The effect of the geometric scale on the critical heat flux for the top of the reactor vessel lower head

Abstract

The critical heat flux (CHF) on the top part of the reactor vessel lower head external wall was measured using three test sections of a two-dimensional slice. The top region (inclination angle of 90°) of the test section, which simulates the reactor vessel lower head for the in-vessel retention through the external reactor vessel cooling (IVR-ERVC) strategy for severe accidents, was primarily considered in this study. The purpose of this study was to produce experimental data to assess the CHF limits on the top of the external vessel wall for the metal layer and investigate the geometric scaling effect on the CHF. Three test sections, with respective radius of curvatures values of 0.15m, 0.25m and 0.5m were used. The width and gap sizes were 0.03m and 0.03–0.06m, respectively. The material of the test section was type 304 stainless steel (SUS304). The CHF results were acquired under an inlet subcooling of 2 and 10K with mass fluxes between 50 and 400kg/m2s. The experimental results were compared with the results of large-scale experiments. The ULPU data were generally greater than the CHF data in this study, which is due to the surface property. Using the CHF data from Jeong et al. (2005), the CHF results were discussed based on the local condition for scales between R=0.15 and 2.5m. However, the local conditions cannot suitably explain the relationship between the R=0.25m and R=0.5m test cases. To eliminate the inconsistency in the CHF relationship of the mass flux and exit quality conditions, a flow visualization technique was used, and the formation of a two-phase boundary layer flow was observed. Applying the scaled-down gap size, the exit quality values for the experimental results were modified. CHF correlations were developed with the data from this study and from Jeong et al. (2005).