Experimental Study on Performance Influencing Factors of Small Modular Reactor Separated Heat Pipe Heat Exchanger

The separated heat pipe (SHP) is a novel passive cooling technology gaining attention in the third-generation nuclear power systems due to its high heat transfer performance, low cost, simple structure, and flexible layout adaptability. The ACP100 is China’s independently developed next-generation small modular reactor (SMR). In its passive safety design, SHP is considered key heat transfer component in both the passive spent fuel pool cooling systems (PSFS) and the passive main control room cooling system. The purpose of this study is to investigate in depth the heat transfer characteristics of the SHP by examining the impact of several key influencing factors. The research employed a fundamental experimental test facility, PEX00+, designed for PSFS applications. This facility was extensively instrumented with numerous temperature and pressure sensors. Adjustable temperature settings of the hot and cold sources, along with a variable-height hot source tank, provided the experimental basis for studying parameter effects. The study consisted of three parts. The effect of the filling ratio was examined first, concentrating on the startup behavior and operational stability of the SHP. Subsequently, the influence of the evaporator structure and material on the heat transfer performance was investigated. Three evaporator structures were evaluated, including a single-row tube evaporator, a double-row tube evaporator, and a circumferential tube evaporator. Additionally, helical tube evaporators fabricated from three different materials were tested, including 316 stainless steel, titanium alloy, and red copper. In the third phase, the impact of the working fluid type on heat pipe performance was explored, through a comparison of water, ethanol, and ethanol-water mixtures. The experimental results reveal several key findings. The startup process occurs in two distinct stages, and the establishment of a stable circulation loop is marked by a characteristic drop in the evaporator inlet temperature to near the condenser outlet temperature, where it subsequently stabilizes. For water, the heat transfer power first increases and then decreases with the filling ratio, reaching an optimum at 40%. Regarding evaporator structure, the double-row tube evaporator achieves a 4.2% higher average heat transfer power than the single-row design, while the circumferential tube evaporator exhibits degraded performance due to increased resistance. Among the evaporator materials tested, the titanium alloy evaporator achieves the highest heat transfer power, while the stainless steel evaporator displays superior startup characteristics. Regarding working fluids, ethanol exhibits superior heat transfer performance under the operating temperature conditions of passive cooling systems. Water demonstrates significantly better startup capability, operating effectively over a wider range of filling ratios. The findings derived from this experimental research provide guidance for future design studies of SHP.