Advanced boiling water reactor, the next generation

Advanced boiling water reactor, the next generation

(Parte 1 de 2)

Steven A. Hucik, GE Nuclear Energy 175 Curtner Ave., San Jose, CA. 95125

Abstract

The ABWR is an advanced light water reactor designed by an international team of engineers and designers to address the utility and public needs for the next generation of power plants. It incorporates major innovations and includes the best from BWR designs in Europe, Japan, and the US.

The major emphasis in the design of the plant has been on improved operability and maintainability. This has led to an overall plant design that is simpler to build, maintain, and operate and at the same time has significantly enhanced safety features including several features that are inherent to BWR designs and ensure "passive" responses to transients and accidents. The ABWR design has several additional features that ensure that offsite doses would be extremely low following a severe accident.

This paper also discusses the other key features of the

ABM: internal recirculation pumps, fine motion control rod drives, digital control and instrumentation, multiplexed fiber optic cabling network, pressure suppression containment, structural integration of the containment and reactor building, and advanced turbine/ generator with 52" last stage buckets.

The 1356 MWe ABWR design is being applied as a two unit project by the Tokyo Electric Power Co., Inc. at its Kashiwazaki-Kariwa site in Japan. On May 15, 1991, Japan's Mmistry of International Trade and Industry (MITI) formally announced the granting of the "Establishment Permit" to Tokyo Electric Power Company for constructing two ABWRs at the Kashiwazaki site. This licensing milestone culminates the successful safety review in Japan and clears the way for construction of the two ABWR units. Construction began September, 1991 and commercial operation is scheduled for 1996, with the second unit one year later.

The ABWR design is based on design, construction and operating experience in Japan, USA, and Europe, and was jointly developed by the BWR suppliers, General Electric Company, Hitachi, Ltd., and Toshiba Corporation, as the next generation BWR. The Tokyo Electric Power Co. (TEPCO) has provided leadership and guidance in the establishment of the ABWR and has combined with five other Japanese utilities

(Chubu Electric Power Co., Chugoku Electric Power Co., Ho :uriku Electric Power Co., Tohoku Electric Power Co., and Jagm Atomic Power Co.) in participating and providing support for the test and development programs.

0-7803-0513-2/92$0.3.O0 QIEEE 137

The ABWR development program was initiated in 1978, with subsequent design and test and development programs started in 1981. Most of the development and verification tests of the new features have been completed. Conceptual design followed by detailed design engineering of the ABWR has progressed to the point where the Tokyo Electric Power Company, Inc. announced the selection of GE

Nuclear Energy, Hitachi Ltd., and Toshiba Corporation to design and construct the two lead ABWRs, Units 6 and 7 at the Kashiwazaki-Kariwa Nuclear Power Station. The three companies form an international joint venture to design the plant and supply equipment.

The evolution of the BWR has occurred in two major areas - the reactor system and containment design. This evolution resulted from design enhancements and experience gained from operating reactors (including abnormal occurrences) and testing programs.

Throughout the BWR evolution, there has been an ovemding trend toward simplification and optimization. The ABWR is the result of this progressive design simplification of the BWR and its containment structure. Systematic review of both the technical merits and cost have played a key role in achieving designs that meet all objectives. This thorough evaluation by the designers and subsequent review by the utility sponsor, TEF'CO, has helped keep the effort focused and achieve excellent results.

The major ABWR plant objectives were defined very early in the ABWR development in close cooperation with

TEPCO. These overall plant objectives were selected to mainly improve the performance and safety and reduce costs.

The major plant objectives which guided the selection of new technologies and features of the design are:

(1) Enhance plant operability, maneuverability and daily load following capability.

(2) Increase plant safety and operating margins.

(3) Improve plant availability and capacity factor. (4) Reduce occupational radiation exposure.

(5) Reduce plant capital and operating costs. (9 Reduce plant construction schedule.

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The ABWR design represents the integration of eight years of conceptual development and design along with an extensive confiiatory test program.

Increased Plant Output and Turbine Design

The ABWR plant is designed for a rated thermal output of 3926 MWth which provides for an electrical output in excess of 1350 MWe. In order to improve plant efficiency, performance and economy, the turbine design incorporates a 52-inch last stage bucket design. Combined moisture separator/reheaters remove moisture and reheat the steam in two stages. Also to help increase plant output and reduce cost, the design has incorporated the concept of both high pressure and low pressure pumped-up drains. Rather than cascading the heater drains back to the condenser, the pumpup drain system takes advantage of this waste heat and injects it back into the feedwater ahead of the heaters. This concept has increased the generator output nearly 5 MWe, has helped to reduce the capacity of the condensate polishers, and has also reduced the size requirements for both the high and low pressure heater areas. The overall design has made optimal use of these design improvements to maximize plant output and reduce cost.

Improved Core and Fuel Design

The ABWR core and fuel design goal was improved operating efficiency, operability, and fuel economy. This was accomplished primarily by utilizing PCI-resistant (barrier) fuel, axially zoned enrichment of the fuel, control cell core design, and increased core flow capability. The use of minimum shuffle fuel loading schemes reduced refueling times, while fuel burnup increased to higher values provides

for longer continuous operating cycle capability and improved fuel cycle costs.

The axially zoned fuel, with higher enrichment and less gadolinia (Gd-absorber) content in the upper half of the fuel rods, allows the axial power distribution to be kept uniform throughout the operating cycle. This feature assures a higher thermal margin that together with the other core design features results in improved fuel integrity, plant capacity factor, and operational flexibility. The axially zoned fuel eliminates shallow control rods which control the axial power shape and the control rods are only used to control reactivity.

The core design employs the control cell core concept successfully applied to many of the operating plants. In this design all of the control rods are fully withdrawn throughout an operating cycle except for those in the control cells. Each control cell consists of four depleted fuel bundles surrounding a control rod. Only these control cell rods move to control reactivity. This minimizes the operator's tasks of manipulating control rods during the cycle to control reactivity or for power distribution shaping. This design also improves capacity factor since the control cell eliminates the need for rod sequence exchanges. The flat hot excess reactivity minimizes rod adjustments during the cycle.

The capability for excess core flow above rated of greater than 1% provides for several benefits. Daily load following from 100% to 70% to 100% power (in a 14-1-8-1 hour cycle) is easy using core flow rate adjustment and no control rod movement. For both maximum use of the excess flow and slight control rod adjustment, load following of 100%-50%-100% is easily attainable. In addition, the excess flow capacity allows for spectral shift operation to provide additional bumup with all rods out to increase operational flexibility, extend operation, and reduce fuel cycle costs.

Reactor Vessel Incorporating Internal Pumps

The most dramatic change in the ABWR from previous BWR designs is the elimination of the extemal loops and the incorporation of internal pumps for reactor coolant recirculation. The reactor pressure vessel (RFV), and core internals have been optimized for the internal pump concept. As shown in Figure 1, all large pipe nozzles to the vessel below the top of the active fuel are eliminated. This alone improves the safety performance during a postulated Loss of Coolant Accident (LOCA) and allows for decreased ECCS capacity.

SPRAY m HEAD VENT

Figure 1. RPV and Internals 1378

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The RPV is 7.1 m in diameter and 21 m in height.

The reactor vessel height and total volume have been minimized, which has resulted in reduced volume requirements for the containment and reactor building @/El). In-service inspection (ISI) has been reduced due to the incorporation of internal pumps and elimination of the recirculation pipe nozzles and reduced amount of vessel welding during vessel fabrication. The RPV has a single forged ring for the internal pump mountirlg nozzles and the conical support skirt. Forged rings are also utilized for the core and upper regions of the vessel shell sections. The elimination of the extemal recirculation piping and the use of vessel forged rings has resulted in over a 50% reduction in the weld requirement for the primary system pressure boundary.

The reactor vessel has been designed to permit maximum IS1 of welds with automatic equipment. This will help minimize manpower and reduce radiation exposure. Other features of the ABWR RPV design include main steam outlet nozzles containing a reduced diameter throat and diffuser which is used to measbre steam flow. This also acts to restrict flow and reduce loads on the reactor internals and reduces the containment loads during postulated LOCA. The steam dryers and separators are of the improved lower pressure drop design developed for BW6. This lower pressure drop contributes to increased stability margins and lower pump power costs.

The ABWR incorporates ten internal recirculation pumps (see Figure 2) located at the bottom inside of the RPV. This simplifies the nuclear boiler system and allows for compact space requirements in the RCCV and R/B. Elimination of the extemal recirculation loops has had many

PUMP IMPELLER c REACTOR VESSEL PURGE WATER INLET

Figure 2. Reactor Intemal Pump (RIP) advantages. Key advantages have been the reduction in containment radiation level by over 50% compared to current plants, lower pumping power requirements. The excess flow provided by the pump design has enhanced plant operation and allows for full power operation with one pump out of service.

The internal pump is a wet motor design with no shaft seals. This provides increased reliability and reduced maintenance requirements and hence, reduced occupational radiation exposure. These internal pumps have a smaller rotating inertia, and coupled with the solid-state variable frequency power supply, can respond quickly to grid load transients and operator demands. These pumps are now accumulating plant operating experience in several European BWRs. Improved designs have also been tested in Japan in testing programs now underway.

Fine Motion Control Rod Drives (FMCRD)

The ABWR incorporates the electric-hydraulic FMCRD, which provides electric fine rod motion during normal operation and hydraulic pressure for scram insertion. Reduced maintenance with reduced radiation exposure is a feature of the new drive. Integral shootout steel built into the FMCRD replaces the external beam supports of the current BWRs and improves maintainability and reduces radiation exposure.

The drive mechanism operates to allow fine motion

(18 m step size) provided by the ball screw nut and shaft driven by the electric motor during normal operation. The electric motor also provides increased reliability through diverse rod motion to the hydraulic scram and acts as a backup with motor run-in following scram. The fine motion capability allows for small power changes and easier rod movement for bumup reactivity compensation at rated power.

It also reduces the stress on the fuel and enhances fuel rod integrity. Ganged rod motion (simultaneous driving of a group of up to 26 rods) and automated control significantly improves startup time and power maneuvering capabilities for load following.

The ABWR FMCRD design has been improved from the European design by reducing the length and diameter, and by adding the fast scram function. Other refinements in the ball-screw assembly and seal designs have led to less maintenance requirements. Other design features include: (1) a two-section design for the CRD housing with the wearing parts concentrated in the shaft seal housing section in order to provide ease of maintenance time and radiation exposure. (2) The FMCRD scram water is discharged directly into the reactor vessel eliminating the scram discharge volume and associated valves and piping. This reduces radiation exposure and eliminates a potential source of common mode failure for the scram function. (3) The drive maintains a continuous purge into the reactor thus eliminating the accumulation of radioactive crud in the drive and reduces exposure. (4) Continuous full-in indication. (5) Dual safety grade separation switches to detect rod uncoupling and a new bayonet coupling to help eliminate the control rod drop accident. (6) Ganged hydraulic control units (HCU) with two CRDs per HCU. (7) A brake mechanism to prevent rapid wind down of the screw

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(Parte 1 de 2)

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