A major safety advantage of passive plants versus current or evolutionary light water reactors (LWRs) is that long-term accident mitigation is maintained without operator action or reliance on off-site or on-site AC power.
The AP1000 PWR uses extensively analyzed and tested passive safety systems to improve the safety of the plant. The Advisory Council on Reactor Safeguards (ACRS) and the U.S. NRC have scrutinized these systems and ruled that they meet the U.S. NRC single-failure criteria, and other safety criteria such as Three Mile Island lessons learned, and generic safety issues.
The AP1000 plant’s passive safety systems require no operator actions to mitigate design-basis accidents. These systems use only natural forces such as gravity, natural circulation and compressed gas to achieve their safety function. No pumps, fans, diesels, chillers or other active machinery are used, except for a few simple valves that automatically align and actuate the passive safety systems.
With less safety-grade equipment, the seismic Category 1 building volumes needed to house safety-grade equipment are greatly reduced. In fact, most of the safety equipment can now be located within containment, resulting in fewer containment penetrations.
The AP1000 plant’s passive safety systems include:
- Passive Core Cooling System (PXS)
- Containment Isolation
- Passive Containment Cooling System (PCS)
- Main Control Room Emergency Habitability System
- High Pressure Safety Injection with Core Makeup Tanks (CMTs)
- Medium Pressure Safety Injection with Accumulators
- Low Pressure Reactor Coolant Make from the IRWST
- Passive Residual Heat Removal
- Automatic Depressurization System