A major contributor to the core damage frequencies (CDF) of current BWRs is staion blackout. To address this issue head on, the ABWR has a steam driven, high pressure safety system. It does not rely on ac power and provides diverse protection against common mode failure. For added protection, the ABWR has an on-site, dedicated combustion turbine to provide a diverse source of emergency power to any of the three divisions of safety systems.
Because the ABWR has more design margins, the frequency of transients resulting in reactor scram or initiation of a safety system has been greatly reduced. Fewer initiating events directly reduces the ABWR's CDF. Furthermore, the impact of such failures, or accident scenarios, in mitigated because there is no core uncovery for any design basis accident; because there is a third, high pressure system (compared to two for previous BWRs) to provide diverse protection; and becau se each safety system has dedicated heat removal capability to ensure recovery from and accident.
ABWR come equipped with features to passively mitigate the consequences of a severe accident. One of these is a system which automatically floods the containement area below the reactor vessel should there ever be a "core on the floor". The radiative heat of the core debris melts a fusible valve, thereby releasing water from the suppression pool in the ABWR. This quenches the core debris and limits the amount of non condensable gas which is generated from the concrete-core reaction. The ABWR also includes a system to prevent catastrophic failure of the containment. When containment pressures near design limits, a rupture disk located in a hardened vent opens. This creates a path from the air space above the suppression pool to the atmosphere for steam and heat to be released until the operator manually closes valves in the vent. Most fission products are retained in the suppression pool.