Analyses of design extension conditions for DEMO tokamak building: Ex-VV LOCA accident of breeding blanket heat transfer system

The European DEMOnstration plant is currently undergoing conceptual design, and the early assessment of potential safety issues is a key activity guiding design improvement. The identification and assessment of accident bounding Design Extension Conditions (DEC) that can challenge the tokamak building integrity and confinement function is of paramount importance to investigate plant robustness and resilience. Moving from such perspective, an ex-vessel loss of coolant accident for EU-DEMO Water Cooled Lithium Lead (WCLL) Breeding Blanket (BB) First Wall (FW) and Breeding Zone (BZ) primary heat transfer system loop has been selected as a potentially bounding case in force of the enthalpic content of the cooling loops of such system. The Postulated Initiating Event (PIE), therefore, assumes a double-end guillotine pipe break co-occurring both in FW and BZ primary Heat Transfer System (HTS) coldleg/cold rings in the Upper Pipe Chase (UPC) area. The accident falls into a DEC category since these loops are separated in the considered baseline design, hence resulting as independent failures. The simulation has exploited the severe accident code MELCOR 1.8.6 for fusion applications. A confinement model for the Tokamak building has been implemented, and transient analysis is presented. Released water inventory to exVacuumVessel(VV) containment volumes,which consist of the UPC, the Lower Pipe Chase (LPC) and Vertical Shaft (VS), from both breaks is about 425 tons released within 143 s. This results in a quick pressurization pattern reaching over 8 kPa on the 0.1 s scale, spreading to connected volumes through relief pathways or leaks (PHTS room, Galleries, dome). Ex-VV rooms pressure and walls temperatures pattern are also presented together with source term (HTO, Activated Corrosion Product (ACP)) inventory mobilized through the building and to environment.