The effect of initial and boundary conditions on the reactor vessel Pressurized Thermal Shock (PTS) was analyzed. For example unit 1 of South-Ukrainian Nuclear Power Plant (NPP) with WWER-1000/V-302 reactor was selected. Thermal-hydraulic analysis was made in accordance with methodology of Reactor Pressure Vessel (RPV) brittle fracture analysis by using RELAP5/mod3.2 code. Also, a detailed model of downcomer, which provides a realistic behavior of the fluid, including the mutual mixing of flows with different temperatures was used for these calculations. As an Initial Event (IE) the large secondary leak, namely Steam Generator (SG) Main Steam Line (MSL) break, was considered. According to the International Atomic Energy Agency (IAEA) recommendations all calculations were performed under conservative approach. Also, the asymmetric overcooling was considered. As a result of a series of thermal-hydraulic calculations the most conservative scenario for a group of IE associated with the secondary leaks was obtained.

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