Passive safety systems are considered as the systems which improve safety and reliability of the Nuclear Power Plants (NPP). Although they seem helpful, for the nuclear technology, passive systems require special attention, because of the disadvantages for their use due to the large difficulties in the thermal-hydraulic design compared with the active systems. One of the important dynamics of the operational principle of passive systems is natural circulation. Natural circulation phenomenon plays an important role in energy transfer from hot zones to the cold zones without using a mechanical pump. In all light water reactors, natural circulation is an important passive heat removal mechanism. In this study, the natural circulation phenomenon was studied with reference station blackout scenario in VVER type NPP. Thermal-hydraulic calculations of Passive Residual Heat Removal System (PRHRS) were performed using RELAP5 system code. This study was performed to evaluate the natural circulation performance of PRHRS for the VVER type NPP.