Analyses of loss of vacuum accident (LOVA) in ITER

A loss of vacuum accident (LOVA) is a unique event in fusion reactors. A reference event for ITER was postulated which was a failure of a penetration line (0.02 m2 cross-section) into a room with stack ventilation. Behavior of ingress air and environmental release of the inventory were calculated by an accident analysis code (MELCOR). No in-vessel component cooling was assumed because of a non-safety system and 1350 g-T as tritium (HTO) and 30 kg as tokamak dust (tungsten) were set in the vacuum vessel (VV) as the initial mobile inventory. Operation of the maintenance detritiation system (MDS) after 1 h was credited to limit the release. The analytical results showed that the environmental release of tritium (19 g-T) from the stack was a factor of 5 below the accidental release limit for ITER, and the release of dust (21 g) from the stack is a factor of 25 below the release limit. To investigate the ultimate safety margin, a hypothetical event was also analyzed which was a failure of a penetration line (0.2 m2 cross-section) into a room with ground level ventilation. Since the best estimate analysis assumed in-vessel cooling, which resulted in cooling down the walls, the mobile tritium inventory was reduced to 390 g-T. The large break size caused fast pressurization of the VV and thus the whole dust inventory (110 kg) was expected to be mobile. The ground level release was less than half of the no-evacuation limit under conservative weather conditions.