Quantifying the effects of break source flow rates on AP600 containment stratification

The Westinghouse AP600 containment structure is a steel containment vessel surrounded by a thick concrete shield building. A passive containment cooli ng system applies gravity-drained water to the outer surface of the steel c ontainment shell to remove heat by evaporation and convection. Mass transfe r is the dominant means of containment hear removal on both inner and outer steel shell surfaces. On the inside, condensation on the containment shell dominates heat removal and is influenced by the distribution of steam and noncondensible gases. The AP600 design basis analysis for containment does not rely on fan coolers or sprays to homogenize the internal atmosphere. Du ring the post-blowdown phase of a loss-of-coolant accident (LOCA) transient , mixing due to break momentum may be neglected by assuming momentum to be dissipated within the break compartment, conservatively minimizing source m omentum-induced mixing. One or more buoyant plumes will rise from openings in the operating deck, and a wall boundary layer induced by heat and mass t ransfer to the containment shell will flow downward. Both the plume and wal l layer entrain bulk mixture, acting to circulate the bulk mixture. The flu id dynamics leads to a time-averaged vertical gradient of steam concentrati on. Simple integral entrainment relations have been examined to assess the order of magnitude of vertical steam concentration differences that may occ ur in the AP600 containment during the long-term LOCA transient.