Visualization study on molten metal jet breakup phenomena in horizontal and vertical injections for SFR with metallic fuel

In the sodium-cooled fast reactors (SFRs), the hypothetical energetic core disruptive accident (HCDA) is one of the major concerns for the safety of the SFRs. If the molten fuel is not fragmented and dispersed, there would be a possibility of recriticality of significant energetic reaction. This means that it is necessary to identify whether the molten jet is broken and fragmented well enough in the channel or core. In particular, metal fuel is known to have relatively lower possibility to reach up the HCDA compared to oxide fuel. The current Korean SFR program emphasizes the adoption of the metal fuel design for the reason. In order to investigate the dominant phenomena for the jet breakup and the fragments, the visualization experiments were conducted by injecting the molten jet into the coolant in a pool tank. The wood's metal and water were used to simulate the molten jet and the coolant, respectively. The experimental data showed that the jet breakup and the size of fragments were dependent on the jet velocity, the jet diameter, the initial temperatures of two simulants, and the injection gap length. High speed camera was used to observe the breakup behavior of the molten jet for each experimental condition. Thermal effects on the fragmentation were observed only with the initial temperature conditions. Other experimental variables showing hydraulic effects were closely related to the jet breakup phenomena. After each experiment, the debris was characterized to analyze characteristics of the jet breakup. In addition, the current study shows effects of the horizontal and narrow-down injection into a subchannel of a fuel assembly. (author)