Numerical investigation on startup characteristics of high temperature heat pipe for nuclear reactor

•A numerical code is carried out based on the three-stage model of frozen startup of the heat pipe.•The heat transfer performance of a NaK heat pipe is simulated during startup from the frozen state.•Sensitivity analysis and heat transfer limitations of the NaK heat pipe are carried out. Heat pipe cooled reactor (HPR) has a good adaptability to portable power system, which is popular in recent years. Due to the particularity of alkali-metal working fluid, the deep understanding of startup characteristics of high temperature heat pipe from frozen state is essential for the development of HPRs. In this paper, a three-stage frozen startup model is developed to describe the thermal behavior of heat pipe during the startup process, and the continuum flow in the vapor space is modeled as a one-dimensional compressible flow. A numerical code is carried out, in which the governing equations are discretized by Finite Element Method (FEM), and then the code is used to simulate the startup performance of a NaK heat pipe in HPRs. Numerical results indicate that the heat pipe startup behavior can be well described by the three-stage model. The NaK heat pipe is successfully started with a final consistency temperature of 834 K, although the entire second stage is restricted by the sonic limitation. The startup lasts 1550 s in total, and enters the second and third stage at 230 s and 650 s, respectively. After 1500 s, the maximum Mach number of vapor flow is lower than 0.1, which verifies the rationality of the one-dimensional compressible flow model of vapor. This work could provide a reference for the design and application of HPRs.