Many of the active safety-related systems in existing and evolutionary PWR designs are retained in the AP1000 plant but are designated as non safety-related.
The AP1000 PWR active non safety-related systems support normal operation and are also the first line of defense in the event of transients or plant upsets. Although these systems are not credited in the safety analysis evaluation, they provide additional defense-in-depth by adding a layer of redundancy and diversity. In addition to contributing to the very low core damage frequency (CDF), the non safety-related, active systems require fewer in-service inspections, less testing and maintenance, and are not included in the simplified technical specifications. For defense-in-depth, most planned maintenance for these nonsafety systems can be performed while the plant is operating.
Examples of non safety-related systems that provide defense-in-depth capabilities for the AP1000 plant design include the chemical and volume control system, normal residual heat removal system, and the startup (auxiliary) feedwater system. These systems utilize non-safety support systems such as the standby diesel generators, the component-cooling water system, and the service water system. The AP1000 PWR also includes other active non safety-related systems, such as the heating, ventilation and air-conditioning (HVAC) systems, which remove heat from the instrumentation and control (I&C) cabinet rooms and the main control room. These are, in simpler form in the AP1000 plant, familiar systems that are used in current PWRs as safety systems. In the AP1000 plant, these HVAC systems are a simplified non-safety first line of defense, which are backed up by the ultimate defense, the passive safety-grade systems.
This defense-in-depth class of systems includes the containment hydrogen control system, which consists of the hydrogen monitoring system, passive autocatalytic hydrogen recombiners, and hydrogen igniters (powered by batteries).