The proposed safety feature considered in this study aims to increase the safety margins of nuclear power plants by proposed water tanks located inside or outside the upper zone of the containment to be utilized for (a) residual heat removal of the reactor in case of station blackout or in case of normal reactor shutdown and (b) beyond-design-basis accidents, in which core melt and debris-concrete interaction take place, associated with accumulative containment pressure increase and partial loss of the active systems. The proposed passive containment system can be implemented by a special mechanism, which can allow the pressurization of the water in the tanks and therefore can enable an additional spray system to start in case of increasing the containment pressure over a certain value just below the design pressure. A conservative case study is that of a Westinghouse 3411-MW(thermal) power station, where the proposed passive containment cooling spray system (PCCSS) will start at a pressure of 6 bars and terminate at a pressure of 3 bars. A one-dimensional lumped model is postulated to describe the thermal and hydraulic process behavior inside the containment after a beyond-design-basis accident. The considered parameters are the spray mass flow rate, the initial droplet diameters, fuel-cooling time, and the ultimate containment pressure. The overall heat and mass balance inside the containment are carried out, during both the containment depressurization (by the spraying system) and pressurization (by the residual energies). The results show that the design of the PCCSS is viable and has a capability to maintain the containment below the design pressure passively for the required grace period of 72 h. Design curves of the proposed PCCSS indicate the effect of the spray flow rate and cooling time on the total sprayed volume during the grace period of 72 h. From these curves it can be concluded that for the grace period of 72 h, the required tank volumes are 3800 and 4700 m3, corresponding to fuel-cooling times (time after shutdown) of two weeks and one week, respectively. This large quantity of water serves as an ultimate heat sink available for the residual heat removal in the case of station blackout. The optimal spraying droplet diameter, travel, and mass flow rate are 3 mm, 30 m, and 100 to 125 kg/s, respectively.