Dynamic simulation and analysis of steam dump valve failure using secondary side model of PWR

The safety system reliability is a key issue for the nuclear power plant (NPP), and the steam dump system (SDS) is an important pressure stabilizing system used in Hualong Pressurized Reactor (HPR1000) to enable nuclear steam supply system to accept loss of load without incurring a reactor trip and to remove stored energy and decay heat following a reactor trip without actuation of the steam generator safety valves. The SDS is consisted of many steam dump valves, and its reliability is dominated by the reliability of steam dump valves. During the steam dump interlock inspection test of a NPP, the pressure of the main steam pipe in the second loop appeared shock, which triggered the protection signal because of the high main steam pressure difference between the second and other main steam pipes. The purpose of this study is to simulate the failures and explore the mechanism of SDS failure process. A dynamic simulation model is developed with APROS version 6.10, which includes components between the steam generator (SG) and the main steam valves of NPP. The compressed air leakage failure for the steam dump valve, which causes valve position reduction (0%, 10% and 20%) and adjust speed change, is included in the dynamic simulation, and it causes fluctuations and steady-state errors in system parameters such as the main steam pressure and valve opening position. In the case of 10% and 20% valve position reduction, the main steam pressure fluctuates between 7.3 MPa and 7.45 MPa, and the steady-state errors are about 0.02 MPa. Then, the safety valve will jump in the fluence of water hammer due to the unsteady mass flow rate of condensate water. The pressure can drop to 6.75 MPa caused by safety valve jump, and the safety injection system will be triggered to form a serious accident reaction if the take-off action sustains over 0.13 s. Results of this study would be helpful for the understanding of the SDS failures process and dynamic characteristics of the SDS for nuclear power units, and provide guidance for failure identification and safety system improvement work.