As one of the few European countries opting to retain its nuclear power generation portfolio, Sweden is upgrading existing civil nuclear assets to meet the increasing demand for low-carbon power. Under the latest ‘Project PULS’ Alstom is currently involved in the major upgrade and life-extension of Oskarshamn Unit 3, a 20 year-old nuclear plant operated by OKG AB.
Amel Akhtar, Alstom project director
lectricity consumption in Sweden has increased continuously over many decades to reach a current level of around 18 000 kWh per head of population, making Swedes among the biggest consumers of electricity in the world. About half of all domestic power production is derived from nuclear generation, the remainder being supplied from hydroelectric plants, although actual hydro production is dependent on weather conditions.
Following the controversial closure of two 600 MWe boiling water reactors, Barseback-1 in 1999 and a second twin unit in 2005, there are now ten nuclear power plants located principally at coastal sites in the south of the country, representing an installed capacity of 9000 MWe. These include three units sited on the Simpevarp peninsula, north of Oskarshamn in the south east of Sweden, owned and operated by OKG AB. Power produced from the country’s nuclear generating plant topped 75 TWh in 2004, comprising around 51 per cent of total electricity production in Sweden.
A public limited company, the Swedish power utility OKG AB forms part of the giant German-based E.ON Group’s power generation business unit, with 54.5 per cent ownership by parent company E.ON Sverige AB. The remaining 45.5 per cent of the shares in OKG AB are held by majority state-owned Finnish energy group, Fortum Oy. E.ON Sverige AB has approximately 5500 employees and a yearly turnover of approximately SEK26 billion ($3.5 billion). In addition, OKG operates the Central Interim Storage Facility for Spent Nuclear Fuel (Clab) on behalf of Svensk Kàƒ¤rnbràƒ¤nslehantering AB (SKB). The company has more than 850 employees and has a turnover of SEK3 billion. Its three existing nuclear units at Oskarshamn account for more than ten per cent of the total electrical energy generated in Sweden.
The PULS project
As part of the expansion programme the Nuclear Power Inspectorate, SKI, approved a 250 MWe up-rating of OKG’s Oskarshamn 3 reactor from 1200 MWe to 1450 MWe gross – the decision to go ahead with the project being confirmed subsequently by the government. The project was initiated originally during 2003 and, after a further two years of far-reaching concept and design studies, economic evaluations of alternative schemes and the drafting and evaluation of tenders, contracts were finally signed in January 2006. Under the finalized arrangements for the PULS (Power Up-rate with Licensed Safety) up-rating project, Alstom was awarded a contract valued at around €170 million ($231 million) to supply and install the power island, comprising steam turbine, generator and associated balance of plant, with Westinghouse Electric being responsible for the upgrade of the reactor.
Figure 1. OKG’s Oskarshamn Unit 3 is undergoing a major refurbishment and life extension project as part of Sweden’s plans to meet future energy demand
Since the plant was first synchronized with the Swedish grid in 1985, Oskarshamn 3, originally constructed by Asea-Atom (now Westinghouse Electric), has performed with exceptionally high reliability and with availability reaching 89.4 per cent. Following completion of the present up-rating project over the next two years, the unit is expected to produce a generated output of more than 600 TWh during its planned lifetime of 60 years. This is more than twice the original estimated maximum through-life design output. The high performance of the plant to date is not only due to the very high operating standards and planned maintenance schedules demanded by OKG, but is also the result of the robust basic design, which includes ample operating margins for the entire plant.
Oskarshamn Unit 3 is a fourth generation boiling water reactor, based on Westinghouse’s BWR-75 concept. The plant was connected to the grid for the first time on March 3, 1985 and in May the same year full power operation was reached, which at that time was 3020 MWth. In October 1989 the plant was granted the necessary permits to up-rate the power output to 109.3 per cent corresponding to 3300 MWth, to give a maximum electrical output of 1200 MWe gross (1160 MWe net). The reactor core contains 700 fuel elements having a total average fuel cycle of five years. A combination of 169 control rods and the water-flow from eight internal recirculation pumps provides effective control of reactor power.
The Oskarshamn Unit 3 reactor and associated generating plant is designed in conformance with internationally accepted norms and criteria. These include 10CFR50 general design criteria, ASME and IEEE regulatory guides and specifications. In addition, a number of specific national criteria affecting the plant design have also been included, such as the requirement for a filtered pressure relief system in the containment structure and the application of the “30 minute criterion”. This forms an integral part of the active protection systems, which take automatic control in the event of a reactor-related fault and require that no operator intervention is permitted for 30 minutes after the malfunction, allowing sufficient time for the fault to be located, diagnosed and the necessary remedial action put into operation.
Figure 2. The original instrumentation and control systems related to the operation of the reactor systems are being retained
The plant layout is designed around four physically separated sub-divisions housing independent, automatic, safety-related systems providing quadruple redundancy. Each of the four systems is powered independently by auxiliary diesel generator sets, with a number of additional battery back-up supplies. Auxiliary power for two of the four sub-divisions is also supplied from gas turbine-based generating plant to provide increased diversification and security.
The scope of the PULS project covers the upgrading of Oskarshamn 3 to increase the power output of the plant to 3900 MWth, providing a minimum electrical power output of 1450 MWe gross. This corresponds to 129 per cent of the original design output rating of 3020 MWth. Due to re-enter full commercial operation in late 2008, the upgrades will enable the plant to operate with increased output and enhanced safety for a planned further 40 years.
The project is divided into two separate functional packages comprising:
- Reactor systems
- Turbine systems and electrical power systems.
In addition to plant and equipment replacements and upgrades, it also covers the necessary re-training of OKG’s technical, operating and maintenance staff, including full-scale simulator training planned to start about one year prior to re-commissioning.
The general development of the new, up-rated turbo-generator equipment has been carried out by Alstom in close co-operation with reactor design engineers from Westinghouse and other systems suppliers, as part of the overall development of the whole plant. Preparatory work is currently taking place at the site prior to installation of the new plant and equipment, scheduled during 2008.
The new 1450 MWe turbine, being designed and built by Alstom as a direct replacement for the existing machine, will consist of a double flow HP stage and three double-flow LP turbine-stages, with a total exhaust area of 155 m2. Although similar to the layout of the existing steam turbine, the uprated replacement unit will involve modified bearings, lube oil and jacking oil systems and other associated sub-systems and equipment. The turbines and the generator will be mounted in-line on a common shaft 72 m in length and will have an operating speed of 1500 r/min.
Live steam will be supplied to the HP turbine from the reactor at 2115 kg/s through four steam lines via four upgraded steam inlet valves to reduce pressure-drop. Corresponding feedwater temperature will remain at 218à‚°C. The moisture separator has been installed directly under the HP turbine to give optimally-shortened flow-paths for the wet steam from the HP turbine exhaust. Both the condensate and the feedwater pump sets consist of three 50 per cent electrically-driven units. The speed of each of the feedwater pumps is controlled through hydraulic couplings. Intercept and emergency stop valves providing overspeed protection are installed in the crossover pipes between the re-heater and the LP turbines. Full steam flow at the uprated conditions will be capable of being temporarily dumped to the condenser during start-up, load rejection or in the event of turbine tripping. The turbine incorporates electro-hydraulic speed governing and safety systems with fully automatic and run-up facilities.
To meet the increased output of 1450 MWe, with approximately the same temperature increase, main cooling water flow is being increased by a minimum of 20 per cent to around 55 m3/s. Modifications will be made to both moisture separator and steam re-heaters, including complete replacement of existing re-heater tube bundles. The outer rows of tube bundles in the turbine condenser are also being replaced, using modern corrosion-resistant material to reduce pitting. Condensate and feedwater systems are also being up-rated to cope with the increased flow-rates and provide adequate control margins. Existing turbine protection is being replaced by a modern digital system, modified to meet the “two out of three” fault-trip requirement common to the turbine plant as a whole.
The major item of plant being supplied by Alstom as part of the electrical systems upgrade package is a completely new, hydrogen-cooled, 25 kV generator, together with both new and upgraded auxiliaries including the excitation system, transformers and sealing oil system. The generator replaces the existing 20.5 kV Brown Boveri machine currently in use. Rated at 1670 MVA with a power factor of 0.9, the new generator will have a minimum output of 1450 MWe and will have the capability of supplying one-third as reactive power to the Swedish national grid.
The higher output power and voltage ratings of the new generator make it necessary to replace the existing main generator step-up transformer with three new single-phase 25 kV/400 kV step-up transformers, each rated at 560 MVA. The increase in generator voltage also requires the two auxiliary transformers, which feed the 10 kV auxiliary power busbars, to be replaced. Initial technical assessments have shown that the existing Isolated Phase Busduct (IPB) and Generator Circuit Breaker (GCB) can operate at the planned higher nominal voltage. The auxiliary electrical power distribution system will also be adapted to meet the new requirements. This includes installing new circuit breakers and undertaking the necessary verification of system capacity and performance. Nevertheless, the ability to retain many existing systems will result in significant benefits to the customer in terms of reduced installation times and costs.
Figure 3. Alstom was awarded a contract valued at around €170 million to supply and install the power island, comprising steam turbine, generator and associated balance of plant
The mechanical configuration of the new generator comprises bearing pedestals with journal bearings and endshield housings at drive and non-drive ends, together with brush-gear and end-bearing pedestal. The stator housing is designed to be mounted on the baseplate to allow for horizontal linear expansion in both directions from the centre. The entire replacement machine has been designed specifically to fit the existing foundation, enabling the installation to be completed rapidly without the need for any modifications to the structure, reducing installation time further.
The uprated reactor systems will be optimised by Westinghouse to allow for operation at the increased power of 3900 MWth, with a core flow of 14 500 kg/s and a maximum humidity of 0.1 per cent by weight of the steam leaving the reactor vessel. The nominal reactor pressure will remain at 7.0 MPa. In order to achieve the higher output conditions, modifications include replacing the existing core spray with a diversified two-train system comprising two core cooling distributors and two boron injection distributors. Other major replacements include the installation of a new moderator tank cover, new steam separator and steam dryer. The main steam line isolation valves will be replaced with new ones with lower pressure drop.
Although dating from the 1980s, the original instrumentation and control systems related to the operation of the reactor systems are being retained, eliminating the need to renew formal licensing for the I&C “1E” systems. While employing the same basic operational and supervisory I&C control room technology, new operating parameters such as increased water and steam flow-rates, higher steam conditions and higher output power levels from the up-rated plant, will require significant modifications. These will include changes to control-points, measuring ranges and modifications to a number of automated power control systems.
Figure 4. Oskarshamn Unit 3 is a fourth generation boiling water reactor, based on Westinghouse’s BWR-75 concept
All changes and modifications to the operating systems will be incorporated in the software for the full-scale simulator and included in the staff training programme. The new turbo-generator plant is classified as “non 1E”. The existing obsolescent turbine protection, instrumentation and control systems will be replaced by the company with a modern, three-channel, programmable digital system.