Determination of Geometrical and Operating Parameters of PRHRS for VVER Reactors: Cooling by Natural Circulation of Atmospheric Air H¨ useyin Ayhan Hacettepe University Hacettepe University, Beytepe Campus, Nuclear Engineering Department 06800, Ankara, Turkey huseyinayhan@hacettepe.edu.tr Cemal Niyazi S ¨ okmen Hacettepe University Hacettepe University, Beytepe Campus, Nuclear Engineering Department 06800, Ankara, Turkey cemalniyazi.sokmen@hacettepe.edu.tr ABSTRACT Passive safety concept was proposed to improve safety and reliability of nuclear power plants. However, for the nuclear technology, passive systems require a special attention, since the disadvan- tages for their use come from the difficulties in their thermal-hydraulic design compared with active systems. One of the strong dynamics of the working principle of passive systems is natural circu- lation. Natural circulation phenomena play 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 phenomena are studied with reference to station blackout scenario in a VVER type nuclear power plant. Thermal- hydraulic calculations of Passive Residual Heat Removal System (PRHRS) were performed in order to evaluate the natural circulation performance of the VVERs. For many geometrical combinations and boundary conditions were investigated. Keywords: Natural Circulation, PRHR Systems, VVER 1 INTRODUCTION Thermal-hydraulic design of passive systems is more complicated and difficult compared to active systems. So passive systems have to be designed carefully. One of the mechanism of the passive systems is Natural Circulation (NC). Extended use of the NC principles is seen in nuclear technology especially in the reactor cooling by passive systems. With the help of this driving mechanism of nature, we gain several advantages. For example simplifying the systems, reducing the costs and increasing the safety level. After shutdown, the residual heat generation rate of a reactor is about 6-7% of the nominal reactor power. After a couple of hours, it is about 1% of the nominal power and it decreases with time. Since residual heat generation continues for a long period after shutdown, core cooling must be continue. The Passive Residual Heat Removal System (PRHRS) is considered as an important safety sys- tem. Because it is designed for the decay heat removal and reactor plant cool-down after reactor stopped, since there is no power in the system for a long time. So Passive Residual Heat Removal (PRHR) via steam generators is designed for long-time residual heat removal from the core to the ultimate heat sink via the secondary side at beyond design basis accidents. 222.1