## 2 The Adopted Methodology  
As stated in the previous section, the WCLL COB design passed through the DEMO pre-conceptual gate review was not detailed enough to allow reliable thermal-hydraulic analysis for the detailed calculation of its thermal field. At the same time, the availability of a reliable and detailed 3D temperature spatial distribution for the whole segment is crucial for the assessment of its thermomechanical performances in different loading scenarios. Hence, in this study, an alternative procedure was applied to get a reliable estimation of the thermal field arising within the WCLL COB segment without performing its whole thermal-hydraulic assessment.  
To this purpose, it has to be observed that each BB segment could be ideally subdivided into 7 poloidal regions [3; 8], each characterized by a certain set of distributed thermal loads (namely, plasma heat flux, nuclear heating and decay-heat power density) representative of the operating conditions of the considered region (Figure 1). In this study, the equatorial radial-toroidal elementary cell of each of the 7 COB poloidal regions (as depicted, as an example, for the O5 region in Figure 1) was assessed from the thermal point of view (Section 3.1), taking into account the 3D nature of the pertinent thermal loads and the reference geometric layout, described below. To this purpose, considering the suggested Eurofer maximum temperature limit equal to 550 \({}^{\circ}\)C within the structural components [3], it was found that a thermal optimization of the WCLL COB segment's cooling layout (namely first wall channels and breeding zone cooling tubes) was necessary in order to obtain, for each of the 7 equatorial cells assessed, a thermal field fulfilling the abovementioned requirement minimizing the number of tubes.  
Hence, alternative geometric layouts were conceived and assessed assuming, for each of the 7 poloidal regions, the corresponding set of thermal loads (Section 3.2). Once an optimized geometric layout for the cooling system and the pertinent thermal field fulfilling the design requirement was obtained for each of the 7 equatorial cells assessed, an interpolation procedure was performed in order to carry out 7 different sets of interpolating functions of the radial and toroidal variable capable of reproducing the calculated thermal fields with a high level of confidence. Then, the found sets of interpolating functions were applied to the 7 poloidal regions (namely to the whole O1, O2,..., O7 region) so as to reproduce, on the whole COB segment, a detailed 3D thermal field originated by the radial-poloidal dependent thermal loads, fulfilling the prescribed design thermal requirement.  
At this point, the structural assessment (Section 4) of the whole WCLL COB under the obtained optimized 3D thermal field was performed, under the nominal and accidental steady state loading scenarios described below. The obtained results were used to assess the displacement and von Mises-equivalent stress fields and to perform a stress linearization procedure in some critical areas, so as to verify the fulfilment of the structural design criteria, namely, the criteria prescribed by the RCC-MRx structural design code.