Safety analysis of NET magnet system: LIVA accident

The possible failures and related consequences on plant safety for the NET (Next European Torus) system have been analyzed. The event tree initiators which have been identified are superconductivity quench propagation, short circuit and arcing due to coil damages and coil breaks, loss of power to some coils, loss of coolant accident (LOCA) in helium pipes, and loss of isolation vacuum (LIVA) in the cryostat. The authors present a discussion of the LIVA sequence, which represents a possible cause of coil generalized overheating. An ingress of water into the cryostat is assumed as a consequence of the rupture of a divertor cooling system line. Due to the fast and nonuniform overheating of the coil, some conductors can break, allowing helium to spill into the cryostat and, at the same time, generating electrical arcs in series connection. The energy dumping system is assumed to fail. Helium entering the cryostat is heated and can pressurize the cryostat. A parametric study has been performed to evaluate the cryostat pressure transients