Traditional design of CPR1000 nuclear plants has a lot of studies on engineering safety facilities which are important for Design Basis Conditions (DBC). Based on more recent studies on safety, such as Defense in Depth (DID) concept, Probabilistic Safety Analysis (PSA), and especially after lessons from Fukushima Nuclear Accident, more and more attentions are paid to Design Extension Conditions (DEC). This paper has studied the abilities and requirements of CPR1000 Nuclear Plants for DEC accidents mitigation. Some important modifications on mechanical systems are introduced.
DEC accidents are separated into DEC-A, and DEC-B. DEC-A accidents are caused by multiple failures. A main reason for a multiple failure is Common Cause Failure (CCF), such as CCF on Emergency Feed Water System (EFWS) pumps, Emergency Core Cooling System (ECCS) pumps, Emergency Diesel Generators or Cooling chain. Diverse design should be considered to deal with CCF. Several modifications are applied, such as enhancing EFWS turbine pumps, fast-backup between ECCS and Containment Spray System (CSS), mobile water makeup and mobile injection for primary and secondary side, etc. For DEC-B, which means severe accidents, several dedicated systems are added to CPR1000 design, such as hydrogen monitor and control, containment venting filter and In-Vessel Retention systems. Thanks to all the modifications, CPR1000 can sufficiently improve its defense for DEC accidents, which will help CPR1000 to reach a new safety level.