Study on pressure variation and its impact on the essential service water system of nuclear power plants

The Essential Service Water (SEC) system of the CPR1000 unit serves as the ultimate heat sink for the nuclear island system of the power plant. Its distinctive redundant configuration (with standby trains configured) gives rise to specific operational transients, which may induce water hammer phenomena. To investigate this issue, this paper establishes six typical operating conditions of the SEC system using the one-dimensional thermal hydraulic system code RELAP5. It conducts a comparative study on (1) the impact of the operating status of the standby pump on water hammer during startup and shutdown transients, and (2) the impact of gas accumulation in the horizontal pipe after the pump on transient water hammer pressure and the system. The findings demonstrate that pump commissioning triggers nonlinear water hammer effects, and the pressure peaks do not follow a linear superposition principle. Of particular significance, gas accumulation acts as a primary risk amplifier, which notably exacerbates pressure oscillations and elevates pressure peak magnitudes. This research provides critical insights and empirical evidence for optimizing the operational strategy of the SEC system and mitigating water hammer risks, thereby safeguarding the reliability of the ultimate heat sink of CPR1000.