Experimental investigation of 3-D ERVC process for IVR strategy - CHF limits and visualization of boiling phenomena

In-vessel-retention (IVR) of molten core debris by external reactor vessel cooling (ERVC) technique has been proposed as the severe accident management strategy for advanced PWRs like CAP1400. CHF limits decide the maximum heat removal capacity of ERVC which have been widely tested by many investigators. Several researches have proved that the CHF limit and its mechanism are strongly related to the vapor motions and two phase flow characteristics. However, the two phase flow characteristics in the downward facing boiling process of ERVC have been rarely reported in the literature. In the present research, a three-dimensional visual experimental apparatus is developed to investigate the downward facing boiling phenomena and CHF limits for ERVC. The vapor morphology at different heat flux levels and inclination angles of the heating surface is carefully studied. The visual results show significant differences in vapor morphology between 3-D ERVC and 2-D slice ERVC experiments in the vessel bottom region. Vapor motion characteristics corresponding to the occurrence of CHF are examined by determining the boiling cycle frequency and maximum vapor height. The effect of inclination angle on the vapor morphology is identified and the local variation of CHF along the vessel outer surface is measured. Vapor motion characteristics are investigated quantitatively by advanced image processing technics. The dependence of boiling cycle on inclination angle is revealed. In addition, the effect of bottom heating on the downstream CHF behavior is investigated, and the mechanism responsible for the influence that is revealed by the vapor morphology is determined. The present study provides an in-depth physical understanding of the 3-D downward facing boiling process during ERVC that can be useful for hydrodynamic modeling of the CHF phenomenon.