Ulchin 3 and 4: the first Korean Standard Nuclear Power Plants30 April 1998
Ulchin unit 3 – the first unit of the Korean Standard Nuclear Power Plant (KSNP) series – is currently undergoing power ascension tests, with commercial operation scheduled for 30 June 1998, while unit 4 is undergoing hot functional tests. Ulchin 3 and 4, both 1000 MWe PWRs, are envisaged as the lead units in a series of more than ten reactors, including two reactors in North Korea. As well as the birth of Ulchin 3 and 4, the KSNP development programme, which has been underway for more than a decade, has also resulted in Korean self-reliance in nuclear power technology.
Korea has been actively promoting nuclear power since 1978 when its first nuclear plant entered commercial operation. The country takes pride in the outstanding performance of its nuclear plants, which will account for a major share of the national energy mix in the future.
As of the end of 1997, the total installed capacity of the twelve operating commercial nuclear units (ten PWRs and two PHWRs) was 10 339 MWe. A further six units (four PWRs and two PHWRs) totalling 5400 MWe are under construction, and there are plans for another ten units to be in commercial operation by 2010. The nuclear power plants in operation, under construction and planned in Korea are listed in Table 1. With one of the currently operating units, Kori 1 (595 MWe PWR), due to be taken out of service for decommissioning in 2009, the twenty-seven units planned to be in operation in 2010 are expected to account for 33% of the country’s installed generating capacity and to provide 45.5% of the electricity generated. However, at the time of writing the mid and long-term construction plan for nuclear power plants to 2015 was under review by the government (reflecting changed circumstances for nuclear plant construction in Korea).
Focused efforts have been made to improve the availability and safety of nuclear power plants. As a result, the performance of Korean nuclear power plants has shown a remarkable improvement over the past several years. For instance, in 1995, the average capacity factor for Korean nuclear power plants was 87.3%, while the world average was only 71.6%, and in 1996 the figure for Korea was 87.5%, compared with a world average of 72.9%. The capacity factor has been over 80% for the past five years.
A comprehensive programme has been put in place to improve plant capacity factors. Short term measures include: reducing unplanned scrams caused by human error and equipment failure; shortening of the refuelling outage duration; enhancing the quality of operating plants.
THE KSNP PROGRAMME
The KSNP nuclear technology self-reliance programme was formulated in 1983 in direct response to recognition of the need for a stable and reliable electricity supply through the 1990s and into the 21st century. The KSNP programme started with technology transfer from foreign vendors, but aims to contribute to self-sufficiency in electricity supply by achieving self-reliance in nuclear power related technologies.
The KSNP programme (see Figure 1) culminated in a utility requirements document (Korea Standard Requirement Document, K-SRED), which sets out the requirements of the utility in terms of design, construction and maintenance of nuclear power plants, and in the Korea Standard Safety Analysis Report (K-SSAR), which gives a complete description of the standard design and presents detailed safety analysis for a complete nuclear power plant. Detailed information on KSNP was presented in Nuclear Engineering International, August 1992.
Beginning in 1987, and still ongoing, Korean entities were the prime contractors, with technology transfer from foreign vendors acting as subcontractors. Within this framework, design and engineering work has been a co-operative effort by domestic and foreign groups, with growing responsibility being taken by Korean entities.
The government assigned to the following four major entities the tasks involved in becoming self-reliant in nuclear power plant design and technology:
• The Korea Electric Power Corporation (KEPCO): overall project management, construction management, start-up and operation of the plant.
• The Korea Power Engineering Company (KOPEC): nuclear steam supply system (NSSS) design, architect engineering and balance of plant (BOP) design.
• The Korea Heavy Industries and Construction Company (Hanjung): major component manufacturing and the turbine generator.
• The Korea Nuclear Fuel Company (KNFC): fuel design and manufacturing.
The first two units in this self-reliance programme were Yonggwang 3 and 4, which began their commercial operation in March 1995 and January 1996, respectively. The project received the “Project of the Year Award” in 1995 from Power Engineering International for its excellence in design, construction and operation.
Yonggwang 3 and 4 are performing well, generating about 6730 MWh and 7600 MWh in 1996, respectively, while the average capacity factor during the first and second operating cycles was a remarkable 99.8%.
Ulchin 3 and 4 have been designated the first KSNPs and are being built by the four companies listed above. They benefit from the experience gained with Yonggwang 3 and 4, but incorporate improved technology.
The six other 1000 MWe PWRs to be constructed under the KSNP programme will incorporate experience gained from Ulchin 3 and 4. Yonggwang 5 and 6 are currently under construction and Ulchin 5 and 6 are in the design stage, while the two units to be built in North Korea under the Korea Peninsula Energy Development Organization (KEDO) Light Water Reactor (LWR) project are at the site preparation stage.
ULCHIN 3&4 PROJECT ORGANISATION
The organisation of the Ulchin 3 and 4 project is shown in Figure 2.
The Korea Electric Power Corporation (KEPCO) is the owner. It is in charge of the total project management, BOP equipment procurement, construction management, and start-up and operation of the units. KEPCO owns and operates 95% of the country’s operating power plants including all nuclear power plants, and produces some 97% of electricity in Korea.
The Korea Power Engineering Company (KOPEC) is in charge of NSSS design, architect-engineering services, and balance of plant design for Ulchin 3 and 4. With wide experience in Korea, it is the country’s sole qualified organisation in the areas of nuclear plant NSSS design and architect-engineering and has performed all the design and engineering work on the project.
The Korea Heavy Industries and Construction Company (Hanjung) is in charge of supplying the NSSS major components and turbine generator (T/G). This includes manufacturing of the reactor vessel, steam generators, pressuriser, reactor coolant piping and other equipment. Hanjung also participates in installation of mechanical and electrical equipment for Ulchin 3 and 4.
The Korea Nuclear Fuel Company (KNFC) is in charge of supplying the nuclear fuel and reactor core design for Ulchin 3 and 4.
The Dong-Ah Construction Industrial Company is in charge of construction, performing the civil/architectural work and equipment installation for Ulchin 3 and 4.
Foreign companies such as ABB Combustion Engineering (ABB-CE) and General Electric (GE) act as subcontractors. ABB-CE is in charge of supplying the reactor internals, control element drive mechanisms, reactor coolant pumps and I&C equipment. GE in co-operation with Hanjung is supplying the turbine generator.
One characteristic of the Ulchin 3 and 4 project is that lead roles are played by the Korean participants in the KSNP programme.
The major project milestones for Ulchin 3 and 4 are shown in Table 2.
Contracts to Korean entities as prime contractors in NSSS and A/E design and engineering were placed in July 1991. Site excavation was started in May 1992, and first concrete was poured in July 1993 for unit 3 and November 1993 for unit 4. For Ulchin 3 initial fuel loading was completed in November 1997, initial criticality was attained in December 1997, and the unit was synchronised to the network in January 1998. Ulchin unit 3 is scheduled to be in commercial operation in June 1998, which corresponds to a construction time of 60 months (from first concrete to start of commercial operation). Ulchin unit 4 completed its cold hydraulic test in December 1997, and is scheduled to be in commercial operation in 1999.
The basic design philosophy for Ulchin 3 and 4 is to:
• Apply advanced technology and use improved design concepts.
• Feedback experience gained from the long operating history of nuclear power plants in Korea.
• Incorporate significant improvements in operation safety and construction performance.
• Comply with international codes, standards and regulations.
• Make maximum use of experience at foreign nuclear power plants to achieve technical competence and economic performance.
• Establish a basis for the future export of nuclear power plants and related services.
Features that could offer significant improvements in safety and performance have been selected and incorporated into the new design in compliance with new licensing requirements.
Building on proven technology, Ulchin 3 and 4 provide increased simplicity, safety and operating margin, improved man– machine interface and greater operational reliability.
An evolutionary approach has been taken to the development of Ulchin 3 and 4, which are based on KSNP technology and also benefit from the experience gained in the construction and operation of existing plants in Korea.
n Reactor and reactor coolant system (RCS)
The reactor is a two-loop pressurised water reactor with a plant design life of 40 years. The RCS design, loop configuration and the design of the main components (reactor vessel and internals, steam generators, reactor coolant pumps, pressuriser and primary piping) are very close to those of the KSNP design as presented in Nuclear Engineering International, August 1992.
The Ulchin 3 and 4 safeguard systems comprise the safety injection system (SIS), the safety depressurisation system (SDS), the shutdown cooling system, the emergency feedwater system and the containment spray system. The safeguard systems represent the area in which the most significant improvements have been made in Ulchin 3 and 4 relative to KSNP and to Yonggwang 3 and 4. They are designed not only to accommodate design basis transients and accidents, but also to prevent and mitigate postulated severe accidents.
Safety injection system
The safety injection system delivers borated water into the RCS to flood and cool the core following a loss of coolant accident. The SIS is also designed to remove core decay heat and to restore the RCS inventory in the event of a total loss of feedwater.
The major components of the SIS are two low pressure and two high pressure safety injection pumps, four safety injection tanks, a refuelling water storage tank, and associated piping and valves.
Safety depressurisation system
The safety depressurisation function of the SDS is one of the major improvements in the KSNP and Ulchin 3 and 4 designs. The aim of the SDS is to provide a manual means of rapidly depressurising the RCS in the highly unlikely event of a total loss of feedwater.
The core damage frequency estimate is significantly reduced when account is taken of feed and bleed capabilities should normal and emergency feedwater supplies be unavailable for removing heat through the steam generators.
The bleed function is accomplished via the remote, manual opening of the motor operated valve in the bleed line of the system. As RCS pressure decreases, the SIS initiates feed flow to the RCS and restores the RCS water inventory.
Shutdown cooling system
The shutdown cooling system (SCS) is used to provide a forced circulation path for decay heat removal from the RCS, and to transfer the heat to the component cooling water system at RCS temperature and pressure below or equal to 350°F and 410 psig.
The SCS makes use of the low pressure SIS. The design pressure of the SCS has been increased to 900 psig, compared with an SCS pressure of 485-750 psig for Yongg-wang 3 and 4, in order to reduce the possibility of an interfacing loss of coolant accident if the SCS is exposed to the full RCS pressure.
High reliability for the SCS during mid-loop operation has been achieved through the installation of a permanent level indication system, sightglass with alarm, ultrasonic level indication system, temperature indication function and SCS low flow alarm (following USNRC GL88-17).
The containment provides a leaktight barrier to preclude uncontrolled reactivity release in the event of postulated accidents. The containment also provides protection for safety related systems and equipment from external hazards such as flooding or aircraft crash.
The containment building is a cylindrical structure with a hemispherical dome. It is constructed of pre-stressed concrete clad with a steel liner plate to withstand thermal and dynamic loads arising from postulated accidents.
Modern technologies using software-based control, automated testing, multiplexing, alarm prioritisation, fault tolerance and computer driven displays have been used in the Ulchin 3 and 4 control room.
Human factor engineering has been used to enhance operator effectiveness. The main control room contains a compact control board with multiple display and control devices, providing organised, hierarchical access to alarms, displays and controls.
The compact control board is fully capable of performing all main control room functions as well as supporting division of operator responsibilities.
The plant protection and control systems employ modern digital technology, including multiplexing and fibre optics for both monitoring and control functions.
SAFETY AND PERFORMANCE IMPROVEMENTS
The main technical data for Ulchin 3 and 4 are summarised in Table 3. The design features implemented to improve plant safety and performance are summarised below.
Enhanced plant safety
• Installation of safety depressurisation system (SDS) to provide a manual means of rapidly depressurising the reactor coolant system in the highly unlikely event of a total loss of feedwater flow to both steam generators.
• Installation of alternate alternating current (AAC) to cope with station blackout (SBO).
• Provision of hydrogen control capability – 18 hydrogen igniters – in the containment for use under accident conditions.
• Reactor cavity design improvement in order to mitigate severe accidents by increasing the floor areas of reactor cavity, by installation of a sump to collect the core debris, and by installation of piping penetrations for a Reactor Cavity Flooding System (RCFS).
• Reinforcement of plant safety during mid-loop operation in compliance with USNRC GL88-17, especially by providing additional monitoring functions such as permanent level indication system, sightglass with alarm, ultrasonic level indication system, temperature indication function and low flow alarm in Shutdown Cooling System (SCS).
• Increase in SCS design pressure from 485 psig to 900 psig in order to decrease the possibility of an Interfacing System Loss of Coolant Accident (ISLOCA).
• Performance of a full scope level 2 Probabilistic Safety Assessment (PSA). The core damage frequency (CDF) estimate for Ulchin 3 and 4 is 2.78 x 10-5/reactor-year.
Enhanced availability and reliability
• Application of human factor engineering for the Main Control Room (MCR) design.
• Use of a Core Operating Limit Super-visory System (COLSS) to improve the core monitoring function and to gain reactor operating margin.
• Increase in the number of charging pumps from three to four to improve charging system reliability.
• Installation of generator circuit breaker (GCB) to increase the reliability of on-site power.
• Expanded use of plant computer.
• Use of Inconel-690 for penetration nozzles of the reactor coolant pressure boundary.
Improved control system design
• Establishment of alarm priority.
• Minimisation of alarm indications and instruments through adoption of a discrete indication and alarm system.
• Expanded use of digital signals.
• Expanded use of multiplexer, reducing use of hardwired cabling.
• Application of advanced turbine generator control system (Mark-V), based on a digital control and monitoring system, with digital automatic voltage regulator (AVR).
• Minimisation of pipe whip restraints and the number of snubbers through application of the leak-before-break (LBB) concept in RCS piping and pressuriser surgeline.
• Improved solid waste management system through application of a dry treatment system for concentrated waste water.
LICENSING AND THE IAEA MISSION
Ulchin 3 and 4 were licensed in accordance with a two-step licensing procedure: construction permits (CP); and operating licence (OL). The key dates were:
• CP approved in July 1993 by the government (Nuclear Safety Committee and Korea Institute for Nuclear Safety (KINS)) after review of the Preliminary Safety Analysis Report (PSAR).
• OL for Ulchin 3 approved in November 1997 after review of the Final Safety Analysis Report (FSAR).
Measures to fulfil additional licensing requirements, including submission of the environmental report, full scope level II PSA report, technical specifications, radiological emergency plan and drawing up of emergency operation procedures, have been carried out by Korean entities.
The safety aspects of Ulchin 3 and 4 were evaluated by an IAEA expert mission in May 1995.
This expert mission evaluated the reactor core and fuel design, the reactor system design, the system mechanical integrity, the engineering safety features and accident analysis, the instrumentation and control, human factors issues, the PSA and the treatment of severe accidents. The evaluation concluded that Ulchin 3 and 4 represent significant accomplishments in the continuous improvement of safety, have added numerous design improvements, and have established the use of a reference plant concept.