[Objective] Small nuclear power plants, which find applications in various fields, are associated with such characteristics as compact size, low investment cost, and high flexibility. Analyzing the off-design performance of the secondary circuit in small modular reactor (SMR) power plants is crucial in terms of optimizing the operation and enhancing the overall performance of nuclear power units. Moreover, doing so serves as a valuable reference point for investigating prospective thermal utilization processes, including nuclear hydrogen production and urban heating.[Methods] This study examines the secondary circuit system of an SMR power plant, establishing a comprehensive and validated system model using the software EBSILON. The simulation is based on the principles of mass, momentum, and energy conservation, employing the Gauss-Seidel iteration method. The primary and secondary circuits of the power plant employ helium and water as coolants, respectively, with the main steam pressure and temperature set at 13.24 MPa and 566℃, respectively. The main steam pressure remains constant under off-design conditions. The system comprises two steam generators, a steam turbine, a condenser, a shaft seal heater, two high-pressure regenerative heaters, three low-pressure regenerative heaters, a deaerator, and pumps. The analysis focuses on exploring the effects of power level variations, turbine back pressure changes, and different regenerative heater removal methods on the system's performance. The power level ranges from 30% turbine heat acceptance (THA) to 100% THA, while the steam turbine back pressure varies from 2.5 kPa to 4.5 kPa. Four different heater removal methods are considered. Notably, the fourth method involves the normal operation of all heaters, serving as a control group for comparison purposes.[Results] The simulation results showed that:(1) Quantitative analysis revealed that power level changes considerably impacted various parameters, such as power generation efficiency, heat consumption rate, regenerative extraction coefficient, and high-/low-pressure cylinder output distribution. As the power level decreased from 100% THA to 30% THA, the power generation efficiency decreased by 5.427%, while the heat consumption rate increased by 1 210.487 kJ·(kW·h)-1. (2) Under the 100% THA operating conditions, reducing the turbine back pressure by 2.0 kPa boosted the power generation efficiency by 1.444%, decreased the heat consumption rate by 272.338 kJ·(kW·h)-1, and increased the power output by 7.120 MW. (3) Taking the main feedwater temperature as an example, partial or total removal of high-pressure heaters caused a decrease of more than 30℃ in the main feedwater temperature; however, the removal of only the low-pressure heater L1 slightly affected the main feedwater temperature. (4) Among the three variables, the power level had the most prominent impact on the thermal performance of the system.[Conclusions] The system's thermal performance is optimized under 100% THA conditions. It is conducive to improving the performance of the units by appropriately reducing the back pressure of the steam turbine. Additionally, cutting off the regenerative heaters can affect the system's thermal performance, but the impact varies with different cutting-off methods. Total removal of high-pressure heaters has a more prominent impact on the system's thermal performance than partial removal of high- or low-pressure heaters and causes a decrease in the feedwater temperature. This study can serve as a valuable reference for the operation and future expansion of SMR power plants. © 2024 Press of Tsinghua University. All rights reserved.
