Experimental study to assess effects of ballooning and fuel relocation on the coolability of fuel rod bundle

For a large break loss-of-coolant accident (LB LOCA) in a pressurized water reactor, the clad temperature increases until the reflood phase. This causes ballooned fuel rods and fuel relocation, thereby reducing the flow passage area of the subchannels and redistributing the flow and heat transfer in the subchannels. The fuel relocation induces augmentation of power and temperature at the ballooned region simultaneously. In this light, research on the coolability of the deformed fuel and fuel relocation is very important for preventing and mitigating severe accidents. In the present study, reflood experiments were performed to investigate the coolability in the ballooned fuel rod geometry and fuel relocation condition under a LB LOCA for a Korean nuclear power plant, APR1400. The coolability experiments were carried out using three kinds of test sections. The first experiments using a 6 × 6 intact rod bundle were performed as a reference case. The second experiments simulated the deformed fuel using a 5 × 5 ballooned rod bundle. The last experiments were performed using another ballooned 5 × 5 rod bundle with a local power increase to simulate fuel relocation. The reflood phenomena were scrutinized for these three rod bundles. While there were not substantial differences in the peak cladding temperature between the intact and ballooned cases, the fuel relocated bundle showed high peak cladding temperatures.