In nuclear safety operations, condensing heat transfer area is placed in a large pool of liquid to accommodate passive thermal decay. Such systems are subject to pool boiling and thermal stratification. Velocity and temperature measurements were carried out in a 300mm i.d. vessel with a central tube as the heat transfer area. For this purpose, particle image velocimetry (PIV) and hot film anemometry (HFA) were employed. Further, CFD simulations of this system were performed. An excellent agreement was found between the experimental measurements and the CFD simulations. For modeling, the boiling was an extension of the model of Krepper et al. (2007). The lift force was described according to the recommendations of Zeng et al. (1993). The stratification occurring inside the pool has been quantified in terms of a dimensionless number (stratification number). It has been observed that, for higher heat input rates stratification occurs in a shorter time period. The effect of submergence of the condenser tube in the large pool has been studied and it has been found that, for any height of submergence, vapors form at the top of the pool but placing the condenser near the bottom may reduce stratification to a certain extent. The model was extended to the real size (50 000mm I.D.) passive decay heat removal system.