The analysis of the characteristic of the CHF on the outer surface of the lower head

The in-vessel retention (IVR) strategy to accommodate the molten corium in the lower head of reactor pressure vessel (RPV) in core degradation accident through external reactor vessel cooling (ERVC) had been adopted in several types of advanced reactors, such as AP1000, APR1400 and Hualong One etc. It is crucial to maintain the heat flux on the outer surface of lower head lower than the critical heat flux (CHF) during the ERVC process. In this study the distributions of the CHF along the orientation angle of the lower head under the pool boiling, natural circulation flow and forced circulation flow condition during the ERVC process have been analyzed respectively. The results show that the variation of CHF with orientation angle would be non-monotonic. In the inlet and outlet region of lower head the thermal equilibrium quality xeq had the significant negative effect on CHF. In the middle region of lower head the CHF would generally increase with increasing orientation angle. (authors)