The Westinghouse AP600 containment structure is a steel containment vessel surrounded by a thick concrete shield building. A passive containment cooling system applies gravity-drained water to the outer surface of the steel containment shell to remove heat by evaporation and convection. Mass transfer is the dominant means of containment heat 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. During 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 momentum-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 transfer to the containment shell will flow downward. Both the plume and wall layer entrain bulk mixture, acting to circulate the bulk mixture. The fluid dynamics leads to a time-averaged vertical gradient of steam concentration. Simple integral entrainment relations have been examined to assess the order of magnitude of vertical steam concentration differences that may occur in the AP600 containment during the long-term LOCA transient.