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Background In the 1980s, Westinghouse embarked on a development program to advance pressurized water reactor (PWR) technology to the next generation. The design objectives of this program, while maintaining field-proven features of the PWR design, were to improve availability, generation economics, and operation and maintenance with respect to the best performing plants in the world. The technology improvements and design simplifications achieved in the next-generation development program were used to establish the AP600, a 600 MWe pressurized water reactor utilizing passive safeguard systems and extensive plant simplifications. The AP600 design uses field-proven technology, integrates modifications based on lessons learned or operating experience, and incorporates changes only where clear benefit is measurable.
Industry Operating Experience The Advanced Light Water Reactor (ALWR) Utilities Requirement Document (URD) was established through the efforts of the U.S. Electric Power Research Institute (EPRI) and the participation of both U.S. and international utilities. The URD uses the large experience base from existing light water reactors to minimize the risk to the plant owner, provide confidence relative to credibility of costs and schedules, and eliminate the need for a plant prototype. The URD consistently emphasizes the use of proven technology to achieve these objectives. The AP600 program has strictly followed the requirements and guidance specified in the URD, and has incorporated proven technology into all aspects of the design. The entire URD process contributes directly to the use of field-proven technology because the URD captures the lessons learned from over 20 years of operating plant experience, identifies the best practices, and incorporates these as design requirements. Over 5000 design requirements are identified in the URD.
Back to Top of Page A Sound Technological Base The AP600 is a 600 MWe PWR utilizing passive safeguard systems and extensive plant simplifications to enhance the construction, operation, and maintenance of the plant. The plant design utilizes proven technology which builds on over 40 years of operating PWR experience. PWRs represent 76 percent of the light water reactors around the world; 67 percent of the PWRs are based on Westinghouse PWR technology. The AP600 nuclear steam supply system (NSSS) design is based on the standard two-loop Westinghouse PWR designs that have collectively logged more than 100 reactor years of operation and shown capacity factors in excess of 85 percent (as compared to an overall industry-wide capacity factor of approximately 74 percent). The U.S. plants of this design include Ginna, Prairie Island, Kewaunee, and Point Beach. Their operating record is the best among any group of plants in the U.S. Internationally, Korea's Kori-2 plant, which became operational in 1983, is a Westinghouse two-loop PWR that has been among the best performers in the world, and it is this plant design which is the basis, or reference, for the AP600 NSSS design. Westinghouse's two-loop plant designs are both robust and simple. These two features are primarily responsible for the excellent reliability of this class of plants. The AP600 NSSS is based on the same robust and simplified design approach. In addition, the same control and feedback mechanisms (e.g., steam generator level, pressurizer pressure), successfully demonstrated in the operation of the existing two-loop plants, are used in the AP600 design. As a result, the AP600 plant response to system transients is well understood and can be reliably predicted based on the proven performance of the existing two-loop plants. Back to Top of Page
Reactor Coolant System The AP600 reactor coolant system (RCS) consists of two heat transfer circuits, each with a steam generator; two reactor coolant pumps; and a single hot leg and two cold legs for circulating reactor coolant between the reactor and the steam generators. The system also includes a pressurizer, interconnecting piping, and the valves and instrumentation necessary for operational control and safeguards actuation. All equipment is located in the reactor containment and uses major components that have been proven in operating reactors under similar flow, temperature, and pressure conditions.
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Other Major Components
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Passive Safety Systems The features of the AP600 passive safety systems include passive safety injection, passive residual heat removal, passive containment cooling, and passive main control room habitability maintenance. All of these passive systems have been designed to meet the NRC single failure criteria, and probabilistic risk analyses have also been used to verify their reliability. These passive systems employ natural forces and stored energy to operate. They are highly reliable because in the unlikely event of an accident, with an assumed unavailability of non-safety systems, they do not require the starting of motors, pumps, or diesel generators. These passive systems have also been designed to satisfy additional NRC criteria, including Three Mile Island lessons learned, Standard Review Plan, Regulatory Guides, and unresolved and generic safety issues. Several aspects of the passive safety systems have been used in existing nuclear plants. The accumulators are a part of most PWR designs, so their use is well understood. Several early boiling water reactors (BWRs), like Dresden in the U.S., used isolation condensers as natural circulation closed-loop heat removal systems. The AP600 passive residual heat removal heat exchanger was designed with the benefit of this experience. BWRs have used automatic depressurization systems (ADS) and spargers for many years. Use of slow opening valves is a result of understanding the air clearing loads discovered in BWR operation. The AP600 ADS incorporates spargers to allow depressurization of the RCS to the in-containment refueling water storage tank (IRWST) in lieu of the containment atmosphere to minimize the containment cleanup following an ADS actuation. The sparger design incorporates BWR design and operating experience. Back to Top of Page
Instrumentation and Control The AP600 instrumentation and control (I&C) system is based on existing digital technology and hardware, which ensures greater design flexibility; improves plant safety, availability, reliability, and maintainability; and dramatically reduces the quantity of plant cabling. Included in the I&C systems are the main and emergency control boards, the plant protection system, the NSSS control systems, the turbine-generator control system, the BOP control systems, and the plant-wide monitoring systems. Westinghouse possesses a great deal of experience in the design and implementation of advanced digital microprocessor-based I&C systems, and is the recognized world leader in providing process controls for the power industry. The Westinghouse I&C family of digital microprocessing equipment includes both the EAGLE Series equipment for safety grade applications, and the Westinghouse Distributed Processing Family (WDPF) equipment for non-safety grade applications. The EAGLE Series was specifically designed to meet the rigorous requirements of nuclear plant protection systems mandated by the U.S. Nuclear Regulatory Commission (NRC). It is installed and operating successfully in nine U.S. plants, as well as the Sizewell B plant in the United Kingdom. It is also currently being installed in the Temelin plant in the Czech Republic. WDPF equipment is installed in over 1500 major control and monitoring systems on a variety of applications in both the nuclear and non-nuclear industries. Nuclear plant non-safety system applications include distributed plant computer and information systems, feedwater control systems, safety-parameter display systems, radiation monitoring systems, turbine control systems, and numerous balance of plant control systems. Applications for fossil plants, waste water treatment facilities, steel mills, chemical processing plants include boiler control systems, combustion turbine controls, data acquisition systems, burner control systems, and chemical addition systems. The AP600 design takes advantage of Westinghouse's capabilities, technology, and the utility industry direction provided in the URD to provide a reliable, operator friendly, world-class I&C system. Back to Top of Page
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