Evaporator overheating and dryout of a sodium heat pipe with visualization using x-ray radiography

• Cooling intensity and inclination angle alters initial sodium distribution, affecting heat pipe startup. • Pool level at various cooling in sodium heat pipe are visualized. • Heat pipe overheats and dryout at small inclination angles, high cooling intensities, or small filling ratios. • Capillary limit model inaccuracies stem from temperature uniformity assumptions and sodium pooling. Sodium heat pipes, with their passive operation and high thermal transport efficiency, are well suited for integration into nuclear microreactor systems. A comprehensive understanding of their performance across a wide range of operating conditions is essential, especially under scenarios susceptible to evaporator dryout and localized overheating. In this study, these phenomena were investigated using temperature measurements complemented by high-resolution x-ray radiography. The effects of key parameters — specifically sodium filling ratio, inclination angle (including negative inclinations down to -15°), and condenser cooling intensity (up to 360 W/m²·K) — were examined to assess their influence on startup behavior, capillary limit performance, and operable regions. Results indicate that partial wetting of the evaporator facilitates startup, whereas complete dryout leads to severe overheating and potentially startup failure. Excessive cooling was found to promote sodium solidification in the condenser, which impedes liquid return and degrades overall heat transfer performance. Moreover, deviations from classical capillary-limit models were observed under conditions of strong axial temperature gradients, reflecting the influence of temperature-dependent sodium properties on capillary behavior. Under negative inclination, the opposing gravitational head further restricts liquid circulation, thereby intensifying evaporator overheating and limiting operable range. These findings provide new insight into the combined effects of parameters on overheating and dryout, contributing to the development of robust sodium heat pipe designs for advanced microreactors.