## 3 Structural Assessment of the Reference WCLL COB Segment BC Region
### Reference BC Region v0
#### 3.1.1 The FEM Model  
The reference geometric layout of the BC region of the DEMO WCLL COB segment [5;6] has been considered (Figure 2). In particular, only the elementary cell housing the BC plate has been modelled in this phase without including in the model the manifolds region, the BP and the BSS. Moreover, it is important to notice that this elementary cell, housing a BC plate 25 mm thick, has a different poloidal size with respect to the others. Hence, the thermal analysis results reported in [6] show that ten cooling channels are necessary to cool the FW-SWs region, differently than the other elementary cells of the corresponding poloidal region of the COB Segment, equipped with six cooling channels [9]. Moreover, the so-called \"v06b\" DWT geometric layout has been slightly modified because of the different sizes of this elementary cell.  
Figure 1: WCLL COB segment architecture.  
A realistic 3D FEM model has been set up in order to investigate the structural behaviour of the BC region. DWTs, Breeder and cooling water, have not been directly modelled, but their effects have been properly considered by means of relevant loads and boundary conditions. A mesh composed of -2 M nodes connected in -550 k tetrahedral and hexahedral quadratic elements has been set up. Temperature-dependent material properties have been considered, drawn from [10,11]. The following set of loads and boundary conditions has been considered and applied to carry out the structural analysis of the BC region under the NO and OP steady-state loading scenarios [12]:  
* Nonuniform thermal deformation field: The 3D thermal field calculated in [6] has been imported and mapped to the structure, as reported in Figure 3, originating the corresponding deformation field by means of the temperature-dependent coefficient of thermal expansions.
* Mechanical restraints: With the aim of realistic simulating the presence of the entire COB segment acting on the considered BC region, the displacements field, obtained in [9], has been mapped and applied on the upper surface of the structure, as already achieved in previous analyses [4]. In particular, the corresponding displacement field has been exported from the whole COB Segment analysis and applied in accordance with the considered loading scenario. In Figure 4, the mapped displacement field on the upper surface in NO and OP loading scenarios is reported. For the sake of brevity, the deformed vs. undeformed shapes of the entire COB Segment have not been reported, but they can be easily found by looking at Figure 13 of the reference document [9].
* Pressure: The design pressures, calculated as the nominal pressure multiplied by a safety factor equal to 1.15, have been applied to the Breeder-wetted and water-wetted surfaces. In particular, a pressure of 17.825 MPa has been considered for the coolant; instead, a pressure of 0.575 MPa for the PbLi has been assumed. During the NO loading scenario, the coolant design pressure has been imposed on the water-wetted surfaces (i.e., the FW-SWs cooling channels), whereas the breeder design pressure has been set to the internal BZ surfaces. As well as the OP scenario, representing an over-pressurisation condition subsequent to an in-box LOCA event, the coolant design pressure has been assumed both for the water-wetted and the breeder-wetted surfaces.
* Gravity: The gravity load has been applied to the entire structure.  
Figure 2: WCLL COB reference BC region.