Effect of new drywell cooler application for venting start time during severe accidents

Abstract

Nuclear power plants are equipped with a hardened containment venting system and a filtered containment venting system to avoid a primary containment vessel (PCV) break by over-pressurization. Considering the gas release from these venting systems, it is desirable to delay venting start time or to avoid operation of the systems by ensuring sufficient PCV cooling in view of the time needed to evacuate residents and the half-life of the radioactive materials.We have been developing a new drywell cooler specialized for severe accidents (DWC_SA) as one additional PCV cooling method. The DWC_SA can cool the PCV by continuous steam condensation using downward flow induced by natural force without any electrical devices in the PCV. We have already conducted element tests using a small tube bundle to confirm the occurrence of continuous downward flow with the DWC_SA configuration and to measure heat transfer rate of condensation with non-condensable gas (NCG). The heat transfer model was validated using the element test data. Here, to estimate plant performance of a representative advanced boiling water reactor (ABWR) applying the DWC_SA, we carried out a severe accident analysis using the MAAP code. We originally installed the heat transfer rate map of the DWC_SA as a function of pressure and NCG ratio in the code. The velocities using in the heat transfer model were estimated from CFD analysis. According to the severe accident analysis results, the DWC_SA can extend venting start time sufficiently to allow evacuation of residents in the accident scenario (LOCA with large pipe failure + ECCS failure + SBO) with the external water injection into the PCV and it can avoid operation of the containment venting systems in the case of internal water circulation in the PCV.