Differences in LBLOCA thermal hydraulic response between 1D and 3D calculations

Best Estimate (BE) calculation of large break Loss of Coolant Accident (LBLOCA) has been applied to the most of the domestic Nuclear Power Plants (NPP). One of the recent concerns in LBLOCA analysis of APR1400 design was a potential of the 'blowdown quenching', which drastically decreased the fuel cladding temperatures by the downflow of the water from the Upper Guide Structure (USG) of the reactor vessel to the core. Depending on its extent, the cladding temperature response during reflood phase can change greatly, and then the uncertainty of reflood phenomena such as downcomer boiling may be more important. Due to the concern, a more specific analysis such as three dimensional (3D) calculation, has been required for the LBLOCA thermal hydraulic response. The present study is to discuss the 3D calculation and its result in comparison with that from 1D calculation. Several differences and their reasons are also discussed. MARS KS code was used for the 1D and 3D calculation, and the hot channel whose flow area is not greater than area of one fuel assembly was used