Design Verification Program of SMART

Transcription

1 GENES4/ANP2003, Sep , 2003, Kyoto, JAPAN Paper 1047 Design Verification Program of SMART Si-Hwan Kim, Keung Koo Kim,* Ji Won Yeo, Moon Hee Chang and Sung Quun Zee Korea Atomic Energy Research Institute P.O. Box 105, Yuseong, Taejon, Korea, Since July of 1997, KAERI (Korea Atomic Energy Research Institute) has been developing an advanced reactor called SMART(System integrated Modular Advanced ReacTor). SMART is a 330MWt integral type pressurized water reactor that can be used for cogeneration, district heating and seawater desalination as well as electricity generation. Safety enhancement and economic improvement are the two most important considerations in the design of the SMART. The SMART design combines firmly established commercial reactor design technologies with advanced design features. Most of the technologies and design features implemented into the SMART have been proven in industries and advanced design features and technologies have been proven or qualified through SMART design verification program. SMART design verification program includes basic thermal-hydraulic experiments, separate effect test on major components, integrated tests of safety system. The overall performance and safety of SMART will be demonstrated through the one fifth scaled SMART-pilot plant. In June of 2002, SMART Research and Development Center was launched for SMART-pilot plant construction project. The first phase will focus on the design optimization and technology verification by way of tests and experiments, and the second phase to construct SMART-P plant will carry out detailed design and complete the construction by 2008 KEYWORDS: SMART, SMR, Reactor Safety, Advanced LWR reactor, Design Verification I. Introduction Since nuclear power generation became established, the size of reactor units has grown from less than 100 MWe to more than 1400 MWe, with corresponding economies of scale in operation. At the same time, there have been the activities on improving the efficiency and safety of nuclear power plant based on light water reactor technology. As the LWR improvement, various advanced types of Small and Medium sized Reactor (SMR) had been developed worldwide. Generally, it is well known that modern SMRs for power generation are expected to have greater simplicity of design, economy of mass production, and reduced capital costs. Many of SMRs have also beneficial advantages of reactor safety and economics by easy implementation of advanced design concepts and technology.[1],[2] KAERI has been developing an advanced reactor called SMART for a dual purpose: seawater desalination and electricity generation.[3] The conceptual design and the basic design of SMART with a desalination system were completed in March of 1999 and in March of 2002, respectively.[4] The nuclear desalination plant with the SMART can produce 40,000 tons of fresh water per day and 90MW of electricity. SMART design is focusing on the enhancement of the safety and the improvement of the reliability as well as the economics. For the safety enhancement and economic improvement, highly advanced design features enhancing the safety, reliability, performance, and operability were introduced in the SMART design. * Corresponding author, Tel , Fax , . kkkim@kaeri.re.kr Advanced design features implemented into the SMART should be proven or qualified by experience, testing, or analysis and, if possible, the equipment shall be designed according to the applicable approved standards. Some fundamental thermal-hydraulic experiments were carried out during the design concept development to assure the fundamental behavior of major concepts of the SMART systems. Various thermal-hydraulic and mechanical tests are in progress and also planned. In addition, overall SMART performance will be demonstrated through SMART pilot plant construction and operation. This paper explains the SMART design goals and presents various advanced design features adopted in SMART with respect to safety enhancement and economic improvement. Finally the SMART deign verification program is described II. SMART Design Goal In the beginning stage of the SMART development, top-level requirements for safety and economics, as described in Table 1, were imposed for the SMART design. Safety enhancement is one of the most important considerations in development of SMART. The safety requirements on the SMART were top-tiered by the core damage frequency per reactor year less than 10-7 and the large off-site dose release rate of less than 10-8 per reactor year. The most important design principle of SMART is that SMART incorporates passive or inherent safety features which require no active controls or operational intervention to avoid accidents in the event of malfunction, and may rely on gravity, natural convection. As most of SMRs based on light water technology, the SMART is designed as an

2 integral type PWR and reactor pressure vessel contains major primary components. The integral reactor design feature excludes the possibility of the Large-Break Loss-of-Coolant-Accident (LB-LOCA) by the elimination of coolant loops and also reduces the fast neutron fluence on the reactor pressure vessel. In order to compensate economical deterioration of SMRs comparing with that of a large-sized reactor, many possible mechanisms for the economic improvement are adopted in SMART. System simplification and reduction of pipes and valves are possible due to the implementation of advanced passive systems and of highly inherent safety characteristics. Modularization, component standardization, and on-shop fabrication & direct site installation of components are additional characteristics which can contribute to the reduction of construction cost. Table 1. Top Level Requirements Imposed for the SMART Design Category Contents Requirements Safety Economics Performance Core Damage Frequency Less than 10-7 /reactor year Large Radioactivity Release Frequency Less than 10-8 /reactor year Grace Period for Longer than 72 Reactor Cooling in hours Loss of Offsite Power Thermal Margin Greater than 15 % Electric Production Less than that of Cost Gas TB Construction Period Less than 36 Months Availability Greater than 95% Reactor Life 60 Years The innovative design features of SMART include passive pressurizer, fine controlled CEDM, the canned motor pumps and advanced MMIS(Man Machine Interface System). The primary system pressure is passively adjusted by partial pressure of steam and nitrogen gas filled in the self-controlled pressurizer in accordance with variation in pressure and temperature of the primary coolant. CEDM (control element drive mechanism) has a very fine-step maneuvering capability to compensate the core reactivity change caused by fuel depletion during normal operation of the SMART. Modular type once-through steam generator has an innovative design feature with helically coiled tubes to produce 40 C superheated steam at normal operating condition. The canned motor pumps remove the necessity of pump seals and the possibility of the small-break LOCA associated with pump seal failure. Advanced man-machine interface systems using digital technologies and equipments will reduce the human error factors. MCP Steam Outlet Reactor Vessel CEDM Annular Cover PZR Feedwater Inlet SG To meet top level requirements, highly advanced design features enhancing the safety, reliability, performance, and operability should be introduced in the SMART design. Thus, the SMART design combines firmly established commercial reactor design technologies with the use of new advanced technologies which can be provide significant enhancements in safety. Core Bottom Shielding Side Shielding III. Major Design Features of SMART As shown in Figure 1, the SMART reactor assembly consists of proven KOFA (Korea Optimized Fuel Assembly) [5], helical once-through steam generators, a self-controlled gas pressurizer, control element drive mechanisms, and main coolant pumps in a single pressure vessel. four(4) main coolant pumps are installed vertically at the top of the reactor pressure vessel(rpv). The twelve(12) steam generators are located at the circumferential periphery between the core support barrel and RPV above the core. Table 2 explains the key design parameters of SMART major components. Figure 1. SMART Reactor Assembly In order to enhance safety characteristics, many inherent safety features has been adopted in the SMART system, which are low core power density, large negative Moderator Temperature Coefficient (MTC), high natural circulation capability and integral arrangement to eliminate large break loss of coolant accident, etc. The small core size eliminates the possibility of the xenon oscillation instability too. Besides the inherent safety characteristics of SMART, further enhanced safety is accomplished with highly reliable engineered safety systems. The engineered safety systems

3 designed to function passively on the demand consists of a reactor shutdown system, passive residual heat removal system, emergency core cooling system, safeguard vessel, reactor overpressure protection system and containment overpressure protection system. With all those enhanced safety features, the core meltdown frequency is expected to one hundredth of that of a conventional reactor. Even after the beyond-design accidents accompanying core damage, the safeguard vessel of the SMART provides an additional barrier to the radioactive release compared with the current commercial reactors. Also the water inside the internal shielding tank surrounding the bottom-side of the RPV behaves as an external cooling mechanism to mitigate severe accidents. Table 2. Key Design Parameters of SMART Major Components Reactor vessel assembly (RVA) Overall length (m) Outer diameter (m) Average RV thickness (mm) Reactor coolant system (RCS) Design pressure (MPa) Design temperature ( C) Operating pressure (MPa) Core inlet temperature ( C) Core outlet temperature ( C) Steam generator (SG) No. of SG cassettes Tube material Pressurizer (PZR) Operating temperature ( C) Control element drive mechanism No. of CEDM Main coolant pump (MCP) No. of MCP Secondary system Feed-water temperature ( C) Feed-water pressure (MPa) Steam temperature ( C) Steam pressure (MPa) Degree of superheating ( C) at normal operation Once-through helical tubes 12 Ti-alloy Self controlled Gas PZR 100 Linear pulse motor driven 49 Canned motor axial pump > >40 IV. Safety of SMART 1. Safety Characteristics of the SMART Design Besides inherent safety characteristics of SMART, further enhanced safety is accomplished with highly reliable engineered safety systems. The engineered safety systems designed to function passively on the demand consist of reactor shutdown system, passive residual heat removal system, emergency core cooling system, safeguard vessel, and containment overpressure protection system. Additional engineered safety systems include the reactor overpressure protection system and the severe accident mitigation system. In addition, a boron injection system is installed as an active backup system for the emergency case. Four(4) independent passive residual heat removal systems(prhrs) with 50 % capacity each remove core decay heat by natural circulation at any design bases events, and have capability of keeping the core undamaged for 72 hours without any corrective action by operators. When small break LOCA occurs, core uncovery is prevented by two(2) independent emergency core cooling system with 100 % capacity each which automatically operates by pressure difference. The reactor overpressure at the postulated design basis accidents related with a control failure can be reduced through the opening of the PSV (pressurizer safety valve). The safeguard vessel is a leak-tight pressure retaining steel-made vessel intended for the accommodation of all primary reactor systems including the reactor assembly, pressurizer gas cylinders, and associated valves and pipings. The primary function of the safeguard vessel is to to protect any primary coolant leakage to the containment and to confine the radioactive products within the vessel. Thus the vessel has a function to keep the reactor core undamaged during 72 hours without any corrective actions at the postulated design basis accidents including LOCA, with the operation of the PRHRS and ECCS. The steam discharge through the break causes to decrease the primary system inventory and thus causes to increase the pressure in the SV. When the primary system pressure decreases to the safeguard vessel pressure, the break discharge flow ceases and no more system inventory loss occurs. The actuation of the ECCS during a SBLOCA keeps the coolant level well above the top of the core. 2. Safety of Desalination System The most important safety concern in using the nuclear thermal energy for desalination is the radioactivity carry-over into the product water from the nuclear reactor. In the integrated nuclear desalination plant using SMART, two protection mechanisms are provided to avoid any radioactive contamination of the product water. Figure 2 shows the schematic diagram of coupling SMART and the desalination system. Two barriers, the steam generator and the brine heater, along with the pressure reversal between the energy supply and the desalination system, acts as one of the two protection mechanisms implemented in the coupling. In addition, a continuous radioactivity monitoring system will be installed in the water production system to check for any symptoms of contamination, with an immediate system reaction to follow in the case of the detection of radioactivity. Additional monitoring may also be performed in an intermediate loop where the concentration of contaminate is

4 higher than the water plant. Another important safety aspect to be considered in the integrated nuclear desalination plant is the operation and transient issues for system interactions between nuclear and desalination system. Since a direct interaction exists in thermal coupling between the reactor and the desalination plant, any transient in the desalination plants would therefore cause a direct physical feedback in the reactor system. The potential disturbances of the SMART desalination plant depend on the characteristics of the desalination plant as well as the coupled system. For the SMART desalination plant, several events were identified as the potential disturbances of SMART imposed by the desalination plant such as main steam flow increase and decrease in feedwater temperature. The impact of these disturbances on Design Basis Accidents and Performance Related Basis Events of SMART were evaluated. Based on the analyses results, it is concluded that no significant impact on reactor safety is expected by the transients induced by the desalination system. Figure 2. Schematic Diagram of Desalination System IV. Design Verification Program 1. Technology Verification Program The SMART adopts many unique design features to improve the level of safety, reliability, and economics. They are significantly different from the existing LWR designs. Therefore, advanced design features implemented into the SMART should be proven or qualified by experience, testing, or analysis and, if possible, the equipment shall be designed according to the applicable approved standards. SMART design verification program, including comprehensive experiments and development of the analysis model, has been planned and performed to confirm the advanced design features of SMART which have not been proven through the design and operation of the existing PWRs. This program includes basic thermal-hydraulic experiments, separate effect test on major components and integrated tests of safety system. Basic fundamental thermal-hydraulic experiments were carried out during the concept development to assure the key technology of the advanced safety systems. These experimental data has been utilized for the conceptual design of SMART. Basic fundamental thermal-hydraulic experiments includes following experiments; Boiling heat transfer characteristics in the helically coiled steam generator, Containment cooling system using heat pipe, Verification and performance test of hydraulic valve characteristics measurement test, Experiment for natural circulation in integral reactor, The performance tests for key parts of MCP, and Tests for various I&C systems. After SMART concept development, the essential technologies are required for the development of SMART such as helically coiled tube steam generator, self pressurizer, core cooling by natural circulation. In order to develop these technologies, the separate effect tests of SMART major components are performed to get the fundamental database and computer analysis models are developed. For verification purpose of SMART technology, various separate effect tests and experiments on the major component and system were identified, which are included in the following; Critical heat flux measurement test, Core flow distribution test, Test of flow instability in steam generator, Test of self pressurizer performance, Two-phase critical flow test with on-condensable gases, and Water chemistry test Some of these tests are finished during the basic design stage, and others are currently under preparation by installing the equipments and facilities. Figure 3 shows several test facilities designed and installed at KAERI, such as the critical flow test facility to investigate the thermal hydraulic phenomena of critical flow affected by non condensable gas entrained in the two phase break flow, critical heat flux measurement test facility and water chemistry experimental test facility to examine the material corrosion characteristics of SMART. In addition to the separate effect tests, which examine the behavior of particular component, there is an integral thermal hydraulic test in progress, which examines system interaction in the SMART design. While the data from the separate effects tests are being used to develop and verify safety analysis model, the integrated system effect tests is being used to verify the capability of the analysis method and predict the integrated innovative safety systems of SMART. The high temperature and high pressure integral thermal hydraulic test facility was already established and the various comprehensive thermo-hydraulic tests are being performed.

5 technically sound and has sufficient economics incentives for pursuing further development. Thus, the Korean government made a decision to continue to develop SMART and to construct the one fifth scaled pilot plant for demonstration of overall SMART performance by 2008.[6] SMART research and development center was launched in June, 2002 for SMART-P construction project. Many industries and universities in Korea, such as Doosan Heavy Industries and Construction Company (DSHIC), the Korea Power Engineering Company, Inc. (KOPEC), Korea Institute of Nuclear Safety (KINS) and various universities, are participated in the SMART-P construction project. Through SMART-P construction and operation, we will be able to confirm overall SMART performance, industrial applications of SMART and we can obtain operational information and data. Figure 3. Various SMART Test Facilities Besides technology experiments, the followings are also performed to improve safety, economy, design technologies, and design features: Improvement of safety analysis methodology including code verification Improvement and development of computer codes Optimum coupling of SMART and desalination system Application of advanced features for severe accident protection/mitigation Development of manufacturing technologies Also, other small-scale R&D efforts are currently underway to support the design, and any R&D activities required for improvement of the design will be continuously carried out in the next stage. The performance tests for Main Coolant pump(mcp), steam generator and Control Element Drive mechanism(cedm) will also be performed. 2. SMART-P Construction Prior to the commercialization of SMART, technical and economical evaluation of SMART was required to make a decision whether the project of developing SMART should continue to be supported beyond the basic design phase. For a successful completion of a systematic interim assessment of the current status of the project of SMART development, the Steering Committee for SMART Technology Development was formed under the Korean Government, which was composed of 15 members in various expertises from governments, industries, research organizations and academic bodies in Korea. The Committee appointed the Korean Nuclear Society (KNS) to perform the overall assessment from August of 2000 to July KNS organized a team of experts (33 engineers and scientists) to review the technical soundness of SMART technology, and subcontracted the part of its economic evaluation to KOPEC. Based on the results of technical review and economic evaluation, it is concluded that the SMART technology is V. Conclusion An integrated nuclear desalination plant coupled with SMART, an advanced small integral pressurized water reactor which has been under development at KAERI was established. The current design of the plant with SMART is a dual-purpose plant for electricity generation and water production by the MED process. Many advanced design features enhancing the safety, reliability, performance, and operability should be introduced in the SMART design. The SMART design verification program, including a comprehensive experiments and development of analysis model, has been planned and performed to confirm the advanced design features of SMART. In deploying small and medium sized reactors, the successful demonstration via constructing the lead plant is considered most important. In this aspect, it is very prudent that Korea has decided to construct the pilot plant. Acknowledgment This study has been carried out under the nuclear research and development program supported by the Ministry of Science and Technology of the Republic of Korea. The authors are sincerely grateful for the financial support References 1) IAEA, Design and Development Status of Small and Medium Reactor Systems 1995, IAEA-TECDOC-881, International Atomic Energy Agency, Vienna, ) IAEA, Introduction of Small and Medium Reactors in Developing Countries, IAEA-TECDOC-999, Vienna, September ) M.H. Chang, et al., SMART AN Advanced Small Integral PWR for Nuclear Desalination and Power Generation, Proc. of Global 99, International Conference on Future Nuclear Systems, Jackson Hole, USA, Aug Sept. 3, ) Si-Hwan Kim, Keung Koo Kim, and Moon Hee Chang, Design Development and Verification of a System Integrated Modular PWR, Proceedings of the fourth International Conference on Nuclear Option in

6 Countries with Small and Medium Electricity Grids, Croatia, 2002, June, 5) S.H. Kim, J.K. Park and C.K. Yang, The Experience of Nuclear Fuel Development in Korea, Proceedings of the Seventh Pacific Basin Nuclear Conference, San Diego, U.S.A., March ) C.S. Kang, et. al., A Planning Study Project of Developing an Integral Reactor (SMART), KAERI/CM-469/2001, KAERI, Taejon, Korea (2001).