Experimental CHF study for the top of the reactor vessel lower head

Abstract

The critical heat flux (CHF) in the vicinity of an inclination angle of 90° for the reactor vessel lower head external wall was measured using three test sections of a two-dimensional slice. The top 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 first purpose of this study was to evaluate the CHF in terms of local conditions for the top of the reactor vessel lower head using a 2D small-scale test section and especially clarify the geometric scaling effect and the relationship for the CHF between a 2D slice test section and a 3D downward facing hemisphere. The second purpose is to predict the CHF for the realistic conditions of IVR-ERVC strategy and academically investigate the effect of additive, heater material and pressure. For the geometric scaling effect, three test sections, with respective radius of curvatures values of 0.15 m, 0.25 m and 0.5 m were used. To compare with the large-scale experiments, the heater material of type 304 stainless steel (SUS304) was used and the mass flux of 50 and 500 $kg/m^2s$ and the inlet subcooling near the saturated condition were considered. 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 2.5 m. However, the local conditions cannot suitably explain the relationship between the R=0.25 m and R=0.5 m 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...