Plant System Design Features of SMART

Transcription

1 GENES4/ANP2003, Sep , 2003, Kyoto, JAPAN Paper 1049 Plant System Design Features of SMART Juhyeon Yoon*, Seung Yeob Ryu, Byung Seon Choi, Manki Lee and Sung Kyun Zee Korea Atomic Energy Research Institute(KAERI), P. O. Box 105, Yuseong, Daejeon, , Republic of Korea SMART is a promising advanced small and medium category nuclear power reactor for a wide range of applications including seawater desalination and electricity generation. It is an integral type reactor pursuing a sensible mixture of proven technologies and new innovative design features. Industry proven KOFA (Korea Optimized Fuel Assembly) based nuclear fuel is combined with such radically new technologies as self-pressurizing pressurizer, helical once-through steam generators, advanced control and soluble boron free operation concepts. The enhancement of safety and reliability is realized by incorporating inherent safety improving features and reliable passive safety systems. The improvement in the economics is achieved through system simplification, component modularization, construction time reduction, and increased plant availability. Currently, a six year project is under way to design and construct a pilot plant of the SMART (SMART-P) to demonstrate the SMART technologies. This paper reviewed the salient design features of the SMART enhancing safety and economics. KEYWORDS: SMART, integral, safety, desalination, economics I. Introduction Various advanced types of small and medium reactors (SMR) are currently under development worldwide, and some of them are ready for construction. One beneficial advantage of SMR is the easy implementation of advanced design concepts and technology. Drastic safety enhancement can be achieved by adopting inherent safety characteristics and passive safety features. Economic improvement is pursued through system simplification, modularization, and reduction in the construction lead time. The safety of the SMART centers around enhancing the inherent safety characteristics of the reactor and salient features include low core power density, strong negative moderator temperature coefficient, integral arrangement to eliminate large break loss of coolant accident, etc. The progression of emergency situations into accidents is prevented with a number of advanced engineered safety features such as Passive Residual Heat Removal System, Passive Emergency Core Cooling System, Safeguard Vessel, Passive Containment Over-pressure Protection. A small-sized reactor is known to be economically less competitive than a large-sized reactor. However, there are many possible mechanisms for the economic improvement. In SMART, system simplification and reduction of pipes and valves are possible due to the no-boron concept, 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. The overall design parameters of the SMART is shown in Table 1. SMART is expected to fully satisfy the Korean as * Corresponding author, Tel , Fax , yoonj@kaeri.re.kr well as the international safety and licensing requirements. A design and construction project of a SMART Pilot plant is being carried out jointly by several organizations including KAERI in Korea. II. SMART Integral Reactor Design Features SMART is an integral type reactor 1,2,3,4), and all of the major primary system components, such as fuel and core, twelve steam generators (SG), pressurizer and four main coolant pumps (MCP), are housed in a single reactor pressure vessel (RPV). The section view diagram of the SMART reactor assembly is shown in Fig. 1. With this integral arrangement, there are no large size pipe connections and thus no possibility of large break loss of coolant accidents. The primary coolant flows up through the core and then through the MCP s to enter the shell side of the SG from the top. On the secondary side, subcooled feedwater enters the tube side of the helically coiled SG tubes from the bottom and superheated steam exits the SG s. A large volume at the top part of the RPV constitutes the pressurizer. Part of the pressurizer volume is occupied by nitrogen gas, water and steam. System pressure is determined by the sum of the partial pressures of gas and steam. These pressures vary with the total primary coolant volume that is changing according to the temperature distribution within the RPV and reactor power. With an appropriate control of the average primary coolant temperature and pressurizer design features, the pressurizer can self-regulate the system pressure within a desired range without any active control. Twelve SG cassettes are equally spaced in the annulus region between the RPV and core support barrel. To provide a sufficient driving force for the natural circulation of coolant, SG s are located relatively high above the core. This design feature and the lowflow resistanceallow a 25% full power

2 CEDM (49) MCP (4) Annular Cover SG (12) Core PZR Steam F d t RPV Side Screen Fig. 1 SMART Reactor Assembly operation capability with natural circulation. The internal shieldings and large water inventory surrounding the core at the sides and bottom reduce the neutron fluence of the RPV. 1. Fuel and Reactor Core The SMART core consists of fifty seven(57) fuel assemblies which are based on the industry proven 17x17 square array of the Korea Optimized Fuel Assembly (KOFA). Each fuel assembly holds 264 fuel rods, 24 guide tubes for control rods, and 1incore instrumentation thimble. Five space grids hold the fuel rods in position. Top and bottom spacer grids are made of inconel, and the three middle space grids are made of zircaloy. A specially designed bottom end piece offers improved resistance to the debris entering the core. Enriched uranium oxide fuel of 4.95 w/o is enclosed in zircaloy cladding and has sufficient reactivity for a three year or longer operation cycle. The fuel assembly is designed to allow power ramp operations during load-following maneuvers. The SMART core design is characterized by an ultra long cycle operation with a single or modified single batch reload scheme, low core power density, soluble boron-free operation, enhanced safety with a large negative Moderator Temperature Coefficient (MTC) at any time during the cycle, an adequate thermal margin, inherently free from xenon oscillation instability, and minimum rod motion for the load follow with coolant temperature control. SMART fuel management is designed to achieve a maximum cycle length between refuelings. A simple single batch refueling scheme allows a cycle of 985 Effective Full Power Days(EFPD). This reload scheme minimizes complicated reload design efforts. A modified single batch scheme with 20 peripheral assemblies reloaded at every even numbered cycle is also possible, and thus enhances fuel utilization. The SMART fuel management scheme is highly flexible to meet customer requirements. 2. Reactor Pressure Vessel The SMART reactor pressure vessel (RPV) is a pressurized cylindrical vessel containing all major components of the primary system. The RPV consists of a cylindrical shell with an elliptical bottom and an upper flange part welded to the shell. The RPV is closed at the top by the annular peripheral cover and the round central cover that also serves as the cover of the in-vessel pressurizer. The annular cover is fixed onto the vessel flange by means of a stud bolt joint. The vessel-to-annular cover joint is made leaktight by a welded torus sealing. The central cover is fastened to the annular cover by a flangeless joint, using rack-and-gear mechanism. The annular-to-central cover joint is also made leaktight by a welded torus sealing. All penetrations are limited in the vessel head region except for SG feedwater and steam nozzles to assure an ample amount of coolant inventory to eliminate core uncovery in any case of postulated pipe break accident. On the annular cover, there are four seats for MCP s, makeup piping nozzles, resistance thermometers, branch pipes, etc. On the outer surface of the central cover, there are nozzles for the 49 CEDM s, rack and gear drives, branch pipes, etc. 3. Steam Generators SMART has twelve identical SG cassettes which are located in the annulus formed by the RPV and the core support barrel. Each SG cassette is of once-through design with a number of helically coiled tubes. The primary reactor coolant flows downward in the shell side of the SG tubes, while the secondary feedwater flows upward in the tube side. The secondary feedwater is evaporated in the tube and exits the SG cassette nozzle header at 40 o C superheated steam condition of 3.0MPa. In the case of normal shutdown of the reactor, the SG is used as the heat exchanger for the passive residual heat removal system (PRHRS). The SG cassette consists of six modules in which 324 tubes are connected. In the case of a tube leak, each module can be plugged in up to 10% of the total heat transfer area. The nozzle feedwater header and steam header of the cassette are designed to be a single structure located on the lateral surface of the RPV to reduce the vessel penetrations. The design temperature and

3 pressure of the SG cassette are 350 o C and 17MPa, respectively. The strength analysis results at the design condition have shown that the SG cassette satisfies the strength criteria. 4. Pressurizer The pressurizer is located inside the upper part of the RPV, and it is filled with nitrogen gas, steam and water. The pressurizer is connected to compressed nitrogen gas tanks located outside the RPV. The primary system pressure is determined by the sum of the partial pressures of nitrogen gas and steam. To prevent a relatively large variation in pressure caused by power change, SMART employs a method for keeping the average primary coolant temperature constant with respect to power change. The large pressure variation which will occur during power maneuvering is reduced by maintaining the coolant temperature of the gas-steam space low and insensitive to the core outlet temperature variation. For this purpose, a PZR cooler is installed to maintain a low PZR temperature and a wet thermal insulator is installed between the PZR and the primary system to reduce the conductive heat transfer. In the SMART pressurizer, there is no active pressure control mechanism such as a spray or a heater and the complicated control and maintenance requirements are eliminated. 5. Control Element Drive Mechanism The SMART Control Element Drive Mechanism (CEDM) designed for fine-step movement consists of a linear pulse motor(lpm), reed switch type Control Element Assembly (CEA) position indicator, hydro-damper and extension shaft connecting CEDM and CEA. The LPM is a four-phase synchronous DC electric machine with a mover(armature) located in a coolant medium inside a strong housing made of magnetosoft corrosion-resistant steel. The CEDM movement is accomplished by sequentially exiting the phase coils. As a control command comes to the LPM control unit, the LPM phase I turns off and phase II turns on. After that, phase II turns off and phase III turns on, etc. The direction of the armature movement depends on the exciting sequence of the LPM phase. The upper and lower pressure housings of the CEDM s are joined to each other on the flanges with stud bolts, nuts and a ring nut. The CEDM s are located on the reactor central cover and fastened by means of thread and omega seal welding. 6. Main Coolant Pump The SMART MCP is a canned motor pump which does not require pump seals. This characteristic basically eliminates small break Loss of Coolant Accidents associated with pump seal failure which becomes one of design bases events in the reactors using a conventional pump. SMART has four MCPs vertically installed on the top annular cover of the RPV. Each MCP is an integral unit consisting of a canned asynchronous three phase motor and an axial-flow single-stage pump. The motor rotor and the impeller shaft are connected by a common shaft rotating on two radial and one axial thrust bearings, which use specialized graphite-based material. The cooling of the motor is accomplished with component cooling water which flows through the tubes wound helically along the outer surface of the motor stator. The rotational speed of the pump rotor is measured by a sensor installed in the upper part of the motor. To block the reverse flow through the impeller, an anti-reverse flow valve is installed at the discharge. The anti-reverse flow valve is always opened in a normal flow and can be easily closed when a slight reverse flow occurs. III. Safety Systems Besides the 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 a 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. The schematic diagram of the SMART safety system is shown in Fig Reactor Shutdown System The shutdown of SMART can be achieved by a function of one of two independent systems. The primary shutdown system is the control rods containing Ag-In-Cd absorbing material. The shutdown signal de-energizes the CEDM and then the control rods drop into the reactor core by the force of gravity and immediately stop the neutron chain reactions. In the case of failure of the primary shutdown system, the emergency boron injection system is provided as an active back-up by make-up pump. One train of make-up system is sufficient to bring the reactor to the sub-critical condition 2. Passive Residual Heat Removal System (PRHRS) The system passively removes the core decay heat and sensible heat by natural circulation in case of an emergency such as unavailability of feedwater supply or station black out. Besides, the PRHRS may also be used in case of long-term cooling for repair or refueling. The PRHRS consists of 4 independent trains with 50% capacity each. Two trains are sufficient to remove the decay heat. Each train is composed of an emergency cooldown tank, a heat exchanger and a compensating tank. The system is designed to keep the core un-damaged for 72 hours without any corrective actions by operators at the postulated design basis accidents. In the case of a normal shutdown of SMART, the residual heat is removed through the SG to the condenser with a turbine bypass system. 3. Emergency Core Cooling System (ECCS)

4 PRHRS Cooldown Tank External Shielding Tank From SG To SG PRHRS Compensating Tank ECCS Tank From Make-up System HVAC SG PZR Core MCP Concrete Building Safeguard Vessel Steel Containment Reactor Overpressure Protection System Emergency Boron Injection Tank Water Jacket To Make-up Pump Reactor Vessel for 72 hours without any corrective actions at the postulated design basis accidents including LOCA, with the operation of the PRHRS and ECCS. The steam released from the opening of the relief valve of the safeguard vessel at the postulated beyond design basis accidents is sparged into the external shielding tanks and immediately condensed. 5. Containment Overpressure Protection System (COPS) The containment is a steel structure with a concrete building enclosing the safeguard vessel to confine the release of radioactive products to the outside environment in postulated beyond design basis accidents relating to the loss of integrity of the safeguard vessel. In any accident causing a temperature rise and thus a pressure rise in the containment, containment cooling is done, in a passive manner, by removing the heat from the containment. The heat is removed through the steel structure itself, and through the emergency cooldown tanks installed inside the containment. A rupture disc and a filtering system are also provided in the containment to protect the steel structure from overpressure and to purify the released radioactive products at the postulated beyond design basis accidents. Internal Shielding Tank Fig. 2 SMART Safety Systems The SMART design excludes any possibility of large break Loss of Coolant Accident. The largest sized pipes connected to the outside of the RPV is 20 mm. The ECCS is thus provided to protect the core uncovery by mitigating the consequences of design basis events such as small break LOCA through make-up of the primary coolant inventory. When an initiating event occurs, the primary system is depressurized, the valve in the line of the ECCS is automatically opened and the water immediately comes into the core by gas pressure. The ECCS consists of two independent trains with 100% capacity each. Each train includes a cylindrical water tank pressurized with nitrogen gas, isolation and check valves, and a pipe of 20 mm in diameter connected to the RPV. The system provides vessel refilling so that the residual heat removal system can function properly in the long-term recovery mode following the event. 4. Safeguard Vessel 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 confine the radioactive products within the vessel and thus to protect any primary coolant leakage to the containment. The vessel also has a function of keeping the reactor core undamaged 6. Reactor Overpressure Protection System (ROPS) The function of the ROPS is to reduce the reactor pressure at the design basis accidents related with a control system failure. The system consists of three pilot operated safety relief valves(posrv s) which are installed on the gas cylinders respectively. The steam discharge lines of the POSRV s are combined to a single pipeline and connected to the external shielding tank. When the primary system pressure increases over the setpoint, POSRV s are opened to discharge the steam into the external shielding tank through a sparging device. IV. Desalination System The SMART desalination system consists of 4 units of MED combined with a thermal vapor compressor (MED-TVC). Each Distillation Unit is capable of producing 10,000 m 3 /day of distilled water for 24 hours of operation at the maximum brine temperature of 65 o C and the supplied sea water temperature of 33 o C. The MED process coupled with SMART incorporates the falling film, multi-effect evaporation with horizontal tubes and a steam jet ejector. One significant advantage of MED-TVC is its ability to use the pressure energy in steam. Thermal vapor compression is very effective where the steam is available at higher temperature and pressure conditions than required in the evaporator. The thermal vapor compressor enables the low-pressure waste steam to be boosted to a higher pressure, effectively reclaiming it s available energy. Compression of steam flow can be achieved with no moving parts using the ejector. The MED-TVC unit is designed with a performance ratio(pr) of 15 and a motive steam to load ratio of one. The PR and steam to load ratio were determined based on the

5 results of the thermodynamic analysis and economic evaluation for the water production capacity of 40,000m 3 /day and electricity generation of 90MWe. SMART and MED-TVC units are connected through the steam transformer. The steam transformer produces the motive steam using extracted steam from a turbine and supplies the process steam to the desalination plant. It also prevents the contamination of the produced water by hydrazine and radioactive material of the primary steam. The steam transformer is made of horizontal tube bundles. The primary steam flow is condensed inside the tubes at its saturation temperature. The feed brine is sprayed outside of the tube bundles by a recycling pump. Part of the sprayed water is evaporated and the produced steam is used as the motive steam for the thermo-compressor of the evaporator. Part of the condensate in the first cell of the evaporator is used as make-up for the steam transformer, and this make-up water is preheated by the condensate of the primary steam before being fed into the steam transformer. The preheater is a plate type heat exchanger made of welded titanium. Fig. 3 Coupling Concept of SMART and Desalination System V. Economic Improvement Features The SMART design adopts a unique design approach to overcome the economies of scale. Economic competitiveness is achieved through system simplification, component modularization, factory fabrication & direct site installation of components, and reduced construction time. A simplified modular design approach is applied to all SMART primary components. This approach enhances the fabrication and construction, which leads to cost and schedule reduction. The major components of the reactor coolant system such as steam generators, reactor coolant pumps, and a pressurizer are assembled in a single pressurized vessel. This compact and integral primary system eliminates both the complexity and extra components and materials associated with the conventional loop-type reactors. The optimized and modularized small-sized components also allow the easy factory fabrication and the * Indonesian National Nuclear Energy Agency direct installation at site, and thus lead to shortened construction time and schedule. These features allow a construction period of less than three(3) years from first concrete to fuel load. The adoption of the simplified passive systems provides a net reduction in the number of safety systems, and also drastically reduces the number of valves, pumps, wirings and cables, pipes, etc. The soluble boron-free design is one of most important design features that largely contribute to the system simplification by allowing the removal of associated systems and components required for boric acid processing, chemical volume and control systems. This feature also minimizes the liquid radwaste generation and thus simplifies the associated processing systems. SMART is designed for a sixty(60) year life and for a three(3) year fuel cycle with a single batch or modified one-and-half batch refueling scheme. The neutron fluence to the reactor vessel is greatly reduced by specially designed side and bottom shielding. Other features also contribute to economic improvements. SMART uses an advanced on-line digital monitoring and protection system that provides the significant advantages of increasing the system availability and operational flexibility. The adoption of advanced man-machine interface technology leads to a reduction of human errors and to a compact and effective design of the control room with respect to minimizing the staff requirements. As a part of an international cooperation project between BATAN*-IAEA-KAERI, a comparative economic assessment study has been carried out for co-production of electricity and potable water 5). For the purpose of the study, nuclear and fossil energy sources are considered. Desalination Economic Evaluation Program (DEEP) was used to evaluate the economics of desalination 6). A 330 MWt integral reactor, SMART, has been taken as a nuclear energy option for seawater desalination. A gas/ fuel-oil fired combined cycle (CC), gas turbine (GT), and pulverized coal (PC) power plant are considered as fossil energy options. Among the power options, coal power plant (PC) has the lowest electricity generation cost. The second one is for nuclear option (SMART) with electricity generation cost of 4.06 cent/kwh, and the highest reached by gas turbine (GT) using oil fuel. Water cost, desalination process coupled with SMART is 1.04 $/m 3. Among alternatives coupled, the lowest water cost was found in desalination plant which is 0.98$/m 3 coupling with Combined Cycle. Economics of water costs are dependent on parameters assumed in the calculation. The major parameters, which have great effects on the economics are identified to be discount rate, the escalation of gas/oil price, uncertainties in the costs of new design of plants and equipment as well as the reliability and availability of both the energy source and seawater desalination plants. Water costs in the study seem to be relatively high comparing with other studies. These results mainly come from very small capacity of desalination plant. In this specific study, the water production capacity is

6 assumed to be 4,000m 3 /day according to an user requirement, lacking economies of scale. This study showed that nuclear desalination can be an economically viable option if nuclear and seawater desalination plants are proven to be technically sound and reliable. VI. Conclusion SMART design features are reviewed. The SMART has many unique design features for enhancing safety and overcoming the economies of scale. The basic design of the SMART was finished in As a part of an international cooperation project, it is demonstrated that nuclear desalination using the SMART can be economically competitive compared with other energy option for seawater desalination if related technologies are proven to be reliable. Korean government and KAERI established a SMART R&D Center last year, which is a special organization to expedite nuclear desalination plant construction. A 6-year project has been kicked off last year to demonstrate the SMART technologies by constructing a pilot plant of the SMART. Many Korean industries, institutes and universities will be involved in this project under the strong support of government. Acknowledgment This study has been carried out as a part of the Development of Design Technology for an Integral Reactor Program supported by Ministry of Science and Technology. The authors are sincerely grateful for the financial support. References 1) D.J. Lee, et al., "Design and Safety of a Small Integral Reactor (SMART), Int. Workshop on Utilization of Nuclear Power in Oceans, Tokyo, Japan (2000). 2) KAERI, SMART for Electricity Generation and Desalination (2001) SMART brochure. 3) IAEA, Design and Development Status of Small and Medium Reactor Systems 1995, IAEA-TECDOC -881, International Atomic Energy Agency, Vienna (1996). 4) 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 (1999). 5) SMART R&D Center, KAERI, The 1st PRM, Preliminary Economic Feasibility Study of Nuclear Desalination in Madura Island, Indonesia, BATAN-IAEA-KAERI, Sept. (2002). 6) IAEA, Desalination Economic Evaluation Program (DEEP), Computer User Manual Series No. 14, Vienna, (2000). Table 1. SMART Design Data Information GENERAL INFORMATION Reactor Name SMART Reactor Integral PWR Thermal Power (MWt) 330 Electric Power (MWe) 100 Design Life Time (yr) 60 FUEL AND REACTOR CORE Fuel 17x17 Square FA Active Fuel Length (m) 2.0 Fuel Material 4.95 w/o UO2 No. of Fuel Assembly 57 Core Power Density (W/cc) 62.6 Refueling Cycle (yr) > 3 REACTIVITY CONTROL No. of Control Element Banks 49 No. of Absorber Elements per CEDM 24 Material of Absorber Elements Ag-In-Cd Burnable Poison Material Al2O3-B4C & Gd2O3-UO2 REACTOR PRESSURE VESSEL Overall Length (m) 10.6 Outer Diameter (m) 4.6 Average Vessel Thickness (mm) 264 Vessel Material SA508, CL-3 REACTOR COOLANT SYSTEM Cooling Mode Forced Circulation Total Coolant Mass (kg) 45,780 Design Pressure (MPa) 17 Operating Pressure (MPa) 15 Core Inlet Temperature ( o C) 270 Core Outlet Temperature ( o C) 310 STEAM GENERATOR Helically-coiled Once-through No. of SG Cassettes 12 Tube Outer Diameter (mm) 12 Feedwater Pressure (MPa) 5.2 Feedwater Temperature ( o C) 180 Steam Pressure (MPa) 3.0 Steam Temperature ( o C) 274 Superheat ( o C) 40 PRESSURIZER Self-controlled Total Volume (m 3 ) 16.0

7 CONTROL ELEMENT DRIVE MECHANISM Step Motor Driven No. of CEDM 49 Design Pressure (MPa) 17 Design Temperature ( o C) 350 Moving Distance per Pulse (mm) Moving Speed (mm/s) 0-15 MAIN COOLANT PUMP Glandless Canned Motor Pump No. of MCP 4 Design Pressure (MPa) 15 SECONDARY SYSTEM Main Steam Flow Rate (kg/h) 549,270 Feedwater Pressure (MPa) 5.2 of Feedwater Pump Multi-stage No. of Feedwater Pump 3 of Startup Pump Multi-stage No. of Startup Pump 2 of Condenser Shell and Tube No. of Condensate Pump 2 MAKE-UP SYSTEM No. of Trains 2 No. of Make-up Pumps 4 Volume of Make-up Tank (m 3 ) 42.8 Design Pressure (MPa) 20 COMPONENT COOLING SYSTEM No. of Trains 1 Coolant Pressure (MPa) 0.5 Coolant Temperature ( o C) 40 CONTAINMENT Pressurized concrete with steel lining Design Pressure (MPa) 0.3 Design Temperature ( o C) 120