Study on characteristics of steady natural circulation flow for external reactor vessel cooling

The In-Vessel corium Retention (IVR) through the External Reactor Vessel Cooling (ERVC) is known as an effective accident management strategy for maintaining the integrity of the reactor pressure vessel (RPV) during severe accidents. The natural circulation, which is adopted in design of IVR, not only determines the amount of heat carried off from corium pool, but also has significant influence upon critical heat flux (CHF) of wall of RPV lower plenum. Thus, characteristics of steady natural circulation flow under external reactor vessel cooling (ERVC) condition are investigated analytically. The analytical model for circulation velocity is established by proper hypothesis and simplification. The mass flow rate of the natural circulation and void fraction for different inlet temperatures, core corium powers, gap clearances and submersed depths are obtained through solving analytical model. The results show that established model is feasible to estimate the natural circulation flow rate. Capability of natural circulation is enhanced when core corium heat power increases or inlet temperature increases. The gap clearance between insulation and reactor vessel has few influences on circulation flow rate after it exceeds certain value. The circulation flow rate decreases as submersed depth increases and this influence is obvious when inlet temperature increases. Keyword: severe accident; natural circulation; analytical model; external reactor vessel cooling