. For the mock-up model itself, a preliminary calibration of the pressure loss coefficients was performed. In addition, a comparison between the two Ansys CFX models and the RELAP model was performed for the steady-state case with heat load. A detailed description of the RELAP model itself and the calibration procedure can be found in [15]. The nodalization of the large pipes in the test section introduced in Figure 2 is shown in Figure 7. In this model, the pipes connecting the manifolds (TS-LP-021, -020, -050, and -051) and the mock-up had simplified geometries since their detailed design, mostly driven by mechanical stress considerations, was not available at that time. The  
Figure 6: LOFA channel flow rate drop (**a**) and maximum surface temperature evolution (**b**) for the partial LOFA and total LOFA. values V-1 and V-2 were represented as control valves (V945 and V944 in Figure 7) with linear characteristics and a flow coefficient (K\({}_{\
u}\)) of 25. Flow meters 1 and 2 were modeled as fully opened valves (V946 and V947 in Figure 7) with a concentrated pressure drop coefficient calibrated based on the calculation sheet from the flowmeter manufacturer, and with a flow coefficient of 32. Due to high pressure losses in channels 1 and 2 [11], we decided to carry out the experimental investigations by only heating up the central region of the mock-up surface, i.e., channels 3, 4, 7, and 8 (Figure 8). In the transversal direction (perpendicular on the channels axis), the loading profile in the RELAP5-3D model was constant at 300 kW/m\({}^{2}\) along the central channel length, and null on the peripheral channels. The heating profile was also shaped to avoid the heating of the channel's bend region. The LOFA scenario was then simulated starting from a steady-state condition with flow rates in the channels of interest close to the nominal value of 50 g/s (see test section parameters in Table 1). The channels under LOFA conditions are highlighted in yellow in Figure 8.  
Figure 8: Mock-up surface loading. Figure 7: RELAP5-3D model of the experimental set-up introduced in Figure 2.  
From these values it can be seen that having the valve V-1 in one of the flow paths resulted in a lower flow level through the corresponding channels (channels 1-5) even if the flow characteristics of the channels were similar (Table 2). The fluid (Figure 9) and the heat structure temperature (Figure 10) profiles during the initial steady state were determined by the heat flux shape imposed as a boundary condition. The heated channels (3, 4, 7, and 8) and the related heat structures were subjected to the highest temperature, with channels 3 and 4 having the highest temperature due to the lower mass flow rate crossing them. The sudden drop in the temperature of the heat structure segments corresponding to the channel bend region (Figure 10) was due to the absence of surface heating in that area. The loss of flow was simulated by fully closing V945 valve, which therefore reduced the mass flow rate of channels 1-5. The time for the complete closure of the valve was set to 10 s to simulate the coast-down of the HELOKA circulator.