For the helium-cooled pebble bed breeding blanket concept improved in 2016 and the associated primary heat transfer system (PHTS) following EU DEMO Baseline 2015, an ex-vessel loss-of-coolant accident (LOCA) has been investigated with the assumption of a double-ended guillotine break of a main pipe in an outboard (OB) loop of the PHTS. The break leads to helium blowdown into the tokamak cooling room. A fast plasma shutdown followed by a plasma disruption is activated after the detection of the LOCA due to the design basis accident. Regarding three affected first-wall (FW) areas in one or two OB loops, three main cases are considered. If the FW temperature reaches the defined temperature limit of 1000°C, the FW is assumed to be failed such that an in-vessel LOCA results. In total five scenarios are simulated using MELCOR 1.8.6 for fusion with respect to the affected FW areas, mitigated or unmitigated plasma disruption conditions, the options of the dry or wet suppression tank, and the transport of source terms performed in the case of the beyond design basis accident without the plasma shutdown. The transient results are discussed for the time evolution of the accident sequences, pressurization in the systems, temperature behavior in volumes and structures, and tritium and dust transport behavior.
