Analysis of Reflux condensation phenomena during small break loss of coolant accident in the Bushehr VVER-1000 reactor

In this study, the Reflux condensation phenomena are investigated during the small break loss of coolant accident in the VVER-1000 nuclear reactor. The accident is chosen as 25mm and 100mm of pipeline break in the cold leg between the main coolant pump and reactor inlet nozzle. The analysis is performed using the RELAP5/Mod 3.2 Code for nodalization and simulation of the nuclear power plant. The designed model for calculation is based on standardized performances of VVER-1000 reactor type. So, modeling of the reactor pressure vessel, the primary system (Main coolant pipeline, reactor coolant pumps, pressurizer and, emergency core cooling system), steam generators and secondary system (Steam-line, feed-water, safety valves) is carried out to achieve a complete model of the BNPP. For a conservative analysis of the accident, the loss of power to the NPP and the failure of two diesel generators are considered when SB-LOCA occurs. Those limitations cause the malfunctioning of two channels of emergency cooling systems. Also, by considering the single failure criteria, one accumulator is failed throughout the accident process. The results show a high probability of this phenomenon at the 100 mm break. Therefore, by converting steam to the liquid after the reactor shutdown, some of the decay heat is transferred to the secondary circuit and the reactor vessel is filled with water sooner. These factors provide better safety for the fuel rods and reactor core.