In-vessel retention for the AP600 design during severe accidents

The AP600 design features simplify the plant design, reduce operator intervention, and provide additional safety. Core melt frequency analyses (Bruschi and Vijuk, 1990) show that the AP600 core damage frequency is well within the NRC safety goal for future LWR plants. This conservative estimate indicates the ability of the AP600 design to satisfy the forward-looking safety goals. Even given the lower likelihood of core damage events for this design, the AP600 containment design features also provide a mechanism for maintaining the reactor coolant system boundary and thereby extend the defense in depth concept. The Westinghouse AP600 design features result in substantial water accumulation within the containment during numerous, postulated accident sequences. Based upon the AP600 containment configuration and the several, passively injected safety systems, during accident conditions the containment water level would not only be sufficient to submerge the reactor pressure vessel (RPV) lower head but also to submerge a substantial fraction of the vessel cylinder. Under these conditions, should an accident progress to slumping of core debris into the vessel lower plenum, significant energy could be removed through the vessel wall with nucleate boiling on the vessel outside surface. Thus, the reactor vessel could be sufficiently cooled to prevent downward failure of the reactor vessel and release of the core debris into containment. This would eliminate consideration of ex-vessel events such as: direct containment heating, core-concrete attack, ex-vessel debris coolability, ex-vessel steam explosions, ex-vessel fission product release