Heat removal analysis in the AP1000 reactor's refuelling process

Abstract
A study has been conducted to find out how much heat removal in the refueling process in AP1000 reactor. The background of this research is, operation at a Nuclear Power Plant is strongly tied to periodical fuel replacement to guarantee future operations. The replacement of fuel assembly can be done by one-third of core or a half of core refueling. After the reactor shutdown, the spent fuel assemblies still generate decay heat that must be removed to prevent from fuel damage cause of heat buildup. The process of heat removal can be done by natural and forced convection modes. The purpose of this study was to obtain the cooling times after shutdown so that the fuel assembly can be transferred to the spent fuel storage pool (SFSP) with natural convection mode, and how minimum flow rate was required for cooling the 7 days spent fuel assembly by forced convection cooling in the SFSP. The requirement in the cooling was no boiling occurred in the channel. The calculation was carried out by assuming the inlet coolant temperature of 30°C and by using COOLOD-N2 code. The calculation results showed that for the one-third of core refueling and for cooling with natural convection, it took 21 days after shutdown, the new spent fuel assemblies could be transferred to SFSP, while for the forced convection cooling required minimum flow rate of 15 kg/s in the SFSP. The calculation result on the hot channel showed the outlet coolant temperature of 101.88°C, the maximum temperature of the outer cladding and the center fuel meat of 104.63°C and 105.21°C, respectively. As for a half of core refueling and for cooling with natural convection, it took 42 days after shutdown, the new spent fuel assemblies could be transferred to SFSP, while for the forced convection cooling required minimum flow rate of 25 kg/s in the SFSP. Calculations on the hottest channel showed the outlet coolant temperature of 90.54°C, the maximum temperature of outer cladding and the center fuel meat of 93.16°C and 93.75°C, respectively.