Design improvement for APR1400 inherent RPCS

The APR1400 RPCS (Reactor Power Cutback System) is designed to rapidly reduce the core power to eliminate the need for a reactor trip following a large load rejection or a loss of two main feedwater pumps at high power. According to GDC (General Design Criteria) 25, CPCS (Core Protection Calculator System) will apply a big power penalty (∼1.3) to the minimum DNBR (Departure from Nucleate Boiling Ratio) and the maximum LPD (Local Power Density) in case of any 12 finger CEA (Control Element Assembly) drop for the SAFDL (Specified Acceptable Fuel Design Limit) actually not to be exceed, and reactor may immediately trip by CPCS low DNBR and high LPD during high power operation. In order to avoid unwanted reactor trips due to a 12 finger CEA drop, the inherent RPCS design has been improved and evaluated for the design and plant adaptability for APR1400. In this development, the CPCS algorithm has been modified to send RPCS actuation signal and not to apply the penalty factors due to 12 finger CEA drop for a short period, and the safety and CPCS performance evaluations are performed. Safety and CPCS performance evaluation results show that the CPCS reactor trip is not occurred during RPCS operation after a 12 finger CEA drop. Also, the long period plant behavior is analyzed to establish plant recovery procedure after a 12 finger CEA drop. With the inherent APR1400 RPCS, it will be possible to enhance plant operability by reducing the unnecessary reactor trip due to a 12 finger CEA drop