The structure and characteristics of the sodium heat pipe in metrology were described. The flat-front startup model for the heat pipe was established and used to analyze the starting characteristics of the heat pipe. As an example

with the input power of 1000 W

the axial temperature distribution in the wall at the startup of heat pipe was calculated and compared with the literature values

indicating that the flat-front startup model is applicable to the sodium heat pipe in metrology. Turning point temperature is an important parameter in startup process of high temperature heat pipe. The turning point temperature of the sodium heat pipe and the start-up time under different input power were analyzed. With the vapor channel diameter of 100 mm and vapor density 0.31×10-4kg·m-3

the transition temperature is 628.2 K. With the input power of 800 W

1000 W

and 1200 W

the time for the hot-zone to move 300 mm in the heat pipe is 1200 s

919 s and 681 s

respectively. The sodium heat pipe has a larger diameter

which makes the heat pipe start easily without considering the limitations of heat pipe generally.