## 6 Conclusions  
The present paper summarizes the findings of an experimental campaign investigating the behavior of a HCPB FW mock-up under different LOFA scenarios, including the pre- and post-test analysis that were performed. Thus, prior to the testing, several numerical studies were performed using both Ansys CFX for detailed CFD analysis and RELAP5-3D models for system-wide studies. The results of these analyses provided the basis for the definition of the tests matrix and the configuration of the experimental set-up.  
The LOFA event in the mock-up was reproduced by reducing the flow rate in the respective branch when closing a valve on the outlet manifold of the channels 1-5. Six different scenarios were investigated:  
* Valve 60% open;
* Valve 40% open;
* Valve 20% open;
* Valve 10% open;
* Valve 5% open;
* Valve fully closed (full LOFA).  
Each scenario was performed for two different heat loads (300 kW/m\({}^{2}\) and 330 kW/m\({}^{2}\)) and each case was repeated several times to provide a robust set of data. The main outcomes of the experimental campaign were:  
* The confirmation of the capability of the adopted coolant strategy to limit the heat excursion of the FW temperature if one of the branches suffers a partial LOFA;
* The demonstration that in the case of severe LOFA the FW temperature might exceed the safety limit in few seconds, thus hampering the integrity of the primary system. A prompt action of the plasma shutdown system would then be necessary to drive the system to a safe condition.  
Finally, the data gathered from this experimental campaign were used for the validation of the RELAP5-3D code. A best-estimate methodology was also applied to ease the distinction between the user's assumptions and code limitations. Instead of simulating a specific case, \"averaged\" runs were investigated to identify the average behavior of the FWMU. The validation focused on three code physical models: the hydro-dynamic model, the fluid-wall heat transfer correlation (Gnielinski correlation [18] as implemented in RELAP5-3D [20]), and the conduction enclosure model. This activity reached the expected goals, proving the validity of these three models in DEMO-relevant conditions.  
Under a more general point of view, the approach adopted to set-up, perform, and analyze the data of the experimental campaign was successful. The pre-test analysis gave fundamental insight into the set-up and the boundary conditions of the experimental campaign. In turn, the strategy adopted to execute the experimental tests allowed the creation of the robust \"average\" tests investigated in the validation activity. Then, this validation activity demonstrated the capabilities of the RELAP5-3D code in DEMO-relevant conditions of nuclear fusion application.  
These activities also contributed to the creation of a relevant theoretical and practical experience that will be used in the on-going studies concerning incidental transients in real-plant scenarios.  
Conceptualization, B.-E.G., B.G., A.K., V.D.M.; methodology, M.I.-B., X.Z.J.; validation B.G.; writing--original draft preparation, B.-E.G.; writing--review and editing, B.G.; visualization, B.G.; supervision, M.I.-B., X.Z.J., R.S. All authors have read and agreed to the published version of the manuscript.  
This work was carried out within the framework of the EUROfusion Consortium and received funding from the Euratom research and training programme 2014-2018 and 2019-2020 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission.  
Not applicable.