Countercurrent Gas-Liquid Flow and Heat Transfer Charicteristics in Narrow Gap with Closed Bottom

Abstract

For severe accident assessment in a light water reactor, heat transfer models in a narrow annular gap between the overheated core debris and the reactor pressure vessel (RPV) are important to evaluate the integrity of the RPV and emergency procedures. This paper discusses countercurrent flow limitation (CCFL) in a narrow gap and heat transfer on heating surfaces based on CCFL calculations and quenching tests. Calculations of the momentum balance between gas and liquid phases showed that CCFLs are greatly affected by flow patterns in a narrow gap. Quenching tests in a verti-cal annulus gave almost the same heat transfer rate and CCFL constant as the quasi-steady state CHF (critical heat flux) tests when the flow patterns were the same in both tests. During the quench-ing of a heated wall with the superheated opposite-side wall, which simulated the cooling of the heated RPV wall facing the superheated debris, the CCFL constant was almost the same as the CCFL constant with the one-side heated wall. The results suggest that the correlation based on quasi-steady state tests with the one-side heated wall could be used to evaluate the total heat transfer rate and the coolant penetration flow rate into a narrow gap during the cooling of core debris.