KSNP+ construction permit review underway1 October 2003
KSNP+, the Improved Korean Standard Nuclear Power Plant, is the culmination of Korean nuclear technology: safer, easier to build, and far more cost-effective than its predecessors. The lead KSNP+ units, Shin-Kori 1 and 2 and Shin-Wolsong 1 and 2, are in the design stage and are expected to have construction permits soon. By Jae Young Yang, Jun Seok Yang, Hwan Chol Bae, Myung Youl Yoon, Ho Taek Yoon and Myung Jin Kim
Korea has been actively promoting nuclear power since the start of commercial operation of Kori 1 in 1978. A total of 18 nuclear units have now been put into operation providing 15,716MWe - about 40% of the total electricity capacity in Korea. By 2015 a further 11,600MWe will be in service, when a further six units now under construction and four more units planning stage, are completed. By then, nuclear power will fill more than 45% of the country's need for electric power. Table 1, showing the status and plan of nuclear power plants in Korea, reflects this.
The Korean Standard Nuclear Power Plant (KSNP) has been developed between 1983 and 1991. Six nuclear units of this design have been built: Ulchin 3-6 and Yonggwang 5 and 6. In May 1995, the International Atomic Energy Agency (IAEA) acknowledged that the KSNP represented a significant accomplishment in improving the safety of commercial nuclear power plants, after the agency carried out a safety review of Ulchin 3 and 4 as one of its expert missions. The details of the KSNP and Ulchin 3 and 4 designs were presented in the August 1992 and April 1998 issues of Nuclear Engineering International, respectively. Incremental design improvements have been continuously added to follow-up units of the KSNP.
The safety and economic record of these units have been excellent. Table 2 shows that the average load factor of the nuclear plants in Korea is over 90% - far higher than the world average - and the number of unplanned shutdown is less than or equal to 0.5 per reactor-year each year since 2000, demonstrating the reliability of the KSNP.
In January 1998, in recognition of the limitation in enhancing plant economics by incremental design improvements and stimulated by the strong competition in the world nuclear market, the KSNP Improvement Programme was launched. The new model, the Improved Korean Standard Nuclear Power Plant (KSNP+), was developed over a four-year period. The development was a co-operative effort by the Korean nuclear industry with the initiative of the utility, Korea Hydro and Nuclear Power (KHNP), and the nuclear steam supply system (NSSS) and the balance of plant designer, Korea Power Engineering (KOPEC).
KSNP+, an evolutionary model based on the proven safety and performance of the KSNP, incorporating the design, construction, and operational experiences from that series, is the most advanced of the KSNP series and incorporates the best technologies in the Korean nuclear industry, making it a safer, easier to build, and far more cost-effective plant than its predecessors.
The history and approaches taken for the development of the KSNP+ unit are briefly illustrated in Figures 1 and 2, respectively.
The KSNP improvement programme
The major design requirements guiding the design of the KSNP+ are shown in Table 3. The KSNP Improvement Programme had three distinct phases.
Phase I (January 1998 - January 1999)
In the first phase, a comprehensive conceptual model of the KSNP+ was formulated. In this phase, ideas for improvements were collected from the operator, constructors, vendors, and designers of the nuclear power plants so the experience could be fed back. A total of 323 collected ideas were filtered by preliminary reviews. After the filtering process, 103 suggestions underwent a feasibility evaluation in terms of technical soundness and cost-benefit. The items that passed the evaluation were integrated to form a conceptual model of the KSNP+.
Phase II (October 1999 - October 2001)
In the second phase, the conceptual model was refined and consolidated. Basic design development, design verification, prototype and scale model tests, and licensing activities were performed along with detail design of selected improvements which have a large impact on interfacing systems or require long lead time to manufacture. In the final selection process detailed technical evaluation and intensive cost-benefit analyses were performed on the ideas that had completed the feasibility process, as well as some that had been newly proposed. In a final selection process 91 improvements were chosen.
Phase III (January 2002 - September 2009)
The final phase of the programme, now in progress, is to realise the KSNP+ design through the construction of Shin-Kori 1 and 2. This phase will end when the units are in commercial operation - planned for the end of September 2009.
In collecting the improvement ideas, experience feedback in occupational radiation exposure reduction, safety and performance enhancement, plant outage reduction, operation and maintenance (O&M) improvement, and man-machine interface improvement were considered, along with design optimisation and simplification in plant arrangement, system design, and plant equipment types and capacity. The conventional safety concept of 'defence in depth' and the use of proven state-of-the-art technology were not to be challenged and these aspects were fed into the selection criteria.
HIGHLIGHTS OF KSNP+ DESIGN
The KSNP+ is an evolutionary model based on proven performance and safety of the KSNP. The major highlights of the KSNP+ designs include:
• An integrated head assembly.
• A new plant monitoring and annunciator system.
• A new safety parameter display and evaluation system.
• Optimisation and simplification of NSSS and balance of plant (BOP) systems.
• Optimisation of plant layout.
• A fission chamber based ex-core neutron flux monitoring system.
• The addition of a permanent refuelling pool cavity seal.
• Wide use of steel-concrete composite structures to save time during construction.
• A submerged circulating water system.
• Adoption of an 'area completion' concept to reduce construction period.
The safety concept of the KSNP+ is based on the multiple level defence-in-depth approach: prevention of accidents or deviations from normal operation; detection of accidents through monitoring; control of accidents to prevent propagation into severe accidents; and mitigation of severe accidents. This safety objective is pursued by compliance with deterministic requirements, supplemented by probabilistic methods. The use of improved passive and active engineered safety features further reduces the possibility of the occurrence of severe accidents.
REACTOR AND REACTOR
The reactor is a two loop PWR, as shown in Figure 3, with a design life of 40 years. The main characteristics of the KSNP+ design are summarised in Table 4.
The reactor coolant system (RCS), a barrier to the release of radioactive materials from the reactor core to the secondary system and containment atmosphere, is equipped with a reactor vessel, two steam generators and four reactor coolant pumps. These are symmetrically arranged about the reactor vessel centreline with a pressuriser on one side, all inside the containment building and connected by pipe assemblies. The pipe assemblies are welded using a narrow-gap automatic welding technique to enhance uniformity of the weld deposit, thus ensuring adequate material properties to apply the 'leak before break' concept.
Reactor and internals
The reactor vessel and internals are shown in Figure 4. The vessel is fabricated using a ring forging method, which reduces the number of welds and simplifies the vessel structure, as compared with rolled and welded plate fabrication. The vessel is equipped with an enhanced leak-tight flange; Inconel 690 control element drive mechanism (CEDM) and in-core instrumentation (ICI) nozzles; sufficient annular downcomer space to reduce the radiation of the vessel wall; irradiation surveillance capsules; an optimised bottom-mounted ICI system; and improved flow skirt to enhance the uniformity of coolant flow. The integrity of the reactor internals were verified through a comprehensive vibration assessment programme.
The reactor vessel head insulation is designed so that a robot camera inspection of the head nozzles can be performed for early identification of the possible symptoms of the reactor vessel head degradation similar to the incident that occurred at Davis-Besse in the USA.
The steam generator design is shown in Figure 5. Each one includes a U-tube bundle, evaporator with integral economiser, double main feed lines and single auxiliary feed line, double steam lines with integral flow restrictors, high capacity separators and high efficiency dryers. It is designed to have an 8% tube plugging margin to ensure a longer full power life. The U-tube bundles are fabricated using Inconel 690 thermally treated tubing, which is resistant to primary water stress corrosion cracking. Vertical and horizontal strips are added to support the tubes located at the inner and upper side of the tube bundle, reducing tube wear caused by flow induced and low frequency vibrations.
Improved feedwater recirculation has been achieved by optimising the feedwater shared between the economiser and downcomer region of the steam generator. This results in a reduction of water hammer in the feedwater system, and an increase in thermal efficiency.
The integral retention ring located at the inlet and outlet nozzles for the installation of single nozzle dams provides an increased operating margin during mid-loop operation.
The pressuriser design adopts the Electric Power Research Institute's (EPRI's) advanced light water reactor (ALWR) requirements of large pressuriser concept. Relative to thermal output, the pressuriser volume is about 30% larger than that of CE-designed System 80 plants. This improves plant response to transient and accident conditions. Sufficient RCS depressurisation and overpressure protection capabilities are provided by the reactor coolant gas vent system, safety depressurisation system and three pressuriser safety valves installed at the top of the pressuriser.
Integrated head assembly
The integrated head assembly (IHA) structure joins all the heavy and complex components in the reactor vessel head area of the KSNP that need to be disassembled, reassembled and stored individually during each plant outage for refuelling operations. The use of IHA, shown in Figure 6, reduces the number of steps and the time to handle vessel head area components, compared with the KSNP, thereby reducing refuelling outage length and occupational radiation exposure. The seismic supports inside the IHA reduce seismic loads on CEDM cables to one-sixth of those in KSNP.
Safety injection system
The safety injection system (SIS), with two trains in two separate divisions, provides core cooling in the event of a loss of coolant accident (LOCA).
The SIS also provides inventory and reactivity control during other events which depressurise the RCS, such as steam line breaks and steam generator tube ruptures, and during feed-and-bleed operation.
Containment spray system and combustible gas control system
The containment spray system (CSS) is capable of reducing the containment pressure and temperature, removing radioactive fission products from the containment atmosphere and mixing the containment atmosphere to prevent local accumulation of combustible gases following a LOCA or main steam line break inside the containment.
Adapting passive auto-catalitic recombiners (PARs) in the combustible gas control system (CGCS) of the KSNP+ eliminates the operator action required to control the hydrogen concentration during design basis accidents, as was necessary in KSNP.
Auxiliary feedwater system
The auxiliary feedwater system (AFWS) provides secondary makeup water to the steam generators when the feedwater system is unavailable. The system incorporates cross-tie lines connecting two redundant trains of the system with improved flow path design that ensures the delivery of the makeup water on demand.
Safety depressurisation system
The safety depressurisation system (SDS), which consists of two separate flow paths from the pressuriser to the containment via remotely operated manual isolation and globe valves, provides a means of rapidly depressurising the RCS when both main and auxiliary feedwater systems are unavailable. By rapidly depressurising the RCS, core cooling can be achieved by feed (high pressure safety injection) and bleed (coolant discharge through SDS valves) operation.
Chemical and volume control system
The chemical and volume control system (CVCS) controls the RCS inventory, primary water chemistry and the reactivity. The system has undergone continuous improvements from the early KSNP design. These include incorporating the letdown orifices to avoid unstable thermal and hydraulic transients, using centrifugal charging pumps for easier maintenance and more reliable operation, and using an auxiliary charging pump of positive displacement type to enhance its ability to cope during a station blackout. These changes are retained in the KSNP+. In addition, the sizes of the large CVCS tanks - such as the refuelling water tank and reactor makeup water tank - are optimised, and the RCP seal injection heat exchangers and associated measurement and control devices are removed to enhance the operation simplicity and reduce radioactive wastes.
Shutdown cooling system
The shutdown cooling system (SCS) consists of two redundant divisions, each of which have the capacity to bring the RCS to the refuelling temperature. It provides heat removal capability from the RCS to the component cooling water system at temperatures where heat removal using steam generators is ineffective.
BALANCE OF PLANT
Main steam system
The main steam system (MSS) carries steam from the steam generators to various systems and equipment including the high pressure turbine and main and auxiliary feedwater pump turbines. The system includes the main steam isolation valves (MSIVs), main steam safety valves (MSSVs), main steam atmospheric dump valves and turbine bypass valves. Their functions are to isolate an affected steam generator, provide overpressure protection, support RCS cooldown when MSIVs are closed or when the main condenser is not available, allowing full turbine load rejection without causing a reactor trip or lifting pressuriser safety valves or MSSVs.
Main feedwater system
The main feedwater system (MFWS) supplies feedwater from the deaerator storage tank to two steam generators at the required pressure, temperature and flow rate. The KSNP+ design incorporates three motor-driven feedwater booster pumps and three turbine-driven feedwater pumps. Each combination can provide up to 55% of the system flow requirements. In normal full power operation, all three main feedwater pumps are running, each delivering about 33% of feedwater demand. When one of the three operating pumps stops, the remaining two pumps are capable of providing full flow to maintain the reactor full power. This enables a pump to undergo maintenance during full power operation without reducing the power output.
INSTRUMENTATION AND CONTROL
NSSS control system
As shown in Figure 7, the NSSS control system (NCS) consists of the reactor regulating system (RRS), control element drive mechanism control system (CEDMCS), pressuriser pressure control system (PPCS) and pressuriser level control system (PLCS), feedwater control system (FWCS), steam bypass control system (SBCS) and reactor power cutback system (RPCS). It allows the reactor to follow turbine load changes under normal plant operating conditions. An improved RPCS control logic, providing a reactor power step reduction capability by dropping a selected number of control element assemblies (CEAs) under plant transients such as a large turbine load rejection, turbine trip or partial loss of operating main feedwater pumps, is implemented in conjunction with the MFWS improvement.
Plant monitoring and annunciator system
The plant monitoring and annunciator system (PMAS) combines the plant monitoring system - comprising the plant computer system (PCS) and plant data acquisition system (PDAS) - and the plant annunciator system (PAS) used in KSNP. The system provides the plant status monitoring capability for normal operation and early warning of abnormal conditions, which may lead to shutdown or accidents. It also assists the plant staff in operating the plant within specified limits and in evaluating the performance of reactor core, primary and secondary plant systems.
The safety parameter display and evaluation system (SPADES) replaces the critical function monitoring system used in the KSNP series. It has enhanced safety status monitoring and diagnostic capabilities and new emergency operating procedures. As shown in Figure 8, the system is configured with dual input local area networks (LANs). Each communicates with four separate servers providing system operation, data storage and retrieval, execution of application software, and alarm functions. The system is highly reliable, immune to the failure of any one of the eight servers.
Because of the importance of the PMAS on plant operation, a prototype was fabricated (see picture above) and major functions were tested during the Phase II of the KSNP Improvement Programme.
The PMAS, in compliance with the human factor engineering (HFE) principles, uses pop-up menu bars, has strengthened historical data retrieval and graphical display, provides more analytical tools to assist the operator and has an enhanced man-machine-interface. We believe it is the most operator friendly system in the world.
Plant protection system
The plant protection system (PPS) consists of the digital plant protection system (DPPS) and the core protection calculator system (CPCS). The DPPS initiates reactor trip and sends appropriate signals to the safety features actuation system when it detects non-permissible plant conditions. The CPCS monitors pertinent reactor core conditions and calculates the departure from the nucleate boiling ratio (DNBR) and local power density values and generates a trip signal if the specified acceptable fuel design limit is challenged.
The systems have been upgraded with Common-Q, AC160 platform with the provision of the maintenance and test panels (MTPs) to assist the operators to monitor, test and maintain the system.
The CPCS software is developed using C language-based Advant AMPL, which eliminates the complexities involved in using the assembler thereby providing ample flexibility to maintain and upgrade the software and reducing the possibility of having routine errors in the software performing safety related functions. A rigorous and comprehensive digital software management programme, including integration, installation, and verification and validation programmes, in compliance with relevant IEEE standards, are implemented to ensure the quality and reliability of the software.
Ex-core neutron flux monitoring system
The ex-core neutron flux monitoring system (ENFMS) consists of four safety channels and two startup/control signal processing drawers for reactor control, protection and information display, as shown in Figure 9. It
measures reactor power level by monitoring the neutron leakage from the reactor core. The system is simplified to use safety grade fission chamber detectors only, with a design life of 40 years. Compared to KSNP, whose monitoring system has three types of detectors, fission chamber, uncompensated ion chamber, and BF3 proportional counter, with associated power supplies and signal processing equipment, the ENFMS of the KSNP+ is essentially maintenance-free, thereby reducing O&M costs and occupational radiation exposure.
Digital BOP plant control system
The digital BOP plant control system (BOP PCS) remotely controls and monitors both safety and non-safety related plant equipment. It also indicates the operating status of the plant process systems and equipment, and provides the information required by the PMAS. The microprocessor-based controller enables multi-loop control of plant equipment. In order to reduce the risk of a common mode failure of a software-driven digital system, a hard-wired backup panel is provided. This allows the operator to bypass the BOP PCS and control equipment necessary to achieve hot shutdown during the unlikely event of a BOP PCS failure.
MAIN CONTROL ROOM
Modern technologies using software-based control, automatic testing, multiplexing, alarm prioritisation, fault tolerance, and computer driven displays have been employed in the control room design. The facilities such as main control boards, operating consoles, and panels are designed and arranged in compliance with HFE principles provide the operator error-free, comfortable and safe operating environs.
The plant arrangement, optimised using a twin unit concept with a combination of slide-along and symmetrical arrangement types, is shown in Figure 10. The compound building combines five buildings from the KSNP (two secondary auxiliary buildings, two access control buildings, one radwaste building with two units) making the building arrangement simple and cost effective. The new design eliminates the radwaste tunnel, minimises underground common tunnel and reduces building volume.
Plant arrangement optimisation has reduced the protected area by about 9% and the total building volume by 15%, as well as considerably reducing operator line movement. These contribute greatly to improving plant economy and reducing occupational radiation exposure.
PLANT SAFETY AND ECONOMY
A comprehensive assessment on the plant safety and economy was carried out. A level 1 probabilistic safety assessment (PSA) performed for the 16 initiating events identified 246 core damage scenarios. The core damage frequency (CDF) for the KSNP+ is estimated to be 6.77x10-6 per reactor year, about 20% lower than that of Ulchin 3 and 4, and 32% lower than the target value of 1x10-5 per reactor year set for the ALWR by EPRI.
The results of a comprehensive cost-benefit analysis indicate that the economic goal of KSNP+ - to secure a cost advantage over the KSNP and competitive energy sources, such as coal-fired power generation - is considered achievable because of the significant savings in construction material, O&M costs, and plant outage.
SHIN-KORI AND SHIN-WOLSONG
Contracts for the construction of the first four KSNP+ units - Shin-Kori 1 and 2 and Shin-Wolsong 1 and 2 - were signed on 9 August 2002. The project key milestones are shown in Table 5.
In order to enhance the public acceptance, the two plants have adopted environmentally-friendly design features. The submerged circulating water system, which enhances the diffusion and mixing of plant effluents, the plant and building arrangement, and the colouring in harmony with the plant vicinity (shown in Figure 11) are examples of the efforts taken in this area.
Construction and licensing
As of the end of September 2003, the projects had reached 19.4% and 10.8% of the overall construction schedule for Shin-Kori 1 and 2 and Shin-Wolsong 1 and 2, respectively. The construction permit reviews for these plants are underway by the Korean licensing authority and the permits are expected to be issued soon.
After receipt of the permits, site levelling work will be undertaken and the projects will enter into the full scale construction stage.
The commercial operation of Shin-Kori unit 2 will mark the completion of the KSNP Improvement Programme. The KSNP+ is on its way to proving its technological and economical competitiveness.