Evaluation of penetration length of molten metallic fuel with one dimensional model prepared for the analysis of core disruptive accidents

The extent of penetration of flowing molten fuel in coolant channels before solidification is an important factor in the analysis of hypothetical core disruptive accidents in a FBR (fast breeder reactor). Previous studies indicated that fuel freezing behavior can be classified into two types: conduction-limited and bulk freezing. Considering the properties of metallic fuel, bulk freezing is an adequate mode for metallic solidification. Analytical code has been developed to estimate transient penetration. Calculations have indicated the following safety features of metallic fuel. (1) The critical velocity of molten fuel dispersion from a cladding breach is high enough to result in deep penetration, in which even a fresh pin could yield about 1 MPa pressure. (2) Lower velocity, higher fission gas containment, lower fuel temperature, and lower cladding temperature may result in a shorter penetration distance. (3) Even assuming worse conditions for deep penetrating, a penetration length of 20 cm may be expected, which would correspond to a sufficient negative feedback. (author)