Evaluation for effect of upper head nodalization in OPR1000 Plant

Nowadays, the best estimate method with the uncertainty evaluation has been broadly used worldwide in licensing of NPP. In Korea, the LBLOCA analysis using the best estimate (BE) methods to replace the old conservative evaluation method (EM) was performed by the licensee in several plants such as WH type Units, OPR1000 and APR1400. In the BE method, the nodalization for base calculation was defined from the comparative study of the experimental data and it could influence the accuracy of code for specific phenomena such as ECC bypass and blowdown quenching. The nodalization of upper head for OPR1000 and APR1400 was generally composed of axial several single volumes and the flow recirculation between the upper plenum and the upper head/dome was not considered significantly since the recirculation rate was much smaller than the primary coolant flow rate. However, there are actually the upward and downward flows in the upper head and this recirculation flow can transfer the heat of the upper plenum to the upper dome. Up to now, it was assumed that the temperature of the upper dome was close to that of the cold leg in most LOCA analyses to consider the effect of bypass flow from the downcomer to the upper dome. However, it was recently found that the temperature of the upper dome might be larger than that of the cold leg according to the heat exchange due to the recirculation flow and the detailed design data. Therefore, the high upper dome temperature can influence the modification of nodalization for upper head and the blowdown quenching behavior. In this study, the LBLOCA in OPR1000 was evaluated to identify the effect of upper head nodalization