Simulation of the cladding freezing during the loss of flow accident in a Gas-Cooled Fast Breeder Reactor

The accident condition of the stainless steel cladding melting, relocation and freezing, following a loss of flow situation in gas-cooled fast reactors has not been determined yet. An alloy of 50Pb/50Sn was selected to facilitate experimental procedures because of its workability and its wide phase transition temperature range (183 to 2160C) similar to that of stainless steel (1375 to 15000C). The objective of the experiment is to simulate the motion of liquid alloy through a tube and observe the conditions where it solidifies in the tube. The objective of the theoretical model is to use transient heat transfer analysis to describe the freezing front of the moving liquid metal and the plugging of the channel. Tests were conducted in alloy tubes having inside diameters 0.5 to 0.95 cm. Molten solder was poured through the vertically held tubes. The average falling velocity of the melt was measured to be about 89 cm/sec. The distance in the tube where the molten Pb/Sn solidifed across the diameter was measured. This penetration length varies from 20 to 40 cm as the initial liquid temperature ranges from 216 (liquidus point) to 5000C. The plugging time is calculated for those temperatures under which data on both the penetration distance and the falling velocity were obtained