## 4 Structural Analysis of the Reference COB Segment
### Loads and Boundary Conditions  
The assessment of the thermo-mechanical behaviour of the reference COB segment foresees its evaluation under different steady state loading scenario. In particular, the loading scenarios considered were: Normal Operation (NO), Over-Pressurization (OP) and an Upper Vertical Displacement Event (UVDE). The NO scenario represented the nominal conditions that the WCLL COB is supposed to undergo during flat-top plasma. In contrast, the OP scenario is a load combination derived from the in-box LOCA. This scenario was derived supposing that the occurrence of an in-box LOCA suddenly pressurizes the whole SB. Lastly, the UVDE scenario took into account the very intense electro-magnetic loads due to an uncontrolled vertical movement of the plasma, causing a plasma disruption. The loading conditions relevant to the steady state scenario of NO, OP and UVDE were  
Figure 9: Thermal field obtained for the whole WCLL COB segment by the interpolation procedure. classified as Level A, Level D and Level C, respectively, in the RCC-MRx structural design code. In order to predict the structural response of the COB segment, a set of loads and boundary conditions was assumed to reproduce the three assessed loading scenarios [15]. * internal pressure distribution according to the considered loading scenario;
* mechanical restraints;
* gravity load;
* non-uniform deformation thermal field;
* electro-magnetic loads. Since neither breeder nor water has been modelled, in order to reproduce their mechanical effect, a uniform pressure value was imposed upon all the wetted surfaces. In particular, the design pressure value, calculated as the nominal one times a safety factor equal to 1.15 [15], was imposed onto both breeder-wetted and water-wetted surfaces. So, for NO and UVDE loading scenario, a pressure equal to 17.825 MPa was considered for all the water-wetted surfaces; instead, a value equal to 0.575 was applied to the surfaces wetted by the PbLi. An equal value of 17.825 was considered for all the internal wetted surfaces in the OP loading scenario. Additionally, the reference attachment system consisted of a set of supports aimed at fixing the blanket segments to the VV and, at the same time, withstanding very large electro-magnetic loads, thermal deformations and maintaining a good alignment of the FW. The supports were located at the bottom and at the top part of the segment and each face of these supports worked along a particular direction, as reported in Figure 10, thus ensuring the segment connection with the VV [16]. These contacts were simulated by means of springs, particular Abaqus element, in correspondence to each surface. So, each of the springs was assigned a specific value of elastic constant. Figure 10: WCLL COB attachment system and indication of the directions of action of the springs. The gravitational load has been also applied to the whole model. However, since the liquid metal breeder and coolant were not modelled, their contribution was taken into account using an Eurofer equivalent density, as already done in previous analyses. The equivalent density value was obtained assuming that the percentage of steel, water and breeder were equal to that calculated for the equatorial region. So, temperature-dependent properties of Tungsten and Eurofer structural steel [9; 11] were taken into account and implemented in the model, properly modifying the Eurofer density values. As to the non-uniform deformation thermal field, it arose within the structure as an effect of the imposed temperature spatial distribution (Figure 9) and the volumetric expansion coefficient. Finally, the electro-magnetic loads were also considered for all the assessed steady state loading scenarios. In fact, their action during normal and off-normal operation cannot be neglected and must be included during the investigation of the COB structural response [17]. Since steady state analyses were performed, the EM loads considered and used were relative to a specific instant of time, i.e., time step.