The safety of the containments of both of the conventional and advanced power reactors is investigated by analyzing the thermalhydraulic behavior within the containments following the loss-of-coolant accidents and the steam line breaks. A detailed examination is made of physical phenomena occurring within the containment during the accidents: the distribution of blowdown fluid between the sump region and the steam region, condensation, heat and mass transfer between the liquid and vapor regions and heat transfer to the heat sinks, etc. Sensitivity analysis is performed to Study the effect of the uncertainty of the heat transfer coefficients. The modeling of the spray and fan cooling heat removal system is also implemented in the code. For verification, present code is compared against the CONTEMPT code. In order to study the characteristics of the heat removal of the passive containment, the performance of the containment with the Passive Containment Cooling System(PCCS) is examined using the present code. The PCCS water storage tank is designed to cantain sufficient water to adequately wet the containment surface for 3 days in order to cool the internal containment heated by decay heat. It is formed that the passive containment without the internal spray system is not effective to remove blowdown energy compared to the conventional one with the internal spray cooling system, but to remove decay energy.