Long-term passive residual heat removal characteristics of a large separate heat pipe considering the effect of non-condensable gas

• A 100 kW-class separate heat pipe is developed for passive cooling of spent fuel pool. • Start-up and heat transfer performance are quantified under non-condensable gas. • A prediction model is proposed to estimate the long-term operational performance. • Vacuuming strategies mitigate performance degradation and achieve large energy savings. As a passive cooling device in nuclear energy systems, the separate heat pipe (SHP) can effectively remove decay heat generated by spent fuel, preventing the occurrence of accidents, boiling, drying, and even melting in the spent fuel pool. However, in large SHPs, the inevitable presence and accumulation of non-condensable gas (NCG) over time significantly deteriorate the heat transfer performance, threatening the system’s long-term stability and efficiency. To address this issue, a novel SHP is proposed in this study, featuring a serpentine finned aluminum flat tube for the condenser and a capsule-type plate for the evaporator. Experimental investigations were conducted on the influence of NCG on both the start-up and heat transfer performance, including a 94-hour long-term operation experiment. Results indicate that higher NCG content prolongs the time required for the SHP to reach steady-state operation. When the NCG mass is less than 131.4 g, the measured heat transfer rate reaches 107.2 kW. By introducing a decay factor, an empirical correlation to predict performance degradation considering the initial vacuum degree, standby time and operation time is proposed. On this basis, appropriate operation strategies such as vacuuming before startup and periodic vacuuming are proposed to mitigate performance degradation caused by NCG. For a commercial nuclear power plant with a decay heat load of 6540 kW, compared with the fuel pool cooling and purification (PTR) system, the SHP system can reduce energy consumption by 690,622 kWh annually. These findings provide strong support for the deployment of large SHP in passive cooling systems.