## 3 WCLB BB Performance
### Thermo-Mechanical Performance  
Concerning thermo-mechanical calculations, several analyses have been carried out to support design activities. Studies have been performed on both local (mainly based on the behaviour of one/few elementary cells and the top cap region) and global models (whole segments). The design development of the WCLL BB has been pursued by analysing the different steady-state loading scenarios that the blanket might undergo and by verifying its structural integrity by means of the design criteria reported in the French nuclear standards RCC-MRx [38]. In particular, a stress linearization procedure has been adopted in all the structural analyses performed so far The structural design of the WCLL BB is strongly dependent upon the over-pressurization (OP) scenario, representing an in-box LOCA due to the breakage of one or more DWTs inside the BZ. In such a scenario, the whole system will be pressurized up to the design pressure, defined as a coolant pressure of 15.5 MPa times a safety factor (initially assumed equal to 1.20 and later decreased to 1.15 [26]) intended to consider the PbLi/water chemical interaction. The structural integrity under this scenario is mainly guaranteed by the presence of stiffening plates that help the segment box (composed of FW, SW, and BP) to withstand such a high pressure load inside the BZ. A parametric static thermo-mechanical analysis considering the equatorial region of the module was performed, with the varying thickness, pitch, layout, and radial length of the stiffening plates. Thermo-mechanical analyses were conducted while assuming, for the assessed OP scenario, a uniform pressure of 18.6 MPa (i.e., the nominal pressure of 15.5 MPa times 1.2) on the BZ internal surfaces and the water-wetted surfaces (namely the FW channels and DWTs), as well as considering a non-uniform thermal strain field inferred from calculations reported in [39]. As reported in [17], the best configuration was found to be that characterized by a vertical and horizontal plate thicknesses of 12 and 10 mm, respectively, and a pitch between two horizontal plate of 135 mm. Moreover, the best configuration allows one to save steel while increasing the breeder amount with an increase of the neutronic performance. In particular, the steel amount in the optimized configuration was found to be 88.7% of the previous reference design. Concerning local analyses, a dedicated study was performed in order to design the upper peripheral area of the segment in detail. In particular, attention was paid to the top cap (TC) of the COB segment [22]. Due to the complexity of the geometry and the non-uniform heat load distribution, a methodology based on a design-by-analysis approach was followed. Preliminary analyses led to the selection of a TC thickness of 40 mm (as reported above) and a geometric layout for its cooling channels (with a square cross-section of 10 \(\times\) 10 mm\({}^{2}\)) able to maintain the TC temperature below the EUROFER limit value of 550 \({}^{\circ}\)C. A preliminary layout of the DWTs housed in this region was proposed, and a further set of thermo-mechanical analyses were performed under both the normal operation (NO, characterised by a pressure of 15.5 MPa onto water-wetted surfaces and 0.5 MPa onto BZ internal surfaces) and OP steady-state loading scenarios. Finally, a stress linearization procedure was conducted along some paths located within the most stressed areas of the model in order to check the fulfilment of RCC-MRx design criteria [38]. The obtained results highlighted that some hot-spots are predicted (maximum temperature equal to 570.6 \({}^{\circ}\)C) within SPs, suggesting the necessity of a further review of the BZ cooling scheme, whereas the TC maximum temperature was found to be well-below the prescribed limit (Figure 20). Concerning the stress field (Figure 20) arising within the model, some of the design criteria were not fulfilled along a few paths. A more exhaustive description of adopted models and results was reported in [22].