AP1000™ Breaking the Dependence on Site Characteristics: Electrical Grid and Cooling Water

Abstract

The AP1000™ has been developed to use passive safety systems to address design basis and beyond design basis accidents to minimize impact on calculated plant Core Damage Frequency (CDF) and Large Release Frequency (LRF). Passive safety cases are supplemented with accident mitigation strategy using highly reliable non-safety grade systems. AP1000™ non-safety systems that resemble the safety systems of conventional plants are designed to mitigate accidents, when available. In addition, a number of mitigation schemes make use of non-safety systems together with select passive system features. This design approach has resulted in a small dependence on site characteristics and has minimized their contribution to CDF and LRF. Conventional nuclear power plants are designed to use motor driven safety grade equipment to address design basis accidents. Safety grade diesel generators are used to provide power to safety grade equipment in the event of a loss of offsite power. The contribution to CDF and LRF from Loss of Offsite Power (LOSP) or loss of cooling water events is significantly lower for the AP1000™ than that for conventional plants. The AP1000™ uses passive safety systems in conjunction with non-safety systems to reduce plant CDF and LRF sensitivity to site specific characteristics. Non-safety systems used for accident mitigation make use of high grade commercial components and are provided power from non-safety grade diesel generators upon LOSP. These components are well maintained to increase system reliability and to increase availability for accident mitigation. The effect of the non-safety systems is further enhanced by partial use of features of the passive safety systems for accident mitigation. In this way, events with initiating event frequencies often driven by site characteristics are mitigated with small contribution to CDF or LRF, often without the need to activate any, or some of the passive plant features.