Small Break LOCA Analysis for Optimization of Safety Injection System of Korean Standard Nuclear Power Plant

The Ulchin Nuclear Power Plant Units 3 and 4 (UCN 3&4) design was based on ABB-Combustion Engineeringʹs (ABB-CEʹs) standardized 3800 MWt Nuclear Steam Supply System (System 80). Despite the 25% core power difference between the two plants (3800 MWt for System 80, 2815 MWt for UCN 3&4), several design features have not been changed, especially the capacity of Safety Injection System (SIS). Focusing on this fact, the sensitivity study for UCN 3&4 SIS capacity on the behavior of the postulated small break Loss of Coolant Accident (LOCA) was performed to optimize the SIS capacity. Firstly, the sensitivity study for Safety Injection Tank (SIT) capacity on the postulated small break LOCA was performed by changing the size of SIT from 100% to 60% (100%, 85%, 75%, 60%, respectively) for the 0.5 ft2 (465 cm2) pump discharge leg break. Secondly, the sensitivity study of High Pressure Safety Injection (HPST) pump capacity on the behavior of small break LOCA was investigated by reducing its discharge flow rate of UCN 3&4 to 50% (100%, 85%, 75%, 70%, 60% and 50%, respectively) for the 0.05 ft2 (46.5 cm2) pump discharge leg break. Finally, the spectrum analysis was performed using the acceptable results of these two studies, i.e., the reduced SIT size and HPSI pump capacity to 70% of UCN 3&4 to retain sufficient margin for the small break LOCA. The results of this spectrum analysis show that the peak cladding temperature (PCT) and peak local cladding oxidation (PLO) for the limiting break size of 0.05 ft2 (46.5 cm2) pump discharge leg break were 1459°F (793°C) and 0.486% respectively, which are in compliance with the Emergency Core Cooling System (ECCS) acceptance criteria of 2200°F (1204°C) and 17% specified in 10 CFR 50.46. The results of this sensitivity study can be adapted to the optimal design of SIS for the future 2815 MWt Korean Standard Nuclear Power Plant and be contributed to the increase of economical gain.