The AP1000® pressurized water reactor works on the simple concept that, in the event of a design-basis accident (such as a coolant pipe break), the plant is designed to achieve and maintain safe shutdown condition without any operator action and without the need for ac power or pumps. Instead of relying on active components such as diesel generators and pumps, the AP1000 relies on the natural forces of gravity, natural circulation and compressed gases to keep the core and containment from overheating. However, many active components are included in the AP1000, but are designated as non safety-related.

Multiple levels of defense for accident mitigation are provided, resulting in extremely low core-damage probabilities while minimizing occurrences of containment flooding, pressurization and heat-up.

The AP1000 meets the U.S. NRC deterministic-safety and probabilistic-risk criteria with large margins. Results of the Probabilistic Risk Assessment (PRA) show a very low core damage frequency (CDF) that is 1/100 of the CDF of currently operating plants and 1/20 of the maximum CDF deemed acceptable for new, advanced reactor designs.

The following features contribute to defense-in-depth of the AP1000:
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