The AP1000 is an advanced 1000 MWe nuclear power plant that uses the forces of nature and simplicity of design to enhance plant safety and operations and reduce construction costs.
The Westinghouse AP1000 is a logical extension of its AP600 plant. Design studies have shown that a two-loop configuration could produce over 1000 MWe with minimal changes in the AP600 design. The primary purpose of developing the AP1000 was to retain the AP600 design objectives, design details and licensing basis, while optimizing the power output, thereby reducing the resulting electric generation costs.
PASSIVITY AND SAFETY
As in AP600, the AP1000 design uses passive safety systems to enhance the safety of the plant and to satisfy the Nuclear Regulatory Commission’s (NRC) safety criteria. These systems use only natural forces, such as gravity, natural circulation, and compressed gas. No pumps, fans, diesels, chillers, or other rotating machinery are used in the passive safety sub-systems.
The passive safety systems include passive safety injection, passive residual heat removal, and passive containment cooling. All these passive systems have been designed to meet the NRC single-failure criteria and its recent criteria, including TMI(Three Mile Island) lessons-learned and unresolved/generic safety issues. Probabilistic Risk Assessment(PRA) tools have also been used to quantify the safety of the design.
SIMPLICITY
The passive safety systems are significantly simpler than the traditional PWR safety systems.They do not require the large network of safety support systems needed in typical nuclear plants,such as AC power,HVAC(heating, ventilation & air conditioning), cooling water systems and seismic buildings to house these components. Simplification of plant systems, combined with increased plant operating margins, reduces the actions required by the operator. The AP1000 has 50 percent fewer valves, 83 percent less piping, 87 percent less control cable, 35 percent fewer pumps and 50 percent less seismic building volume than a similarly sized conventional plant. These reductions in equipment and bulk quantities lead to major savings in plant costs and construction schedules.
NUCLEAR STEAM SUPPLY SYSTEM (NSSS) & FUEL
The AP1000 NSSS plant configuration consists of two Delta-125 steam generators, each connected to the reactor pressure vessel by a single hot leg and two cold legs. There are four reactor coolant pumps that provide circulation of the reactor coolant for heat removal. A pressurizer is connected to one of the cold leg piping to maintain subcooling in the Reactor Coolant System (RCS).
The two-loop, 1090 MWe plant retains the same basic design of the AP600. Changes to the design to increase the electricity output have been minimized to allow the direct application of most of the existing design engineering already completed for the AP600. Examples of design features that remain unchanged include the nuclear island footprint and the core diameter.
Major component changes incorporated into the AP1000 design include a taller reactor vessel, larger steam generators (Delta-125), a larger pressurizer and slightly taller, canned reactor coolant pumps with higher reactor coolant flows. The designs for these reactor components are based on components that are used in operating PWRs or have been developed and tested for new PWRs. Performance of the passive safety features have been selectively increased, however, these changes have been accomplished with small changes to the AP600 plant design.
The AP1000 fuel design is based on the 17x17 XL (14 foot) design used successfully at plants in the U.S. and Europe. As with AP600, studies have shown that AP1000 can operate with a full core loading of MOX fuel.
CONSTRUCTION
Like the AP600, the AP1000 utilizes modularization technique for construction, which allows many construction activities to proceed in parallel. This technique reduces the plant construction calendar time, which saves the IDC (Interest During Construction) cost and reduces the risks associated with plant financing. The AP1000 has a site construction schedule of 36 months from first concrete to fuel loading.
LICENSING
Westinghouse has submitted documentation to the United States Nuclear Regulatory Commission (US NRC) thereby initiating the process of obtaining a Final Design Approval (FDA) of the AP1000 from the US NRC. As the AP1000 shares much in common with the AP600, both in technical design philosophy and technical design detail, it is anticipated that the NRC review and approval process will be significantly faster than the six years needed to review and approve the AP600.
TESTING
All necessary testing have been performed in the development of AP600.
AP600 Background
The AP600 is an advanced 600 MWe nuclear power plant that uses the forces of nature and simplicity of design to enhance plant safety and operations and reduce construction costs.
Westinghouse is in the process of obtaining design certification for the AP1000 — a more powerful plant based on the AP600.
The AP600 - A Cooperative Effort
As part of the cooperative U.S. Department of Energy (DOE) Advanced Light Water Reactor (ALWR) Program and the Electric Power Research Institute (EPRI), the Westinghouse AP600 team has developed a simplified, safe, and economic 600-megawatt plant to enter into a new era of nuclear power generation. Designed to
satisfy the standards set by DOE and defined in the ALWR Utility Requirements Document (URD), the Westinghouse AP600 is an elegant combination of innovative safety systems that rely on dependable natural forces and field-proven technologies.
Simplified Plant Systems
The Westinghouse AP600 design simplifies plant systems and significant operation, inspections, maintenance, and quality assurance requirements by greatly reducing valves, pumps, piping, HVAC ducting, and other complex components. The AP600 safety systems are predominantly passive, depending on the reliable natural forces of gravity, circulation, convection, evaporation, and condensation, instead of AC power supplies and motor-driven components.
Extensive Use of Field-Proven Technology
Although the AP600 uses simplified and passive plant systems to an unprecedented extent to enhance plant safety and operations, the effectiveness of the technology has been demonstrated through years of operations and testing. The AP600's major components are based on years of reliable operating experience. The canned motor reactor coolant pumps have been in use by the US Navy for decades. The steam generators, pressurizer, fuel, and reactor vessel are all based on field-proven technology with incremental improvements developed as a result of operating experience. The passive safety systems are an extension of the technology used previously, since Westinghouse-supplied PWRs have had accumulators for injection of core cooling water without the use of pumps for many years. The AP600 is the result of a logical progression in plant design.
Safety and Licensing Certainty
The AP600 provides a high degree of public safety and licensing certainty. It draws upon over 40 years of experience in light water reactor components and technology, so no demonstration plant is required. The AP600 has received final design approval from the US NRC.
Comprehensive Test Programs
During the AP600 design program, a comprehensive test program was carried out to verify plant components, passive safety systems components, and containment behavior. When the test program was completed at the end of 1994, AP600 became the most thoroughly tested advanced reactor design ever reviewed by the U.S. Nuclear Regulatory Commission (NRC). The test results confirmed the exceptional behavior of the passive systems and have been instrumental in facilitating code validations.
Extensive Industry Review
The Westinghouse AP600 reactor has been designed as part of the Advanced Light Water Reactor (ALWR) Program sponsored by the U.S. DOE and EPRI. Following an extensive review by the U.S. Nuclear Regulatory Commission, the AP600 received final design approval on September 3, 1998. A detailed design program (FOAKE-First-of-a-Kind-Engineering) was completed under the sponsorship of DOE, the Advanced Reactor Corporation (ARC), and EPRI.
Passive Safety Features
The Westinghouse AP600 is a 600 MWe reactor which utilizes passive safety features that, once actuated, depend only on natural forces such as gravity and natural circulation to perform all required safety functions. These passive safety systems result in increased plant safety and can also significantly simplify plant systems, equipment, and operation.
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Plant Features
The Westinghouse AP600 isn't just an advanced nuclear power plant-- it represents a new way of doing business that addresses the needs of the new utility environment. The AP600 is helping to
change the way the world views construction of new nuclear power plants.
A Simplified Plant
It is crucial that new nuclear power plants be easier and less expensive to build, operate, and maintain. The AP600 requires 50% fewer valves, 80% less safety grade piping, 70% less control cable, 35% fewer pumps (no safety grade pumps), and 45% less seismic building volume than other conventional reactors.
A New Kind of Safety
Westinghouse commercial nuclear power plants have an excellent safety record. There are more plants safely operating with Westinghouse technology than any other kind. The safety of these
operating plants is achieved by substantial amounts of complex, redundant emergency equipment that requires electrical power and occasional operator actions. These are referred to as active safety features. The AP600 achieves safety by relying on the forces of nature, such as gravity, natural circulation, convection, evaporation, and condensation. By relying on these forces, there is no need for complex active systems and operator actions.
Economics Driven By Simplicity - Not Size
Simplification helps to reduce capital costs and provides a hedge against regulatory driven operating and maintenance costs by eliminating equipment which is subject to regulation. The AP600's capital cost for a twin unit station is expected to be about 15% less than for conventional twin 600 MWe unit stations. Projected operating and maintenance costs are 35% less than the current industry average. Economic performance driven by simplicity rather than scale allows generating companies to add capacity in AP600's smaller increments that more closely match demand growth.
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Passive Safety
AP600 Design Objectives
The primary design objective of the AP600 was to provide a greatly simplified plant design that meets NRC regulatory requirements, meets or exceeds NRC safety goals and those of the ALWR Utility Requirements Document, and addresses past safety issues, while being economically competitive with other power generation systems over the full operating cycle. This objective is to be met using experience-based components so that plant prototype or demonstration models are not required. Simplification of plant systems, combined with increased plant operating margins, reduces the actions required by the operator in the event of an accident. The AP600 design requires no operator actions to maintain a safe
configuration following a design basis accident. The design target for the AP600 is to technically support elimination of the emergency planning zone beyond the site boundary. Simpler systems, combined with the U.S. licensing reforms of 10CFR52, increase the licensing certainty of the AP600 in international markets.
Implementation of the passive safety features greatly reduces the operation, maintenance and testing requirements of the AP600. The AP600 has been designed to have a shorter construction schedule through the use of modular construction techniques that are similar to those applied in ship construction. The construction design objective is a 36-month schedule from first concrete pour to the fuel load. An added benefit of this approach is that a significant portion of the quality assurance inspections can be completed in the factory before the modules are delivered to the construction site.
Passive Safety Features Overview
The AP600 uses passive safety systems to enhance the safety of the plant and to satisfy NRC safety criteria. These systems use only natural forces, such as gravity, natural circulation, and compressed gas. No pumps, fans, diesels, chillers, or other rotating machinery are used in the passive safety sub-systems. A few simple valves are used to align the passive safety systems when they are automatically actuated. In most cases these valves are \"fail safe\" (i.e., they require power to stay in their normal, closed position; loss of that power causes them to open to their safety alignment. This power is normally supplied by class lE uninterruptible power supplies). These passive safety systems are significantly simpler than typical PWR safety systems.
In addition to being simpler, the passive safety systems do not require the large network of safety support systems needed in typical nuclear plants, such as AC power, HVAC (heating, ventilating, air conditioning) and cooling water systems and seismic buildings to house these components. This simplification includes eliminating the safety-grade emergency diesel generators and their network of support systems, air start, fuel storage tanks and transfer pumps, and the air intake/exhaust system. As a result, support systems no longer need to be safety grade and can be simplified or eliminated. The features of the AP600 passive safety systems include passive safety injection, passive residual heat removal, and passive
containment cooling. All these passive systems have been designed to meet the NRC single-failure criteria and its recent criteria, including TMI lessons-learned and unresolved and generic safety issues. PRAs have also been used to quantify the safety of the design.
Passive Core Cooling System
The AP600 passive core cooling system (PXS) performs two major functions:
Safety Injection and Reactor Coolant Makeup for Inventory
The AP600 has a passive residual heat removal (PRHR) subsystem that protects the plant against transients that upset the normal steam generator feedwater and steam systems. Westinghouse analysis results, using NRC-approved codes, has shown that the PRHR subsystem satisfies the NRC safety criteria for loss of feedwater, feedwater line breaks, and steam line breaks with a single failure. Anticipated transients without reactor trip have also been analyzed and shown to result in peak RCS pressures of about 2900 psig, well within NRC criteria.
The Passive RHR Heat Exchanger
The Passive RHR heat exchanger consists of a 100 percent capacity bank of tubes connected to the reactor coolant system (RCS) in a natural circulation loop. The loop is normally isolated from the RCS by valves that are normally closed, but fail open if power is lost. The
heat exchanger tubes are located in the in-containment refueling water storage tank (IRWST). This location places the Passive RHR heat exchanger above the RCS loop so that hot water leaving the RCS hot leg will rise to the top of the PRHR HX where it is cooled. The difference in temperature between the hot inlet water and the cold outlet water drives the natural circulation loop. If the reactor coolant pumps are running, they boost the PRHR HX flow. The In-containment Refueling Water Storage Tank (IRWST)
The IRWST provides the heat sink for the passive RHR heat exchanger. The IRWST water volume is sufficient to absorb decay heat for about 2 hours before the water would start to boil. After that, steam would be generated and enter the containment. This steam would condense on the steel containment vessel and then drain back into the IRWST.
Passive Residual Heat Removal (Passive RHR)
The AP600 has a passive residual heat removal (PRHR) subsystem that protects the plant against transients that upset the normal steam generator feedwater and steam systems. Westinghouse analysis results, using NRC-approved codes, has shown that the PRHR subsystem satisfies the NRC safety criteria for loss of feedwater, feedwater line breaks, and steam line breaks with a single failure. Anticipated transients without reactor trip have also been analyzed and shown to result in peak RCS pressures of about 2900 psig, well within NRC criteria.
The Passive RHR Heat Exchanger
The Passive RHR heat exchanger consists of a 100 percent capacity bank of tubes connected to the reactor coolant system (RCS) in a natural circulation loop. The loop is normally isolated from the RCS by valves that are normally closed, but fail open if power is lost. The heat exchanger tubes are located in the in-containment refueling water storage tank (IRWST). This location places the Passive RHR heat exchanger above the RCS loop so that hot water leaving the RCS hot leg will rise to the top of the PRHR HX where it is cooled. The difference in temperature between the hot inlet water and the cold outlet water drives the natural circulation loop. If the reactor coolant pumps are running, they boost the PRHR HX flow. The In-containment Refueling Water Storage Tank (IRWST)
The IRWST provides the heat sink for the passive RHR heat exchanger. The IRWST water volume is sufficient to absorb decay heat for about 2 hours before the water would start to boil. After that, steam would be generated and enter the containment. This steam would condense on the steel containment vessel and then drain back into the IRWST.
Computer analyses demonstrate that the PXS provides effective core cooling for various break sizes and locations. These calculations show that the PXS prevents core damage for breaks as large as the 8-inch vessel injection lines and provides about 500 degF margin to the maximum peak clad temperature limit for the double-ended rupture of a main reactor coolant pipe.
Passive Containment Cooling System
AP600 Passive Containment Cooling System (PCS) provides the safety-related ultimate heat sink for the plant.
As demonstrated in computer analysis and tests, the PCS is able to effectively cool the containment following an accident such that the design pressure is not exceeded and the pressure is rapidly reduced. The steel containment vessel itself provides the heat transfer surface that removes heat from inside the containment and rejects it to the atmosphere. Heat is removed from the containment vessel by a natural circulation flow of air that cannot be isolated.
In the unlikely event of an accident, the air cooling is supplemented by water evaporation on the outside of the containment shell. The water is drained by gravity from a tank located on top of the containment shield building. Two normally closed, fail-open valves are opened to initiate the water drain. The water tank is sized for three days of operation, after which time the tank is expected to be refilled to maintain the low containment pressure achieved after the accident. If the water is not resupplied after three days, the containment pressure will increase, but the peak is calculated to reach only 90 percent of design pressure after about two weeks.
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Industry Review
Unparalleled Industry Review
The AP600 has been developed with an amount of industry
participation and review that is unparalleled. The AP600 plant has benefited from the participation of a diverse blend of experienced talent in the design process. As a result, the AP600 is a plant with unprecedented safety margins, a plant that can be quickly
constructed, and a plant that is much improved from an operations and maintenance standpoint.
Final Design Approval From NRC
The US Nuclear Regulatory Commission (NRC), after the most thorough new plant review in agency history, granted the AP600 Final Design Approval in September 1998.
The breadth and depth of experienced talent that has been brought to bear on the AP600 design has come from the following major areas:
A Diverse and Experienced Design Team
Industry Guidance From the Utility Requirements Document
(URD)
Industry Guidance From Advanced Reactor Corporation (ARC) International Participation in the Design Process
Design Team
A Diverse and Experienced Design Team
The AP600 plant has benefited from the participation of a diverse blend of experienced talent in the design process. As a result, the AP600 has turned into a plant with unprecedented safety margins, a
plant that can be quickly constructed, and a plant that is much improved from an operations and maintenance standpoint.
The Plant Designers
The breadth and depth of experienced talent that has been brought to bear on the AP600 design has come from companies listed below. Each company has used their extensive experience in plant design to make the AP600 a marked improvement over current plants.
Westinghouse Electric Company (Team Leader) ANSALDO
Avondale Industries BATAN BPPT
Bechtel Power Corporation Burns and Roe Company Chicago Bridge and Iron DTN ENSA ENUSA Initec
MK-Ferguson
Southern Company Services
Shanghai Nuclear Energy Research & Design Institute Tecnatom UTE
In addition to these companies, many other organizations in the international community participated in the design of the AP600.
URD
Industry Guidance From the Utility Requirements Document
The Advanced Light Water Reactor (ALWR) Utility Requirements 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 field-proven
technology to achieve these objectives. The AP600 program has strictly followed the requirements and guidance specified in the URD, and has incorporated field-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.
Areas of Specification
The ALWR program has formulated policies in key areas that are central to achievement of advanced plant program objectives and that have broad, fundamental influence on plant design requirements. These policies help to form the foundation for the overall plant development and provide guidance to plant designers. The key policy statements in the URD cover the following areas:
Safety
Design Margin Simplification
Field-Proven Technology Human Factors
Design Basis vs Safety Margin Operation and Maintenance Reliability and Availability Constructibility Standardization
Regulatory Stabilization Quality Assurance Sabotage Protection Good Neighbor Policy
The AP600 design is in accordance with the requirements outlined in the URD under each of these key areas.
ARC
Industry Guidance From Advanced Reactor Corporation (ARC)
The Advanced Reactor Corporation (ARC) was formed to oversee the development of advanced plant designs including the AP600 during the First-of-a-Kind Engineering phase. The ARC organization
consisted of personnel from operating electric utility companies. The ARC project office was co-located with Westinghouse so that ARC personnel could closely scrutinize AP600 design work to verify that utility industry requirements were addressed. Moreover, Westinghouse solicited input from ARC personnel as part of the on-going design process to assure that the AP600 benefited from operating plant experience.
Participating Utility Companies
The following electric utility companies participated in ARC either through the supply of project personnel, steering committee personnel, or review of AP600 design work:
American Electric Power Service Corporation Carolina Power and Light
Commonwealth Edison Company Consolidated Edison Company Duke Power Corporation Florida Power and Light GPU Nuclear Corporation Pacific Gas and Electric PECO Energy Company
Pennsylvania Power and Light Public Service Electric and Gas Southern Company Services Tennessee Valley Authority Texas Utilities Union Electric
Wisconsin Electric Power
Yankee Atomic Electric Company
International
The AP600 -- A Global Design
The AP600 truly has an international flavor. The design of the
AP600 has been accomplished with a global perspective, thanks to the participation of engineering talent from the long list of nations shown below.
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Modularization
Modularization
Used
to
Reduce
AP600
Construction Cost
Construction costs of commercial nuclear generating plants must be reduced in order to expand the future use of nuclear energy. Two of the drivers of plant construction costs are the cost of financing during the construction phase and the substantial amount of skilled craft labor hours needed on site during construction. The AP600 technique of modularization of plant construction mitigates against both of these drivers.
Parallel Work Processes in Controlled Environments
AP600 modularization allows many more construction activities to proceed in parallel. This reduces the calendar time for plant construction and thereby reduces the cost of money and the exposure risks associated with plant financing. Secondly, with many more construction activities occurring off-site in factory-type and shipyard environments, the amount of skilled field craft labor needed to complete the plant is reduced. Shop labor costs substantially less than field labor. In addition to the labor cost
savings, having more welding and fabrication performed in factory environments increases the quality of the work, increases the flexibility in scheduling, and reduces the amount of specialized tools on site.
Advanced Information Management Techniques to Produce Modularized Plant Design
In order to achieve proper interfaces with the rest of the plant systems and structures, AP600 construction modules are fabricated to more precise tolerances than typical field-run commodities (e.g., piping, duct, raceway) and \"stick-built\" construction techniques. To produce such designs, advanced computer-based tools and information management techniques are used.
Typical AP600 Piping and Valve Module
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Field Proven
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.
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.
Reactor Coolant System
AP600 Reactor Coolant Pumps
The AP600 reactor coolant system (RCS) employs high inertia, high reliability, low maintenance canned motor pumps for circulating primary reactor coolant through the reactor core, piping, and steam generators. Two pumps are mounted directly in the channelhead of each steam generator. This allows the pumps and steam generator to use the same structural support, greatly simplifying the support system and providing more space for pump and steam generator maintenance. The combined steam generator/pump vertical support is a single column extending from the floor to the bottom of the channelhead.
Large Operating Experience Base
Inverted canned motors have been in operation for over 25 years in fossil boiler circulation systems with better operating reliability than upright units because the motor cavity is self-venting into the pump casing, avoiding the potential for gas pockets in the bearing and water regions. Approximately 1300 units have been built and placed into service. The pumps are integrated into the steam generator channelhead in the inverted position.
The advantages of this pump design are significant. The auxiliary fluid systems needed to support a canned motor pump are much less complex than those needed for a shaft seal type pump. The canned motor pump is more tolerant of off-design conditions than shaft seal pumps, and inherently reduces the potential for small loss-of-coolant accidents (LOCAs) by eliminating the shaft seal. The integration of the pump suction into the bottom of the steam generator channelhead eliminates the crossover leg of coolant loop piping; reduces the loop pressure drop; simplifies the foundation and support system for the steam generator, pumps, and piping; and eliminates a potential of core uncovery during a small LOCA.
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.
Instrumentation and Control Coolant Systems
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.
Major Components
Steam Generators
Two model Delta-75 steam generators are used in the AP600. The Delta-75 steam generator is based on standard Westinghouse Model-F technology. The Model-F is a proven design with approximately 75 units currently in commercial operation. The
Model F-type replacement steam generators have an impressive record of less than one tube plugged per steam generator for every four years of operation. This is the highest level of reliability achieved by any steam generator worldwide.
Steam Generator Reliability
These reliability achievements are due to a variety of design enhancements incorporated to improve performance and increase service life. Enhancements include full-depth hydraulic expansion, stainless steel broached tube support plates, the use of thermally-treated Inconel 690 (I-690) for the tube material to improve corrosion resistance, the addition of upgraded anti-vibration bars to reduce wear, upgraded primary and secondary moisture separators, and use of a triangular tube pitch. Steam Generator Materials
Westinghouse led in the development of the I-690 tube material which has been endorsed by the Electric Power Research Institute (EPRI) for use in new steam generators. I-690 has excellent overall corrosion resistance, is resistant to primary water stress corrosion cracking, and has low primary release rates which are expected to reduce primary side radiation levels by at least 50 percent compared to units employing I-600 tube material.
The Delta-75 steam generator used for the AP600 is currently in use at the V.C. Summer plant in the U.S., and has completed one 18-month cycle of operation with the new steam generators. The design enhancements incorporated into the Delta-75 steam generators have provided the plant greater operating flexibility and increased safety margins.
Steam Pressurizer
The AP600 pressurizer is based on the standard Westinghouse design currently used in approximately 70 operating plants worldwide. Westinghouse pressurizers are a technology with over 30 years of successful operating experience. The AP600 pressurizer is 1600 cubic feet, which is about 30 percent larger than would normally be used in a plant of comparable power rating.
The larger pressurizer increases transient operation margins and eliminates the need for relief valve actuation, which results in a more reliable plant that will have fewer reactor trips and that will require less time to recover after a transient. It also eliminates the need for power-operated relief valves, which eliminates a possible source of RCS leakage and maintenance.
The AP600 pressurizer heaters are similar in design to those employed in operating Westinghouse PWRs. The heaters are vertically mounted, extending up through penetrations in the bottom head of the pressurizer shell. They are also individually seal-welded to the penetrations providing the system pressure boundary. The pressurizer heaters are one of the many components that have achieved very good overall performance in operating plants.
RC Pumps
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.
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.
Fuel Design
The AP600 fuel design is based on standard 17 x 17 optimized fuel technology currently being used in approximately 120 operating plants worldwide. Over 25,000 17 x 17 fuel assemblies have already been manufactured. Westinghouse designed and supplied fuel has shown superior performance relative to the rest of the PWR industry. The median coolant activity (a measure of fuel integrity) of Westinghouse supplied fuel is 60% better than the overall PWR industry average. Fuel performance improvements, such as zircaloy grids, removable top nozzles, and longer burnup have been added.
Low Power Density Reactor Core
The advantages of the AP600's low power density are achieved by making the core larger than conventional 600 MWe designs. As a result, the number of fuel assemblies is increased -from 121 to 145, so that many of the important nuclear and thermal parameters are improved by 25 to 30 percent over those of a standard plant of the same power rating. This results in lower fuel enrichments, less reliance on burnable absorbers, longer achievable fuel cycles, and an increase of more than 15 percent in departure from nucleate boiling (DNB) and loss-of-coolant accident (LOCA) margins. A larger gas plenum has been incorporated into the fuel to allow higher burnup.
Improved Overall Reactor Design
The combination of the radial reflector, the low power density core, and optimized fuel assemblies results in a 20-percent fuel cycle cost savings compared to a standard PWR design of the same power rating. The core design allows 18-month refueling cycles to be achieved with an 85-percent capacity factor (approximately 466 effective full power days per cycle) and requires no burnable absorbers other than for the first cycle of operation. Gray Rods For Load Follow Without Boron
Another core design feature of the AP600 is the use of reduced worth control rods, which are termed \"gray\" rods, to achieve daily load follow without requiring daily changes in the soluble boron concentration. While the same control rods are used in existing PWRs for load follow, it is also necessary for these plants to process thousands of gallons of water per day in order to change the soluble boron concentration sufficiently to achieve a daily load follow schedule. The use of gray rods eliminates the need for processing the primary coolant on a daily basis and greatly simplifies operations using the boron systems.
With the exception of the neutron absorber materials used, the design of the gray rod assembly is identical to that of a normal control rod assembly. Thus, the design of gray rods, the fuel assembly, the reactor vessel, and reactor internals are all based on existing Westinghouse PWR technology.
Reactor Vessel
Reactor Design
The core, reactor vessel, and internals of the AP600 are essentially those of a conventional Westinghouse PWR design. Several important features, all based on existing technology, have been used to improve the performance characteristics of the AP600. The AP600 reactor vessel has incorporated improvements over its predecessors, including ring forgings to eliminate vertical weld seams, location of circumferential welds outside high neutron flux beltline region, and control of chemical composition of vessel material to reduce irradiation damage.
Over-sized Reactor Vessel
Although the AP600 reactor has two primary coolant loops, the reactor vessel is similar to the standard 3-loop model. Each primary loop has two cold leg nozzles and one hot leg nozzle, so the standard 3-loop vessel accommodates the AP600 nicely with slight re-orientation of the nozzles. Moreover, the larger reactor vessel on the AP600 accommodates a larger reactor core than that for a traditional two-loop plant. With the larger reactor core and the same power rating, the core power density is reduced which adds safety margin to the plant -- one of the hallmarks of the AP600. Radial Reflector
The use of a radial neutron reflector contributes to lowering fuel cycle cost and extending reactor life. This reflector, which surrounds the core, reduces neutron leakage, thereby improving core neutron utilization and reducing the damaging neutron fluence on the reactor vessel. The reduced fluence is important in view of the 60-year design objective of the AP600. Improved Overall Reactor Design
The combination of the radial reflector, the low power density core, and optimized fuel assemblies results in a 20-percent fuel cycle cost savings compared to a standard PWR design of the same power rating. The core design allows 18-month refueling cycles to be achieved with an 85-percent capacity factor (approximately 466 effective full power days per cycle) and requires no burnable absorbers other than for the first cycle of operation.
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Test Programs
The Most Thoroughly Tested Reactor Design
The AP600 test programs were thorough and exhaustive and the results were valuable. The AP600 is now the most thoroughly tested reactor design ever reviewed by the Nuclear Regulatory Commission. The very sophisticated, detailed, conservative computer codes used to analyze the AP600 have been validated. In some cases, test data was used to refine the codes. The codes predict and the test data confirms AP600 plant safety under a long list of scenarios, including normal, abnormal, and accident. These tests met the very demanding requirements defined at the start by the DOE, the NRC, and the supporting utilities.
Testing Objective
The primary objective of the extensive AP600 testing program has been to supply hard data to verify computer codes. The testing served to verify that the computer codes are able to accurately predict how the AP600's systems will work.
Testing Facilities
To perform the testing, world-leading facilities were used. The major facilities appear below:
Long Term Passive Core Cooling Tests Oregon State University
Long Term Cooling
Long Term Passive Core Cooling Tests
The Oregon State University work was conducted in a brand new facility. The tests went so well that work planned for several months was completed in just six weeks, with the last test performed in August, 1994. The Oregon State testing focused on 30 small-break loss-of-coolant accidents (LOCAs). Using a one-quarter scaled model that replicates the AP600, the tests simulated the transition from LOCA events into long-term cooling, relying on coolant injected from the passive safety systems. The tests validated computer codes, proving that the AP600 core will be adequately cooled at all times.
Full-Height Full-Pressure Core Cooling Tests (SPES-2) Piacenza, Italy
SPES-2
Full-Height Full-Pressure Passive Core Cooling Tests
Other LOCA tests were conducted in Piacenza, Italy, at the SPES-2 facility. These were full-height, full-pressure simulations of both the primary cooling and passive core cooling systems. Thirteen small-break LOCAs were conducted, as well as simulations of ruptures in steam generator tubes and steam lines. Again, the results validated safety analysis computer codes and models. Views of the SPES-2 test facility are shown below.
Reactor Coolant System Automatic Depressurization Tests (ADS) Casaccia, Italy
ADS
Reactor Coolant System Automatic Depressurization Tests
In Casaccia, Italy, full-scale, full-flow tests were done on the automatic depressurization system (ADS). These tests went through two major phases. In the first phase, 21 depressurization runs were conducted. The second phase included 24 depressurization tests. The test results matched computer modeling of the ADS performance.
Containment Cooling Wind Tunnel Tests London, Ontario, Canada's University of Western Ontario
Wind Tunnel
Containment Cooling Wind Tunnel Tests
In London, Ontario, Canada's University of Western Ontario used its wind tunnel to test that winds -- especially high winds -- would not diminish cooling in the annulus of the containment while the natural convection and evaporative forces were doing their job. A detailed scale model of the AP600 was placed in the tunnel. The Ontario tests showed that wind was not a safety factor. The natural circulation of air occurs over the surface of the reactor shield under all wind conditions.
Containment Vessel Water Distribution / Heat Transfer Tests (PCCS)
Westinghouse Science and Technology Center in Pittsburgh
PCCS
Test Programs. PCCS
Passive Containment Cooling System (PCCS) Containment Vessel Water Distribution / Heat Transfer Tests
At the Westinghouse Science and Technology Center in Pittsburgh, containment cooling was studied. First, a flat plate ... three feet wide and six feet high ... that met the specifications for the containment was built. It was used to examine basic thermodynamics. Water ran across its surface and hard data on the heat transfer was obtained under varying conditions. The data was used in containment code safety analysis.
A much larger test device was then built-- a 24-foot high, three-foot-diameter model of the containment. This time, with steam inside, air was moved over the outside as water flowed across the surface. This larger-scale, more-detailed testing provided additional data for computer codes. Then an even larger model was built: a one-eighth scale model of the containment. Again, the entire range of passive safety actions was studied-- using various internal conditions and external air and humidity values. Testing with water applied to the top of the external surface was used, along with testing with a dry surface. This exhaustive, real-world testing shows that the theory of passive containment cooling is valid. Even the toughest internal and external conditions -- and combinations of both -- did an adequate job of cooling. This is a cooling concept that does the job perfectly, relying solely on natural forces.
Core Makeup Tank Tests
At its Waltz Mill test facility site in Madison PA, Westinghouse constructed a scaled version of the core makeup tank. It was used to investigate the thermal-hydraulic behavior of the tank under a wide range of conditions. The data from this series of tests was a major source for the refinement of computer model codes.
Other Tests
The highlights of the AP600 testing program were described above. Also tested were reactor coolant pumps, check valves and incore instrumentation, and a wide variety of flow and heat transfer tests. Every aspect of the passive safety system was tested-- from safety-grade components to the containment cooling and core cooling systems.
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Licensing
Final Design Approval
AP600 — Not a Traditional Design
The AP600 has received final design approval from the United States Nuclear Regulatory Commission (US NRC) following over six years of review, which included over 380 technical meetings and over 7400 requests for additional information. From a regulatory perspective, the AP600 design is a significant departure from traditional plant design and is not directly comparable to all aspects of the traditional review practices.
Extensive Review of Data from AP600 Test Programs
The most significant activity in the AP600 approval process was the execution and review of the extensive test and analysis program that provided the data base used to verify and validate the safety analysis computer codes.
AP600 - The Most Thoroughly Reviewed Plant In NRC History
Completing the design certification review required the NRC to examine not only the compliance of the design with the regulations, but the underlying intent of the regulations as well. As a result of the unique features of the AP600 design, the AP600 has received the most thorough review ever performed by the NRC.
Passive Plant Concepts Intact After Extensive Review
The number of design changes required to obtain the final design approval was small and did not compromise the concept of the passive plant. The number of exceptions to the regulations
required for the AP600 passive design was similar to that required for the evolutionary plant design certifications.
AP600 Design Objective Achieved
The AP600 design objective, of providing a greatly simplified plant that meets the NRC regulatory requirements and meets or exceeds the requirements published by the utilities in the ALWR Passive Plant Utilty Requirements Document using technology that is sufficiently proven that neither a plant prototype nor a demonstration plant was required, has been achieved.
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