## 3 Thermal Analysis of the WCLL COB Segment
### Thermal Analysis of the Equatorial Radial-Toroidal Cell of Each of the 7 COB Poloidal Regions
#### 3.1.1 The Reference Geometric Layout  
In accordance with the Single Module Segmentation (SMS) concept, the WCLL COB Segment is composed of a single elementary cell, identified by two horizontal Stiffening Plates (SPs), that repeats along the poloidal direction from one cap to another that vertically closes the segment. The structure, depicted in Figure 2, is mainly characterized by the Segment Box (SB), divisible into the First Wall (FW), with the 2 mm-thick W-armour (coloured in orange in Figure 2), and the Side Walls (SWs) that laterally close the SB. Moreover, the structure is closed in the back by the complex of Back Plate (BP), water and breeder manifolds and the Back Supporting Structure (BSS), and connected to the attachment system (coloured in grey in Figure 2), aimed to mechanically connect the Segment to the Vacuum Vessel (VV). The breeder flows inside the box, in the region called Breeder Zone (BZ) [3], which is reinforced by means of the SPs. In particular, the SPs grid is composed of horizontal (or toroidal-radial) and vertical (or poloidal-radial) SPs, with a thickness equal to 10 and 12 mm, respectively. A bundle of 22 cooling tubes per cell (Figure 3), Double Walled Tubes (DWTs), immersed in the PbLi, has the role of cooling the BZ; in contrast, the FW-SW-FW region is cooled by means of 7 \(\times\) 7 m\({}^{2}\) cooling channels located into the SB.  
In order to observe the different thermal loads characterizing the poloidal extension of the segment, the number of cooling channels per cell varies along the COB segment [3], following the aforementioned poloidal segmentation. In particular, four channels per cell are foreseen for each poloidal region with exception of O7 region in which six channels are foreseen. Instead, only one DWTs configuration was preliminarily selected for all the elementary cells along the segment. In particular, the chosen DWTs layout, named v06b as reported in [3], consisted of 22 C-shaped double-walled tubes, with a hydraulic diameter of 8 mm and an external diameter equal to 13.5 mm, each with different position and radius. In Figure 3 the DWTs layout in an elementary cell (namely, an equatorial radial-toroidal cell) and the water path inside them (in which red arrows indicate the inlet and green the outlet), with the BZ recirculation scheme (highlighted in orange), is reported.  
Figure 2: COB Segment architecture and detail of the main components.  
In the present work, two realistic 3D geometric models of a couple of elementary cells (Figure 4), equipped with four and six cooling channels, respectively, and the WCLL v06b DWTs layout, representing a portion of the whole COB segment, were set up in order to realistically simulate its thermal behaviour. Given that the reference DWTs layout is the same along the whole segment, the geometric model envisaging four FW channels represents the equatorial radial-toroidal cell of regions from O1 to O6, whereas the model with an FW equipped with 6 cooling channels represents the equatorial elementary cell of the O7 poloidal region.  
Two detailed FEM models have been set-up, each one composed of \(\sim\)13M nodes connected in \(\sim\)5M linear hexahedral and tetrahedral elements, including the Eurofer structure, tungsten layer, DWTs, breeder and water coolant flowing inside FW channels and DWTs.  
Figure 4: Models of the two elementary cells equipped with 4 and 6 cooling channels.  
Figure 3: WCLL v06b DWTs layout and recirculation scheme (inlet in red, first outlet/recirculation in orange, exit in green).