Korea successfully kicks off Standard Nuclear Power Plant program

Yonggwang Units 3 and 4, reference design for Korea`s Standard Nuclear Power Plant program, beat an aggressive construction schedule

By Timothy J. Collier and Deva R. Chari, ABB CE Nuclear Systems

The Republic of Korea`s (ROK) nuclear power program achieved a significant milestone when Yonggwang (YGN) Units 3 and 4 were accepted into commercial operation by Korea Electric Power Corp. (KEPCO). Each YGN unit produces 1,050 MWe, and together they supply approximately 10 percent of the total electric power demand in the ROK. The YGN units use the ABB Combustion Engineering Nuclear Systems (ABB-CE) standard System 80 nuclear steam supply system (NSSS).

This design incorporates features which enhance safety and performance margins. The YGN Units 3 and 4 design serves as the reference design for the Korean Standard Nuclear Power Plant program. Standardization of design features has resulted in many benefits to KEPCO, including construction times on a par with the best in the world. YGN Unit 3 was declared in commercial operation on March 31, 1995, the original date established at the time of bidding and contract signing. During the first cycle of commercial operation (which was completed on Feb. 1, 1996), YGN Unit 3 achieved a capacity factor in excess of 85 percent. The unit is currently in its second cycle of operation. YGN Unit 4 went commercial on Jan. 1, 1996, three months ahead of the contract schedule, and continues to operate at essentially full power.

Project team

An international project team accomplished the design and construction of YGN Units 3 and 4. As the owner and operator of the plant, KEPCO was responsible for providing overall project management, balance of plant equipment and startup services. The NSSS supply team was comprised of ABB-CE, Korea Heavy Industries and Construction Co. (HANJUNG) and the Korea Atomic Energy Research Institute (KAERI). ABB-CE and KAERI worked together to jointly design the YGN NSSS and the nuclear reactor core. ABB-CE and HANJUNG supplied the NSSS equipment and components. HANJUNG manufactured the reactor coolant system (RCS) components, with the exception of reactor coolant pumps and motors, in the ROK. The nuclear fuel was manufactured by Korea Nuclear Fuel Co. using the ABB-CE 16-by-16 fuel design and components. Korea Power Engineering Co. and Sargent and Lundy provided the architect engineering services, and Hyundai Engineering and Construction Company Ltd. constructed the plant. HANJUNG, in conjunction with General Electric, supplied the turbine generator.

Project management

The project team first undertook the task of forming an integrated project schedule (IPS) based on both the construction and startup needs of the site and the need for documentation to support licensing. From those two end points, all deliveries by the different project entities were tied together. This integrated schedule allowed decisions to be made on individual items based on the impact on construction and startup. All entities participated in progress review meetings that were held every six months. At these meetings, status was reviewed against the IPS, and action plans were outlined to recover from unexpected delays. These meetings also served as convenient times for the review and resolution of plant interface issues.

Project schedule

The YGN project began in April of 1987, at which time a commercial operation date of March 31, 1995, was established for Unit 3. Commercial operation of Unit 4 was scheduled to occur one year later. The construction and startup (first concrete to commercial operation) of the first System 80 unit in the ROK was scheduled for, and actually completed in, 63 months. This compares favorably with recent world experience in construction and startup of nuclear power plants. Figure 1 shows the YGN Units 3 and 4 construction and startup milestones. The standardized design features, along with the experience gained from YGN Units 3 and 4, are being applied to further reduce the construction and startup times of the Korean Standard Nuclear Power Plant. These units are expected to require only 56 months for construction and startup, a reduction of seven months from the lead unit.

Design features

The YGN Units 3 and 4 pressurized water reactors each contain two independent primary coolant loops. Each loop has two reactor coolant pumps, a steam generator, an outlet pipe (hot leg) and two inlet pipes (cold legs). Pressurized water circulates through the system by means of motor-driven, single-stage, centrifugal pumps. An electrically heated pressurizer is connected to one of the hot legs, and safety injection lines are connected to each of the four cold legs and the two hot legs. Table 1 lists the major YGN design parameters.

The YGN Units 3 and 4 System 80 NSSS design includes numerous advanced features to enhance plant safety, performance and operability. The design represents an evolution of the System 80 NSSS design employed at Palo Verde Nuclear Generating Station Units 1, 2 and 3, Ariz., USA. These units have set numerous safety and performance records over the past 10 years. Startup and operating experience from Palo Verde was used to further advance the YGN NSSS design. The design changes implemented on YGN Units 3 and 4 were selected to yield maximum benefit while minimizing the licensing and schedule risk. The rated power for the design was changed from 3,817 MWt to 2,825 MWt for YGN Units 3 and 4 to satisfy needs and requirements of the utility. While doing this, the sizes of components and systems important to safety remained the same to provide additional flexibility in operation and increased margins of safety. The significant YGN NSSS design features, which are shown in Figure 2, include the following:

Designers did not reduce the size of the steam generators and pressurizer of the System 80 standard design. This resulted in greater volume per MW for these components, consistent with utility recommendations for advanced designs. The larger volumes increase the range of maneuverability of the NSSS by allowing the RCS to respond more slowly to unanticipated transients, thereby giving additional operating margin and reducing challenges to safety systems.

Designers maintained the larger System 80 standard design high-pressure and low-pressure safety injection pump capacities. The larger pumping capacities relative to core size provide additional safety margin following accidents, keeping the core covered and cooled for a larger range of events.

The reactor pressure vessel fabrication process utilized ring forgings rather than steel plate, eliminating the need for longitudinal welds and reducing the total number of welds. This reduces the amount of weld material exposed to neutron flux, ultimately reducing the time needed for in-service inspection of the reactor vessel welds.

Because of industry concerns and experience with Inconel-600 in pure water, high temperature environments, designers used Inconel-690 for the high temperature, small diameter nozzles on the major RCS components.

YGN incorporates a safety depressurization system to respond to new regulatory requirements on severe accidents (beyond design basis), thereby adding even more protection from the consequences of a severe accident.

The improved steam generator design minimizes the potential for intergranular stress corrosion cracking and incorporates an 8 percent tube plugging margin.

Engineers added an alternate ac power source to address recent regulatory concerns related to station blackout and to minimize the chance that such an event would occur.

Engineers also added the diverse protection system to augment the reactor protection system. This provides independent and diverse logic to initiate reactor trip and auxiliary feedwater actuation, thus protecting the plant from anticipated transients without scram.

Startup test program

As the lead unit, YGN Unit 3 conducted a comprehensive initial test program. The initial test program`s objectives were to:

– demonstrate that components and systems operate in accordance with design requirements,

– demonstrate that the plant can be operated safely and that performance levels can be maintained in accordance with established safety requirements,

– demonstrate that the design methods and analysis models accurately predict NSSS performance,

– verify that the plant is constructed as designed and responds as predicted, and

– provide training and experience to plant operators.

The extensive experience gained during the startup testing of YGN Unit 3 was used to reduce the critical path test of YGN Unit 4. The plant startup (from fuel load to full power operation) was achieved in approximately three months, half the time needed for Unit 3. In addition, the experience gained by the plant operators enabled the entire Unit 4 startup to be conducted with only one reactor trip.

Performance testing

Before commercial operation, a series of performance tests were conducted to demonstrate that the NSSS satisfied specified performance levels at designated conditions. This performance testing demonstrated that the NSSS safely and reliably produced warranted output. The higher steam pressure, along with the moisture removal efficiency, contributed to an overall plant efficiency of approximately 37 percent. This is at least 1 percent higher than most nuclear units. Table 2 shows the results of these tests.

Operations and maintenance

The YGN System 80 design incorporates features that significantly reduce operation and maintenance (O&M) costs. A substantial portion of these savings results from features aimed at increasing plant availability. One such feature is the reactor power cutback system (RPCS), which accommodates load rejections of various power magnitudes up to 100 percent, or loss of one of the operating main feedwater pumps, without challenging plant safety systems. The RPCS has operated successfully at all three Palo Verde units and at YGN Units 3 and 4. YGN Units 3 and 4 represent the first plant in the ROK to incorporate the RPCS.

The YGN System 80 design includes additional features which result in O&M costs, reduced radioactive effluents and lower personnel radiation exposures. Some of these are:

– low cobalt materials in the RCS, which reduce plant activity levels during maintenance;

– advanced fuel cycles with gadolina burnable poison and part-strength control element assemblies, which enhance power maneuvering capabilities;

– backup systems, such as alternate ac power sources, which can be credited to relax operability requirements in the technical specifications for a plant;

– advanced materials and fabrication techniques that extend design lifetimes of components and provide a basis for relaxing surveillance inspection and testing;

– selection of components with high reliability demonstrated by extensive use in the industry; and

– increased steam generator manway diameters for easier maintenance access.

Conclusions

With the commercial operation of YGN Units 3 and 4, several goals of the nuclear power program in the ROK have been realized. YGN Unit 3, the first System 80 NSSS in the ROK and the reference plant for the Korean Standard Nuclear Power Plant, was accepted into commercial operation on the original schedule established more than eight years ago. Unit 4 entered commercial operation three months ahead of its original schedule. The use of a standardized design, which includes advanced features, is expected to reduce the overall construction and startup time of follow-on units by up to the seven months. Finally, the successful transfer of technology during the design, construction and startup of the YGN units ensures that the technical self-reliance program for NSSS manufacturing is continuing on schedule.

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Yonggwang Units 3 and 4

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Authors:

Timothy J. Collier is the YGN project manager for ABB Combustion Engineering Nuclear Systems. He is responsible for all US activities associated with the YGN Units 3 and 4 power plants, including system design and licensing, component design, equipment supply, fuel design and fuel component supply. Since joining the company in 1983, Collier has held positions in licensing and project management. Collier holds a bachelor`s of science degree in electrical engineering from Tri-State University, Angola, Ind., USA, and a master`s of business administration degree from the University of Hartford, Hartford, Conn., USA.

Deva R. Chari is the plant startup manager for ABB Combustion Engineering Nuclear Systems. He has been responsible for the development of nuclear steam supply system-related test and operations guidelines and for providing technical assistance during the startup of YGN Units 3 and 4. Since joining the company in 1977, Chari has held positions in safety analysis and in startup and test. Chari holds a bachelor`s of science degree in physics from American University, Washington, D.C., USA and a master`s of science degree in nuclear engineering from North Carolina State University, Raleigh, N.C., USA.