## 1 Introduction  
The Helium Cooled Pebble Bed (HCPB) blanket concept is one of the European Demonstration Power Plant (EU DEMO) blanket concepts currently under development [1]. At the present stage, the blanket concept employs helium as coolant, Li\({}_{4}\)SiO\({}_{4}\) as breeder material, Be\({}_{12}\)Ti as neutron multiplier material, and EUROFER as structural material [2, 3, 4]. As part of the general strategy [5, 6] for the qualification of the DEMO design and its components, in view of licensing and operation, several experiments have been performed or are planned to be carried out in the HELOKA facility at KIT [7, 8].  
Most of these experiments address issues related to the plasma-facing components, in particular the design and qualification of the First Wall (FW), which faces the plasma and is subject to high heat and neutron loads from the plasma core. Being in front of the breeding zone, the DEMO FW needs to be rather thin to reduce its impact on the reactor breeding ratio while coping with surface heat fluxes ranging from 350 kW/m\({}^{2}\) in the median region and up to 1 MW/m\({}^{2}\) in the upper and lower regions.  
For HCPB, to manage such loads, one uses high-pressure (8 MPa) helium flowing through small channels imbedded in the FW. As it is inside of the reactor Vacuum Vessel, the integrity of the FW is of paramount importance for the DEMO operation. There are several causes that might hamper the integrity of the FW, but Loss Of Flow Accidents (LOFAs) are among the most dangerous ones because, in the case of a reduced flow, the material temperature increases while the pressure inside the channels remains at levels close to the nominal operating values. Depending on the severity of the flow reduction, the temperature of the wall facing the plasma can increase to temperatures where the material strength is significantly reduced, placing the FW outside the operating limits foreseen in its design [9].  
The studies addressing this kind of accident rely on a combination of different types of numerical tools, the system behavior simulated with MELCOR (Sandia National Laboratories, Albuquerque, NM, USA), or RELAP5-3D (Idaho National Laboratory, Idaho Falls, ID, USA), while local detailed simulations are performed with CFD tools such as CFX or Fluent combined with Ansys Mechanical (Ansys Inc., 275 Technology Drive, Canonsburg, PA, USA) for stress and failure analysis. Despite the significant progress in modeling and the computational capabilities, the numerical simulation of fast transients characterizing a LOFA event still needs experimental data to either benchmark the models or to calibrate certain parameters.  
The present paper looks into different details of an experimental campaign carried out in HELOKA to investigate the behavior of a FW mock-up under LOFA conditions. First, the mock-up characteristics is introduced followed by preliminary numerical investigations carried out to define the experimental matrix. The experimental results are discussed afterwards, with emphasis on the calibration and validation the FW model implemented with the RELAP5-3D code as part of the validation of the codes adopted in DEMO safety studies. A best-estimate plus uncertainty methodology are applied to ease the distinction between the user's effects and code limitations (if any).