Structural assessment of the EU-DEMO WCLL Central Outboard Blanket segment under normal and off-normal operating conditions

Within the framework of the EUROfusion design activities concerning the EU-DEMO Breeding Blanket (BB) system, a research campaign has been carried out at the University of Palermo with the aim of investigating the structural behaviour of the DEMO Water-Cooled Lithium Lead (WCLL) Central Outboard Blanket (COB) segment. The assessment has been performed considering three different loading scenarios: the Normal Operation (NO), the Over-Pressurization (OP) and the Upward Vertical Displacement Event (VDE-up). In particular, NO scenario represents the loading case referring to the nominal operating conditions, whereas the OP scenario refers to the loading conditions due to an in-box LOCA accident, listed as one of the BB design basis accidental events. Lastly, the VDE-up scenario is an off-normal event reproducing the plasma disruption caused by an uncontrolled vertical motion of the plasma volume. This event brings the plasma in contact with the upper part of the plasma chamber, generating a sudden energy discharge accompanied by Electro Magnetic (EM) forces acting on the structure. The study has been carried out following a theoretical-numerical approach based on the Finite Element Method (FEM) and adopting the quoted ABAQUS v. 6.14 commercial FEM code. In particular, a detailed 3D FEM model of the whole COB segment, including the back-supporting structure and its attachment system to the vacuum vessel, has been set up. Several simulations have been run to assess the thermo-mechanical performances of the segment under the afore-mentioned loading scenarios, also taking into account the impact of the tungsten (W)-armour on the overall structural response. EM loads have been considered in all the assessed scenarios. In the first two, only magnetization forces have been taken into account, while in the VDE-up scenario Lorentz's forces have been also taken into account. The structural response has been evaluated according to the RCC-MRx structural design rules. The obtained results are herewith presented and critically discussed