Investigation on a Complete Passive Cooling System Using Large-Scale Separate Heat Pipes in Spent Fuel Pool

Abstract

Large-scale separate heat pipes used in the complete passive cooling system (PCS) transfer the decay heat in the spent fuel pool (SFP) efficiently through the two-phase natural circulation without any external power. In this study, a lumped mathematical model for the heat pipes are developed and parameters related to the heat transfer ability are discussed to settle the number of the heat pipes under different heat load. For the condensers at the auxiliary building, the effects of the tube pitch and the fin height are discussed, which are key parameters to the heat transfer performance. Different structural designs of the PCS under typical operating conditions are settled. A larger quantity of heat pipes is required for higher decay heat power conditions. To validate the reliability of the PCS, transient three-dimensional simulations of the SFP with immersed evaporators under different heat loads are conducted. Based on the results, detailed thermal-hydraulic characteristics are captured in the pool. Large natural convection circulations are formed at the steady-state. Single flow circulation is formed in the X-Z plane under low heat load cases while a pair of counter-rotate natural convection circulations under high heat load cases. A larger heat load promotes the natural convection intensity and shortens the response time of the PCS. Proper distance between the heat source and heat sink in both vertical and horizontal directions in the SFP is beneficial to the flow organization, improving the heat transfer efficiency of the PCS. The maximum temperature in the SFP is always below the saturation point after the startup of the heat pipes, which could validate the reliability of the PCS and ensure the safety of the plant under emergency conditions.