عنوان مقاله [English]
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. The results showed the high probability of this phenomenon in the 100 mm break, which was observed after 294 seconds and the fluid velocity reached -3 m/s. The duration of this phenomenon is until half of the reactor core is filled with water (1175 s). Also, in the 25 mm break, when the water level of reactor pressure vessel dropped below the reactor outlet, the liquid velocity was negated in the hot leg of loop no.2. 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.