Experimental study for the effects of fuel relocation on the rod bundle

The injected coolant decreases temperature of the heated fuel rods for 40 - 200 second (reflood). After core vessel is filled by coolant, it is long-term cooling. During the single-phase steam flow in the early phase of the reflood, the cladding temperature may increase and have a peak value due to low heat transfer from the fuel to the steam. The increased temperature can make a ballooned fuel rods. As a result, the flow passage area of sub-channel is reduced and it leads the redistribution of flow and heat transfer in sub-channels. The flow rate through the sub-channel between ballooned fuel rods is decreased while the flow rate through the sub-channel between intact fuel rods is increased. The reduction of flow reduces the capacity of coolability and ballooned fuel rods have higher temperature than non-deformed fuel rods. If a LBLOCA condition and ballooned fuel rods are occurred, the effect of reduced flow passage on the convective heat transfer by single-phase steam flow is important phenomena to analyze the safety of a reactor. During the LOCA condition, accumulation of fuel debris in the ballooned region of the bust cladding, which resulted from fuel fragments slumping from upper regions, can be occurred.. This fuel relocation makes different thermal hydraulic behavior. The present experiments were performed in various Reynolds numbers (about 2600 - 13000) and Heater power (0.14 - 1.12 kW/m) to investigate the effect of the Fuel-relocation on heat transfer phenomena by single-phase steam flow. The experiments were performed in three rod bundles in KAERI reflood ATHER test facility. One is a non-deformed 6 x 6 rod bundle, which consists of 36 non-deformed heater rods. Experimental study of heat transfer phenomena by single-phase steam flow using three type heater bundles (intact bundle, ballooned bundle, fuel-relocated bundle) were performed to investigate the effect of fuel-relocation on single-phase steam flow. Fuel-relocated bundle showed its own characteristics