Effect of regulation valves (RV) installation in high pressure injection system (HPIS) pipelines on the formation of reactor pressure vessel (RPV) thermal stress conditions was analyzed. Modernization is implemented at South-Ukrainian nuclear power plant (SUNPP) Unit 1 within the framework of life extension, which finished by the end of 2013. The main goal of the modernization is to expand the HPIS functionality for small leak accident and protection against the cold overpressurization due to flow rate and primary pressure effectively regulation. The thermal hydraulic model for RELAP5/mod3.2 code with detailed downcomer (DC) model and changes in accordance with modernization was used for calculations. Detailed (realistic) modeling of piping and equipment was performed. Also, an algorithm for the RVs was developed. Applying of cooling water flow rate regulation avoids excessive primary cooling and, consequently, helps to preserve the RPV integrity and to prevent reaching through crack formation, which can lead to a severe accident.

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