## 4 R&D Needs and Further Activities  
The WCLL BB design was deeply revised during FP8 activities in order to improve its global performance. In particular, with respect to the initial layout, the main achievements at the end of the pre-conceptual design phase were:  
1. The fulfilment of the neutronic requirements both in terms of TBR and shielding of components. Figure 23: Stress linearization results under the VDE-up (T = 11.52 s) scenario: **(a)** IED Level C RCC-MRx and **(b)** IPI Level C RCC-MRx design criteria results. Figure 24: Stress linearization results under the VDE-up (T = 11.52 s) scenario—IPFL Level C RCC-MRx design criteria results. The confirmation of BB cooling while maintaining the maximum temperature of structural materials lower than suggested values and respecting prescriptions in terms of maximum coolant velocities. The enhancement of the thermo-mechanical performance, increasing margins against design criteria prescribed by codes. The integration of the different feeding pipes and fluid manifolds while respecting boundaries given by the different BB interfaces. The carrying out of a design that is compliant with constraints given by the manufacturing processes as much as possible. Finally, the work done within the Work Package BB during the FP8 period, as well as that carried out in other DEMO Work Packages, underwent a so-called Gate Review process in mid 2020. A panel of external experts reviewed the design and the supplied documentation in order to provide feedback, highlight criticalities, and suggest improvements to the different design teams. Concerning the presented WCLL BB architecture, the following main gaps have been identified:  
1. The shift of the EUROFER ductile-brittle transition temperature (DBTT) up to values close to room temperature may be an issue, mainly during maintenance operations. 2. The current WCLL BB layout has a low reliability, mainly because of the high number of welds. 3. Modelling tools adopted for the prediction of WCLL-characterising phenomena, such as MHD, PbLi/water interaction, and the thermal-hydraulic modelling of water, are not at adequate levels of maturity. The first two points can be mitigated by adopting proper design choices, such as a careful definition of maintenance operations or the simplification of the WCLL BB layout. The third point is more delicate. Indeed, it requires both the development of numerical tools able to model different phenomena and the set-up of experimental campaigns in order to provide data for the validation of codes and models. For this purpose, the European Framework Program 9 (FP9), also known as Horizon Europe, has foreseen the construction of facilities in order to fill these gaps. A water hydraulic experimental platform (W-HYDRA) will be constructed at the ENEA Brasimone research centre to meet this need. This platform will group different facilities, with the main goal of representing an integral test facility able to perform experiments relevant to the water thermal-hydraulics and PbLi/water interactions. On the other hand, the conduction of experiments adopting MEKKA and MaPLE facilities at KIT will allow for the validation of numerical tools for the modelling of MHD phenomena occurring within the WCLL BZ, such as magneto-convection influence on temperature and tritium distribution, as well as within the manifolds (MHD pressure losses, coupling phenomena, etc.). Moreover, despite the COB segment reaching a quite advanced design status, inboard and lateral outboard blanket segments are worth assessing in more detail. In particular, the thermal-hydraulic and thermo-mechanic performance of some specific regions of these segments need detailed investigations because they significantly deviate from the assessed equatorial region. Indeed, some regions present singularities that may lead to the necessity of envisaging ad hoc design solutions (e.g., the top part of the inboard segment or areas of the lateral segments where the vertical stiffening plate number could change).