Intrusion of Molten Steel into Cracks in Solid Fuel in a Transient-Undercooling Accident in a Liquid-Metal Fast Breeder Reactor

In a transient-undercooling accident in a liquid-metal fast breeder reactor with a small sodium void coefficient, steel cladding melts prior to disruption of the oxide fuel pellets. If molten steel could wet the fuel and penetrate into the interior of the fuel pin through the surface cracks, steel vapor pressure could cause an early breakup and possibly a dispersal of the fuel. The wetting and capillary properties of molten steel in contact with solid oxide fuel have been examined. Based on available indirect in-pile and out-of-pile experimental results, the known surface properties of steel and UO₂, the behavior of similar materials, and direct experiments on steel-UO₂ wetting properties, it is concluded that in an accident situation, molten Type 316 stainless steel will not wet oxide fuel at temperatures below the point of disruption of the fuel. Steel intrusions in the fuel will have no significant impact on the accident sequence.