Molten fuel radial motion during a high temperature transient in LWRS

Molten fuel radial motion during a high temperature transient is of interest to current light water reactor (LWR) safety analyses. Particularly, since partial or complete melting of the zircaloy cladding may occur upon the cladding being contacted by molten UO2-fuel, because of the large difference between the melting point of the fuel and that of the cladding. During a high temperature transient, fuel melting may be initiated at the center of the rods and propagate radially outwards. Fuel cracking, which usually occurs along radial and horizontal planes in the fuel pellet during reactor normal operation provides paths for such molten fuel to relocate outside the central melting zone. This paper presents an analytical study of the transient freezing of a superheated liquid that penetrates an initially empty crack maintained at a constant subfreezing temperature. Results of the analysis are applied to assess the molten UO2 fuel radial extrusion observed in some of the in-pile tests in which power-cooling-mismatch (PCM) conditions in a pressurized water reactor were simulated