## 3 WCLB BB Performance
### Magneto-Hydrodynamic (MHD) Performance
\begin{table}
\begin{tabular}{c c c c c} \hline  & & **Inboard** & & **Outboard** \\ \hline \multirow{3}{*}{\(\bullet\)} & Feeding pipe & 1.557 & (63.9\%) & 0.613 & (38.1\%) \\  & Inside TFC & 1.489 & & 0.338 & \\  & Outside TFC & 0.067 & & 0.276 & \\ \hline \multirow{3}{*}{\(\bullet\)} & Bottom collector & 0.092 & (3.8\%) & 0.122 & (7.6\%) \\ \cline{2-5}  & PbLi manifold & 0.450 & (18.5\%) & 0.258 & (16.0\%) \\ \cline{1-1}  & External & 0.328 & & 0.068 & \\ \cline{1-1}  & Internal & 0.122 & & 0.190 & \\ \hline \multirow{3}{*}{\(\bullet\)} & Breeding zone & Negligible & - & Negligible & - \\ \cline{1-1} \cline{2-5}  & Top collector & 0.076 & (3.1\%) & 0.241 & (15.0\%) \\ \hline \multirow{3}{*}{\(\bullet\)} & Draining pipe & 0.260 & (10.7\%) & 0.376 & (23.4\%) \\  & Inside TFC & 0.229 & & 0.358 & \\ \cline{1-1}  & Outside TFC & 0.031 & & 0.017 & \\ \hline \multirow{3}{*}{\(\bullet\)} & **Total** & **2.435** & **(100\%)** & **1.609** & **(100\%)** \\ \cline{1-1} \cline{2-5}  & & & & & \\ \end{tabular}
\end{table} 
Table 2: MHD loss in WCLL 2019 IB and OB expressed in MPa, with percentage contribution by hydraulic region in brackets. Figure 19: WCLL IB/OB PbLi loop: (**a**) in-vessel PbLi path with details of the manifold and BZ; (**b**) IB/OB collector layout. Coupling effects were found to play a pivotal role in defining both flow features and pressure loss in the manifold since currents leak through the partition wall and cause flow structure rearrangement. This phenomenon was particularly evident at the bottom and top of the manifold. In these positions, either the internal (collection) channel or external (distribution) one is expected to be stagnant. Conversely, it was found that the other channel drove the flow in the duct centre, therefore causing pressure build-up and flow reversal at walls parallel to the magnetic field. Large recirculation zones were expected in the external (distribution) manifold (covering approximately the top 2/3 of blanket poloidal height), and in the collection channel (roughly the lower third of the manifold duct). This latter phenomenon, especially, is a concern for tritium inventory control, since PbLi loaded with tritium could reside for a long time in the manifold channel. Moreover, coupling was found to lead to increase in the manifold pressure loss of approximately 1.59 and 2.41 times the loss in the uncoupled case for, respectively, the distribution and collection manifolds [37]. The OB manifold and collector layout were found to be characterized by an overall pressure loss of 621 kPa, of which 258 kPa only were ascribable to the manifold and 241 kPa were localized in the top collector and its connection with the drainage pipe, located at 2/3 of the blanket poloidal height. Therefore, a 30% pressure loss reduction was accomplished by the revised manifold, which was even more significant due to the addition of the coupling effects, which were previously neglected in [36]. However, this was compensated for by larger losses in the collectors caused by more complex geometry and, specifically, the necessity to integrate with the connection pipes through a long BSS duct that caused a final 35% blanket loss increase. Even accounting for this drawback, OB pressure loss were still considered to be acceptable. The IB pressure loss was found to be affected by larger magnetic field (8 vs. 4 T) and lower velocity due to reduced mass flow rate (5.32 vs. 16.38 kg/s) compared to OB. Moreover, the top IB collector layout was found to be much more streamlined thanks to the possibility to directly drain the breeder from the segment top end. The IB pressure loss was estimated as 618 kPa, of which 450 kPa were ascribable to the spinal manifold and 76 kPa were ascribable to the top collector--values substantially in-line with those reported for the OB and that are likely to be improved in successive iterations. MHD pressure loss in the piping system is a key concern for overall pressure loss since it constitutes the largest contribution.