Comprehensive analysis on suitability of alkali metals with high enthalpy characteristics for space nuclear Rankine cycle

The compatibility between working fluids and high-temperature thermal systems is essential for improving system performance, stability, and safety. This article focuses on exploring the relative adaptability of alkali metals in nuclear high-temperature Rankine systems under three operating conditions: different output powers, different core powers, and different mass flow, evaluating from three dimensions: thermal, economic, and weight. The study finds that the cesium and potassium cycles represent systems for low and high turbine inlet temperatures, respectively. At a turbine inlet temperature of 1100 K, the cesium cycle can enhance thermal efficiency by 32.14 %, while with the turbine inlet temperature increased to 1500 K, the potassium cycle can improve efficiency by 28.19 %. Under varying output power and core power conditions, the high-temperature potassium cycle demonstrates more significant performance advantages. Compared to the cesium cycle, both economic and weight performances are seen an increase of approximately 20 %. Under varying mass flow conditions, the cesium cycle has a significant weight advantage. The mass flow increases from 0.5 to 2.5 kg/s, and the proportion of specific weight for the cesium cycle compared to the potassium cycle decreases from 54.90 % to 12.93 %. However, the output power of the potassium cycle is ten times that of the cesium cycle with the same mass flow. This research offers valuable insights for selecting optimal working fluids in high-temperature thermal systems considering operating conditions and mission requirements.