ABB’s BWR 90 + – designed for beyond the 90s28 February 1998
The next step in ABB’s evolution of BWR technology focuses on meeting the utility needs of the 21st century. Based on its forerunners, the BWR 75 and BWR 90, the BWR 90+ design incorporates the latest safety requirements, including measures for severe accidents, while providing increased reliability and reduced costs. The design development work also supports the modernisation programmes undertaken to upgrade and uprate the earlier generations of ABB plants.
There is a general consensus that global energy demand will rise in the years to come, and this demand growth is expected to involve an increased call for electricity. At the same time, a number of old generating plants, both nuclear and others, will probably have to be replaced due to ageing. Hence, new generating capacity will be needed and nuclear will be one of the options.
ABB’s BWR 90 design was originally developed to meet the 1990’s market demand and has been offered commercially. As such, the design was selected by the European Utility Requirements (EUR) group for a review to evaluate how it compares with the EUR.
ABB is continuing BWR development, focusing on the needs of the next century. The latest design, the BWR 90+, incorporates significant safety improvements and offers reduced costs. Furthermore, the development work provides input to modernisation programmes and other efforts to improve the earlier generations of its nuclear power plants.
ABB Atom has supplied 11 BWR plants now in operation in Sweden and Finland. The first unit, Oskarshamn 1, was delivered in 1972 and the latest, Oskarshamn 3, was taken into commercial operation in 1985.
A major development effort in the 1970s resulted in the ABB BWR 75 design. This design is characterised by the use of internal recirculation pumps, fine motion control rod drives and four-train safety systems. Six nuclear power plants of the advanced BWR 75 design are in operation in Finland and Sweden. The accumulated operating experience of these plants of almost 100 reactor-years provides proof of a successful design with a high operation reliability. The two Finnish BWRs, Olkiluoto 1 and 2, are among the best performing nuclear power plants in the world; the 1997 capacity factors were 94.0% and 94.3%, respectively, and the average over the last 10 years is 93.2%. The plants in Sweden are close behind. The annual energy availability of the BWR 75 generation in Sweden – the Forsmark 1, 2 and 3 and the Oskarshamn 3 units – has averaged 90.1% over the last decade.
The total electricity generation costs are very low for the BWR plants delivered by ABB, as shown by the published production costs for the Forsmark 1, 2, and 3 plants during the last 10 years (see chart on next page).
THE DESIGN FOR THE 1990S
The BWR 90 design development work started in 1986 as a review of lessons learned from previous plant projects. The basic design was completed and offered to Finland in 1991, as one of the contenders for the country’s fifth nuclear power plant. The development work was conducted in co-operation with the Finnish utility TVO (Teollisuuden Voima OY), the operator of the two BWR 75 plants.
The BWR 90 is based on the design, construction, commissioning and operation of the BWR 75 plants in Finland and Sweden. Specific changes were introduced to the established reference design, the Forsmark 3 and Oskarshamn 3 units.
The BWR 90 design, as its BWR 75 predecessor, is characterised by the use of internal recirculation pumps, fine motion control rod drives, and separation of the four trains of safety systems. Modifications were made to take in technological developments and new safety requirements and to achieve further cost savings.
The thermal power rating of the base version is 3800 MWth (providing a nominal 1374 MWe net); this has been supplemented by a smaller version of 3300 MWth (1195 MWe net). The BWR 90 takes advantage of thermal margins gained using a new generation of BWR fuel.
The general arrangement of the buildings as shown is characterised by a division into a nuclear, safety-related part of the plant, containing the reactor building, the diesel buildings and the control building, and a more conventional part that is separated from the former by a communication corridor. The conventional part consists of the turbo-generator and plant auxiliary systems. This arrangement is advantageous when building the plant as well as during plant operation.
The reactor pressure vessel has been redesigned to reduce the number and lengths of welds. The total weld length is only about half of that in the BWR 75 design, and this reduces the amount of in-service inspection to be carried out during refuelling outages. The basic design of the internals is maintained from the BWR 75 design. This means that they are not welded to the reactor pressure vessel, and can easily be removed at refuelling, yielding time savings.
The recirculation system is based on the use of internal glandless pumps driven by wet asynchronous motors, supplied with variable frequency – variable voltage power from individual frequency converters. This type of pump has been operating reliably for more than four million operational hours since 1978 in ABB BWR plants. The internal pumps provide the means for rapid and accurate power control and are advantageous for load following purposes.
The safety and maintenance benefits from a systematic division of the engineered safety system into four independent and physically separated sub-systems (trains) are kept from the previous BWR 75 design. Two out of four trains always provide the safety functions required. Another feature is the ABB BWR control rod drive system that incorporates diversified means of control rod actuation and insertion, by hydraulic pressure and by electrical motor. Together with a generous reactor pressure relief capacity, and combined with a capability of rapid recirculation flow rate reduction, it provides an efficient ATWS (Anticipated Transient Without Scram) countermeasure.
The number of distribution voltage levels has been reduced. All distributions are by AC, and local AC/DC converters are used where needed. The simplifications introduced will reduce maintenance work considerably.
BWR 90 is designed to use digital systems for plant control, including reactor protection. Man-machine communication in the control room is facilitated by the consistent use of video display units, keyboards, and display maps. This design improves the control of the plant, eases the burdens on the operator, and facilitates operation and maintenance. Significant savings are also achieved since less space and fewer cables are needed.
A suitable plant design covers many different aspects, eg the design of the various systems, the choice of materials and components, the installation, radiation shielding, accessibility to components, transport routes, proper routing of ventilation air, general building arrangement etc. The end result represents a compromise between a number of concerns, and the co-operation with the Finnish utility TVO, with its feedback of practical experience, has been of great value in the development of the BWR 90 design.
MODERNISATION AND UPRATING
Development activities focusing on new nuclear power plants have also contributed to creating a framework for modernisation and uprating of the older units. A broad design basis reconstitution programme is underway for the older BWR units in Sweden. The programme reviews the safety case for the plants, and examines the status with respect to new rules and guidelines in order to determine the need for modernisation of plant procedures, structures, systems and components.
The necessity of replacing obsolete equipment is also a contributory factor in initiating modernisation programmes. In particular, old instrumentation and control systems (I&C) are being replaced by modern technology with programmable equipment. The replacement projects entail substantial modification of the I&C, including safety-related logic circuits. So far, three of the Swedish BWRs have signed orders to install ABB Atom’s Nuclear Advantage®, a complete structure for I&C replacement, but available also for new plants. The Nuclear Advantage structure is based on high quality general process automation products with a total operational experience in existing plants of more than 400 years. Recently, a digital system for neutron flux measurement and supervision was introduced in Olkiluoto 1.
Modernisation programmes have been performed or are in progress at most of the plants in Finland and Sweden. The programme at Oskarshamn 1 is of particular interest as all reactor internals were removed to inspect the pressure vessel and control rod nozzles in the bottom of the vessel. The main shroud, steam separators, core spray system and the moderator tank cover will be replaced in 1998.
At Ringhals 1, the reactor coolant pressure boundary has been improved; all materials that are susceptible to environmentally assisted cracking have been replaced.
Built-in performance margins as well as safety enhancement and modernisation have paved the way to increasing the nominal power of the ABB BWR plants. Most of the plants have been uprated on the order of 10-25%.
EUR EVALUATION OF THE BWR 90
The European Utility Requirements group selected the BWR 90 to be evaluated against the EUR documents. This effort, involving an active dialogue with the EUR group, also serves to demonstrate the general applicability of the EUR document to BWR designs.
The work comprises a detailed assessment against the requirements of Volumes 1 and 2 of the EUR document. This activity is part of an effort to develop a Volume 3 of the EUR document with specific requirements for specific designs. The work was started in the Spring of 1997 and is scheduled to be completed during 1998.
The effort also provides input to the development of the BWR 90+ design, with respect to incorporating any adaptations necessary for conformance.
THE BWR 90+ DESIGN
The aim of ABB’s continuing programme is to maintain and develop the BWR as a competitive option for a reviving market in Europe. The main objectives of the project focus on reducing the overall cost of energy production and on providing increased levels of safety, of particular importance for attaining public acceptance.
The BWR 90+ is an evolutionary design, characterised by an optimised mixture of proven and new technology with safety improvements that maintain or enhance the successful construction and operation of the BWR 75 plants. The BWR 90+ design includes all the well proven design features such as internal recirculation pumps, fine motion control rods and a pre-stressed concrete containment.
The development work is conducted in co-operation with the Finnish utility TVO. Swedish utilities have recently decided to join the project focusing on aspects of the work that will benefit the modernisation and improvement programmes at their existing plants.
The design principles of BWR 90+ are based on generally established international codes and standards. In addition, great attention is given to the new requirements of STUK, the regulatory body of Finland. The new EPRI URD (a utility requirements document) and EUR documents are also considered.
The revised edition of the STUK guides addresses the need for very high safety levels. Some examples are:
• The plant shall be designed so that no release of radioactivity will occur during the first period following a severe accident, even if all easily oxidised materials in the reactor core react with water.
• The design should include a containment venting system, but containment venting shall not be the primary way to remove decay heat during a severe accident.
• The safety relief valves shall not be the primary way to control the reactor pressure during an anticipated transient.
• The plant has to be designed for a possible LOCA during plant shutdown conditions, resulting from human errors during refuelling operations.
• Reactivity control systems shall be so designed that a damaged reactor is maintained subcritical in a severe accident.
In response to these demands, a number of fundamental hardware changes have been introduced in the BWR 90+ design, especially regarding the containment (see diagram). The connections between the drywell and wetwell are designed to minimise the potential for condensation pool bypass. Design measures to cope with a degraded core accident have been incorporated by provision of a core catcher arrangement and filtered venting for the containment. There-fore, the public and the environment will be protected even in the event of a degraded core accident; the residual risk for the public is, therefore, practically eliminated.
The new containment design is characterised by robust and easily understandable design principles. The containment structure itself is not the primary barrier to a core melt. An initially dry core catcher is located under the reactor pressure vessel, submerged in the containment pool. In case of a severe accident with a core melt-through, the molten core will be collected in the core catcher and cooled in a passive way by the containment pool water.
The new design reduces the risk of steam explosions and the risk of core–concrete interaction is minimised. The wetwell gas compression chamber volume, as well as the pool water volume, has been increased. The new design will cope with the pressure buildup in a passive way for one day without activation of the overpressure protection system, taking into account hydrogen generation from the zirconium in the core. The containment itself serves as an inherently passive system with practically zero releases to the environment. By activating other passive or active cooling systems, releases can be prevented for a very long time, and by activating the filtered venting the long-term pressure can be reduced without any off-site concerns.
All nuclear power plant containment structures can be characterised as pressure vessels. Logically, and in response to the code requirements for pressure relief equipment on pressure vessels, the containments of all the Nordic BWRs, as well as the BWR 90+ design, are equipped with pressure relief equipment. This consists of a safety rupture disk connected to the wetwell gas atmosphere and a parallel valve that can be opened manually, allowing release of the containment atmosphere via a filtered vent system in the longer term.
In the BWR 90+ design there are no openings or pipe and cable penetrations from the lower part of the drywell. The top of the core is located at or under the level of the upper drywell floor. This design makes it possible to keep the core flooded in a passive way in case of a LOCA during plant shutdown and refuelling operations. In addition, it has a positive influence in the event of a severe accident.
Some of the specific goals for the design and performance of the BWR 90+ are the following:
• Nominal power output: 1500 MWe.
• Construction time from first pouring of concrete to commercial operation: 48 months.
• Average availability: higher than 90%.
• Average refuelling outage: less than 20 days/year.
• Capital cost: in the range of US$1600/kWe.
As increased plant size will make nuclear power more competitive compared with fossil-fuelled plants (coal or natural gas), the capacity has been increased from 1350 MWe for the BWR 90 design to 1500 MWe for the BWR 90+ design.
As interest during construction represents a large portion of the cost for constructing a new nuclear power plant, a short construction time will increase the competitiveness of the nuclear alternative. As in earlier projects, ABB has used a number of methods to reduce the construction times. As an example, the containment liner with associated reinforcement arrangement was built as a very large module in parallel with the erection of the basement structure. The module was slid into the basement and placed in the correct position. This method was applied in Oskarshamn 3, together with a slipforming technique, and reduced the overall time schedule by 3 months.
The construction techniques and the BWR 90+ containment design as well as the reactor service room arrangements have been optimised to allow extensive use of slipforming and modular construction methods. The reactor building and the other auxiliary buildings will be built with an extensive use of prefabrication. The use of new construction methods have been considered in the layout. As a result, the construction time, from pouring of first concrete to start of demonstration run, will be reduced from just under 60 months for the 1350 MWe size to about 48 months for the 1500 MWe size.
A significant contributor to the overall economy for the power plant is availability. The BWR 90+ design incorporates all the design features from the BWR 75 that have recorded high operating reliabilities. In addition, an efficient feedback of operating experience from the Finnish utility TVO will bring improvements regarding operation and maintenance aspects.
Based on operating experience from previous plants the process and electrical systems have been simplified. The number of components have been reduced. The building volume has, despite the increase in plant size, been reduced. This will yield lower costs for the buildings as well as for the installed process and electrical systems.
The BWR 90+ is being designed with modern tools, including 3D-CAD and advanced IMS, which will yield a reduction in engineering costs, savings in time and will minimise the impact of late changes.
|Features of the BWR 90+ containment design|
|• Improved volume ratios to cope with extreme hydrogen generation at core melt. • Elimination of low level penetrations. • Core melt retention and cooling device inside containment. • Decay heat removal by active and passive cooling systems. • Core remains covered by water if loss of coolant occurs during refuelling. • Cylindrical design all the way up.|