A VVER1000 primary side model in Apros

Abstract: Nuclear power plants generate electricity by means of splitting atoms. The basic safety requirements and objectives are to protect the people, society, and the environment from radiological accidents, limit harmful effects of ionizing radiation during operation and take all reasonable practical steps to prevent radiological accidents. Defense in depth is the concept of preventing and mitigate accidents with multiple layers of protection and is applied in nuclear power plants. Safety systems and safety criteria from regulatory authorities are put in place to ensure defense in depth and fulfill the safety requirements and objectives. The high-pressure injection system injects high concentrations of boron acid into the primary side of the plant, reducing reactivity and power. It has three lines connected to the cold leg of three out of four main coolant loops. Each line uses a piston pump to pump borated water from boron tanks into the primary side. The system is designed to suppress positive reactivity without a pressure drop on the primary side. For this work, the high-pressure injection system is activated at 107% nominal power, a condition for when SCRAM normally is activated. The amount of boron introduced to the system is decided by two main factors, the volumetric flow rate and the boron concentration. System codes for modelling and simulation of power plants have long been used for analysis of reactor dynamic behavior. The modelling and simulation software Apros has been developed for the purpose of modelling nuclear power plant systems. This thesis is conducted at Westinghouse Electric Sweden AB with the purpose of modelling the primary side of a VVER1000. The license, learning material and documentation were provided by the company. A sensitivity study of the boron concentration contra volumetric flow rate of the high-pressure injection system was performed to see if one factor had a larger effect than the other on the primary side. The sensibility study explored two scenarios where reactor trip is unavailable. One scenario where all the control rods are extracted and get stuck and another scenario where all the rods are fully withdrawn, increasing power, temperature and pressure, triggering the pressurizer pressure relief system. The analysis focused on the effects on power and reactor outlet pressure. Results showed that volumetric flow rate affects the system more than boron concentration. In particular, when volumetric flow rate increased to 8.3 m3 /h , the pressure relief system did not activate while it did for 7.3 m3 /h , suggesting that for a limited power increase rate and high enough volumetric flow rate, the high-pressure injection system dampen reactivity, and in extension, pressure enough to not activate the pressure relief system. For future work, the natural continuation of this work is to explore a larger range of boron concentrations and volumetric flow rates. Obtaining validation data and validating the model could yield results that are not purely theoretical.