Importance of natural convection to in-vessel melt coolability

In the event of a core meltdown accident, one of the accident progression paths is fuel relocation to the lower reactor plenum. In the Heavy Water New Production Reactor (NPR-HWR) design, the reactor cavity is flooded with water. In such a design, decay heat removal to the water in the reactor cavity and thence to the containment may be adequate to keep the reactor vessel temperature below failure limits. If this is the case, the accident progression can be arrested by retaining a coolable corium configuration in the lower reactor plenum. The strategy of reactor cavity flooding to prevent reactor vessel failure from molten corium relocation to the reactor vessel lower head has been discussed in this document.