Optimal design of heat pipe cooling system of small modular reactor and experimental study of high temperature heat pipe

Abstract

In this study, a 10MW hybrid MMR (H-MMR) was designed to converge with renewable energy and energy storage system (ESS). A 12.1% enrichment hexagonal annular shaped UN nuclear fuel & solid core was designed with 990 heat pipes cooled. The heat pipe was designed with an annular wick structure with an outer diameter of 22 mm, an operating limit of 31.6 kW and a working fluid potassium that removes 20.4 kW of heat for 56 years without refueling. By designing a pool type sodium intermediate heat exchange system that circulates at an operating temperature of 550~650 oC and 150kg/s, heat from the heat pipe was transferred to the supercritical CO2 cycle through PCHE. The U-shaped reactor vessel auxiliary cooling system (RVACS) with a height of 5m is optimally designed to remove about 230 kW of heat through natural circulation of outside air in the event of an accident. By nodalization of the heat pipe system and reactor core, heat transfer and safety analysis tools for steady state and transient states was developed. Although H-MMR's heat pipe was designed through the existing performance correlation, there was no performance experiment and analysis of circular and D shaped heat pipe under supercooled frozen startup conditions. Therefore, sodium heat pipe and wick structure and experimental apparatus were manufactured, and frozen startup limit experiment was performed according to the shape and angle of the heat pipe. As the input power was increased, the sonic limitation due to chocking occurred, and dryout of the evaporator occurred and the temperature increased rapidly due to the capillary limit. "Transient limitation model at frozen startup" was proposed, and the capillary limit was predicted by numerically analyzing the pressure drop and effective heat transfer length and property of sodium vapor. At each horizontal, 45 o, and vertical angle of the circular heat pipe, the temperature surge was observed at the capillary limit of about 500 W, 1500 W, and 2000 W, and analysis was verified through experiments. As a result, capillary limitation may occur prematurely under conditions of overcooling and excessive power rise in frozen startup situations.