## 3 WCLL BB Mock-Up Design  
The designed mock-up reproduces a portion of the breeding zone of the WCLL BB [9], where the water pipes will be externally coated to reduce both the tritium permeation from the breeder into the coolant and the corrosion rate of EUROFER. A summary of the main design parameters of the mock-up is reported in Table 1.  
The mock-up (Figure 1) will be installed in TRIEX-II facility, whose main characteristics are reported in Section 4, and consists of 6 U-shaped EUROFER pipes installed in a dedicated stainless steel box, with a size of 600 \(\times\) 600 \(\times\) 300 mm\({}^{3}\). Each tube has an outer diameter of 13.5 mm, an inner diameter of 8 mm and a total length of 900 mm. The diameter and thickness of the tubes are the same as those used for the double-walled tubes (DWT) of the WCLL BB. One pipe is made of bare EUROFER to obtain the reference value of permeation flux to be used for the calculation of the permeation reduction factor (PRF), see Equation (1). The other 5 tubes will be PLD or ALD coated. The length of the tubes is limited by the maximum length to which the PLD process can be applied. The maximum possible length of the tubes was chosen to increase the permeation area as much as possible, so that it is possible to detect a hydrogen or deuterium flux (Q\({}_{2}\), Q=H, D). In fact, even if the PRF is independent on the permeation area, the measurement of the flux is carried out by a quadrupole mass spectrometer, which can provide the amount of permeated moles of hydrogen isotopes in the unit time (e.g., mol s\({}^{-1}\)); in order to have a signal which can be measured by the mass spectrometer, a sufficient permeation area has to be guaranteed. The tubes are welded to a plate flanged to one side of the box for easy maintenance/replacement. This solution allows direct connection to the mass spectrometer and avoids bends that interfere with vacuum pumping. All the internal parts of the box will be coated by ALD. The coating characterization will be performed in gas phase, in the first part of the experimental campaign, and LiPb in the second part of the characterization. Hydrogen or deuterium are used instead of tritium and will be dissolved in the liquid metal at the appropriate partial pressure (10-1000 Pa) through the saturator column S200 of TRIEX-II, as detailed in Section 4. The tubes are connected to a mass spectrometer to measure the amount of hydrogen permeated from the box. In Figure 2, the Process Flow Diagram (PFD) of the mock-up is displayed. It is constituted by the following main subsystems:  
* Subsystem 100 (test section);
* Subsystem 200 (LiPb interface system);
* Subsystem 300 (vacuum system);
* Subsystem 400 (gas supply system). \begin{table}
\begin{tabular}{l l l} \hline
**Parameter** & **Value** & **Units** \\ \hline Design pressure & 5 & barg \\ Design temperature & 530 & \({}^{\circ}\)C \\ Operative temperature & 300–500 & \({}^{\circ}\)C \\ LiPb mass flow rate & 0.5–4.5 & kg s\({}^{-1}\) \\ Hydrogen isotopes partial pressure & 10–1000 & Pa \\ \hline \end{tabular}
\end{table} 
Table 1: Design parameters of the WCLL mock-to be installed in TRIEX-II. The mock-up is constituted by the stainless steel flanged box, the HV gate valves (FV-09 to FV-20) and the capacitive vacuum meters (PT-03 to PT-08). The choice of capacitive pressure transducers relies on the need to guarantee high-accuracy measurements (0.15% of the measured values). The test section is heated by means of a dedicated heating system. A set of K-type thermocouples will be used to map the temperature in different points of the box and in the pipings (TT-01 to TT-10). The LiPb system interfaces with TRIEX-II facility by means of two 1\" pipes in P22 steel. The LiPb line (1\" nominal diameter) can be intercepted by two ball valves, FV-07 and FV-08. The vacuum system consists of an integrated pumping station (diaphragm and turbomolecular) and its connections.