Dryout heat flux for core debris bed. (II) Effects of particle size mixing and coolant flow.

Abstract

For gaining basic data of post-accident heat removal, coolability of degraded core debris bed was studied with emphases on the effects of particle mixing and coolant flow, as a sequel to the effects of system pressure and particle size in Part (I). A fuel debris bed with decay heat was simulated by steel ball particles induction-heated in a water-filled 50 mm I.D. pyrex glass cylinder. The system pressure was fixed at 0.1 MPa and the bed height at 8 cm. The particle diameters were 3.0, 2.0, 1.0, 0.7 and 0.3 mm. The mixture of 1.0 and 3.0 mm or 1.0 and 2.0 mm particles was used as the simulate debris bed. The dryout heat flux for a mixture bed agreed fairly well with the Lipinski's model when a simple average diameter weighted by number was taken as the equivalent diameter, the same as for homogeneous beds. This means that small size particles with large number govern the dryout. The dryout heat flux increases with mass flux of coolant flow, asymptotically approaching to the complete vaporization of coolant. Coolability of debris bed can be greatly improved by a small amount of flow, which is produced by natural convection due to boiling, when coolant can flow into the bed from the bottom.