Numerical analysis of postulated out-vessel lithium-lead loss in the DEMO WCLL Loop Using MELCOR

DEMO reactor aims to demonstrate the feasibility of electrical energy production from nuclear fusion source in a tokamak configuration. Within the DEMO pre-conceptual design assessment, the water-cooled lithium lead (WCLL) and helium-cooled pebble bed (HCPB) concepts were selected as Breeding Blanket (BB) candidates. Along the research pathway toward blanket selection, safety analyses of these two options were performed using selected postulated initiating events (PIEs) as reference scenarios. For the WCLL BB, the out-vessel loss of liquid metal was identified as a major safety concern. This paper presents a numerical analysis of this postulated accidental scenario, carried out adopting MELCOR code. Large rupture (double-ended 200%) and leak (5%), occurring at two different positions (middle and bottom) of lithium-lead (LiPb) Cold Leg (CL) in both the inboard (IB) and outboard (OB) loop, were simulated implementing a detailed MELCOR nodalisation. The exothermic reactions of LiPb with air and steam present in the DEMO building were evaluated, adopting an in-house conservative numerical model, since only pure lithium-air/steam chemical reactions are available in the default MELCOR version. The LiPb mass flow rate discharged into the building was evaluated along with the LiPb volume transients in the hot and cold leg and segments of both the IB and OB loops. The time-dependent mass evolution of the reactants and products involved in the LiPb-air/steam chemical reactions was also calculated. Moreover, the increase in temperature and pressure within the considered building volume due to the energy released is shown. These numerical analyses do not implement safety or mitigation functions and the results presented should be considered highly conservative.