Experimental Investigation on Heat Transfer Performance of Annular Heat Pipe for Passive IN-Core Cooling System

Abstract

The concept of a hybrid control rod is proposed for a passive in-core cooling system (PINC) for decay heat removal in a pressured water reactor (PWR). The concept aims to combine the roles of heat pipes and control rods in a PWR system. For effective application, the proposed hybrid rod is to be composed of a suitable neutron absorbing material, such as boron carbide, while providing for sufficient heat dissipation. The annular shape of the boron carbide pellets in the heat pipe provides a path for the working fluid. The objective of this paper is to investigate heat transfer performance across these annular spaces. A small scale heat pipe (L: 300 mm, O.D.: 12.7 mm) is analyzed and wall temperature distributions, overall heat transfer, heat transfer coefficients, and capillary limit are determined. Results show a heat transfer reduction due to face friction at the vapor and liquid boundary layers. Theoretical capillary limit of annular flow path heat pipe was slightly lower than concentric heat pipe and then both of predictions were underestimated compared to the experimental results. The liquid pressure drop was dominant for capillary limit, thus the cross-sectional area of vapor space had a low effect on heat pipe dry-out phenomenon. Experimental results indicate that the inner structure of the heat pipe has some effects on performance. It is also confirmed that the annular heat pipe concept of passive decay heat removal works sufficiently.