Experimental and CFD studies of condensation with non-condensable and air natural convection in a passive decay heat removal system

Abstract

Passive safety design concepts have been emphasized to remove decay heat even under the station blackout (SBO). In the first part, the design concept of Air-cooled Passive Decay Heat Removal (APDHR) system was suggested, and its passive heat removal capacity was verified. Up to 3 days after the reactor shutdown caused by non-LOCA accident, water in the Passive Condensate Cooling Tank (PCCT) was used to condense the steam in secondary side. Since then, the steam was cooled by natural convection of air passively and indefinitely. Both, finned and bare heat exchangers (HXs) were considered for the design optimization of the APDHR. By establishing a fin geometry optimization methodology using CFD simulation, reference fin geometry was determined. Then, the sensitivity of several design parameters of APDHR, such as pitch, height, wall temperature of the HXs and the interval of spacer grids, was checked and determined by CFD in the view of better heat removal capacity and economic construction. Then, thermal-hydraulic simulation by the MARS code was conducted to investigate the behavior of the APR+ selected as a reference plant for the simulation. The simulation contains two phases based on water depletion: the early phase and the late phase. In the early phase, the volume of water in PCCT was determined to avoid the water depletion in three days after shutdown. In the late phase, when the number of the HXs is greater than 4,089 per PCCT, the MARS simulation confirmed that the long-term cooling by air is possible under extended SBO.

In the second part, experimental and CFD-based studies on condensation in the presence of non-condensable gas was conducted to analyze the performance of a Passive Containment Cooling System (PCCS). External condensation experiments were performed in 2~4bar, air mass fraction in 0.2~0.7 with 1~3 cylindrical tubes. Horizontal tubes gave 20% higher heat transfer coefficient (HTC) over vertical tubes and smaller diameter tubes gave higher HTC over larger diameter tubes. Based on the experiment, we suggested the single tube condensation correlation which well predicted other condensation experimental data within 15%. However, the the development of the condensation model for the tube bundle is very expensive to be resolved through an experiment. Therefore, CFD simulation was performed to analyze the condensation phenomenon by reflecting a realistic accident condition. The CFD best practice guideline was suggested to give the reliable CFD simulation results. We used the COPAIN experiment to validate physical-based governing equations of the CFD condensation model, mesh scheme, proper turbulence model, and wall boundary condition. With the best practice guideline, a parametric study was addressed to see the effect of the tube bundle in terms of the ratio of HX area and vessel volume. And a bundle correction factor was suggested to deal with 30~50% of HTC enhancement in the tube bundle. PCCS design with the horizontal tube bundle is suggested, which enables us to significantly reduce the number of tubes required compared to vertical tube bundle design suggested by KHNP because of the larger tube surface area with longer tube length, and higher HTC by an inclination enhancement factor.