|Machining of the rotor of the Arabelle steam turbine, which is now designed to work with the new generation of large-scale nuclear reactors|
The Baltic nuclear power plant being built in the Russian exclave of Kaliningrad, located between Poland and Lithuania, is based on Russia’s new generation of VVER pressurised water reactors and will comprise two identical units with individual generating capacities of 1200 MW.
The recent award of the contract for the conventional turbine island marks an important milestone for the project. Apart from featuring the new generation of Russian reactors, the project represents the first nuclear power plant project in Russia to involve the participation of foreign suppliers for its critical components. The conventional island will be built by Alstom-Atomenergomash (AAEM), a joint venture established in 2007 between Alstom and OAO Atomenergomash, part of Russia’s state-owned Rosatom Group.
Under a €875 million ($1.15 billion) contract signed this February, AAEM will supply and install the major equipment, including Alstom’s Arabelle™ steam turbines, generators, condensers, moisture separator reheaters, low-pressure and high-pressure feed water heaters, feed water tanks and various other auxiliary equipment. This is the joint venture’s first large order and the share of equipment to be manufactured in Russia will exceed 50 per cent in accordance with the Arabelle turbine production localisation programme for nuclear power plants based on Russian reactor designs. In the future, this share is expected to exceed 70 per cent.
Under a separate contract, AAEM will deliver conceptual and basic design studies of the turbine island, turbine hall layout studies, process studies and steam and water loop studies to its customer OJSC NIAEP, the general contractor of the project, and again part of the Rosatom Group.
The Baltic nuclear project has two main goals: to provide a reliable supply of electricity to the Russian enclave of Kaliningrad and to offset the lack of electricity in neighbouring nations by exporting power to those countries.
Half-speed turbine technology the natural choice
In a nuclear power plant, the reactor unit size in general dictates the choice of the turbine technology. With full-speed steam turbine technology reaching its limits at around 1000 MW, half-speed technology is clearly the dominant technology for above 1000 MW. At 1200 MW, half-speed technology can be used efficiently under a large range of reactor and site conditions. The decision to move to the new reactor unit size under AES-2006 (VVER-1200) therefore made steam turbines based on half-speed technology the natural choice for the Baltic nuclear power station.
|Half-speed turbine technology is the dominant technology for ratings of over 1000 MW|
Half-speed technology is the only technology that can efficiently handle the very large quantity of relatively ‘cold’, wet and low-pressure steam produced by large nuclear reactors. The lower stress levels achieved with steam turbines that use half-speed technology ensure high reliability and longevity over long periods. Half-speed technology also allows the use of longer last-stage blades (LSB) offering large exhaust areas, which is a key parameter to achieve the best performances in relatively cold areas.
Consequently, around 85 per cent of all nuclear power plants with a power output above 900 MW are equipped with half-speed steam turbines; and all nuclear power plants with a power output greater or equal to 1200 MW are equipped with half-speed turbines.
AAEM was established to equip the turbine islands of nuclear power plants constructed in Russia under the Federal Target Programme with half-speed turbines based on Alstom’s Arabelle technology.
The Arabelle steam turbine is widely acknowledged as the best half-speed turbine on the market. It offers outstanding power output (from 1000 MW to 1900 MW), efficiency and reliability, using a specific architecture and welded-rotor technology developed by Alstom. A fully validated and advanced Arabelle design provides proven reliability and performance for the new generation of reactors.
Previous generations of steam turbines for nuclear plants feature one double-flow high-pressure (HP) cylinder in which the main inlet steam flow is divided into two symmetrical flows. After expansion, the steam is led to the moisture separator reheaters (MSRs), where it is first dried and then superheated by a derivation of the main steam. Superheated steam is then fed to each of the four or six low-pressure (LP) flows (with two or three LP modules respectively, depending on back-pressure conditions) for final expansion down to the condenser pressure.
Arabelle’s Unique IP Section
In an Arabelle turbine, the steam expands in a single-flow HP path and is then divided to feed the two MSRs. The two superheated steam flows are again joined and expanded in a single flow intermediate-pressure (IP) section. This IP section is unique, and Arabelle is currently the only saturated steam turbine for nuclear plants with an IP expansion not integrated in the LP modules. The final split, to feed two or three double-flow LP cylinders, is done at a relatively low-pressure level – around three times lower than in the earlier generation of nuclear steam turbines.
In order to reduce overall turbine length, the HP and IP expansions have been regrouped in a combined HP/IP cylinder, similar to those sometimes used in fossil fired applications except for their much larger size. A saturated steam nuclear steam turbine will accommodate an inlet volume flow roughly five times greater than a fossil fired unit of the same nameplate rating because of the combination of much lower steam inlet pressure and temperature.
|Arabelle steam turbine’s international footprint – successfully commissioned at China’s 2160 MW Ling Ao nuclear power plant|
The most striking feature of the Arabelle technology is an architecture that makes the best use of the high-efficiency single-flow steam expansion. The single-flow arrangement ensures higher efficiency because of the reduction of secondary losses that develop at the root and at the tip of the steam path blades.
With this unique arrangement, the single-flow steam expansion is maintained typically from the inlet pressure of 60 bars to 70 bars down to approximately 3 bars. Sixty per cent of the delivered power output comes from this single-flow steam expansion performed with the best efficiency. The overall gain in efficiency permitted by the single flow architecture as compared to the former architecture is estimated to be at least 1 per cent.
The Alstom turbine’s last-stage blades (LSB) for the Baltic nuclear application have a length of 1430 mm. This type of LSB provides an exhaust area well suited to the steam mass flow provided by the reactor and to the ultimate heat sink conditions found at site. Its design, with an integral snubber for reliable operation and a fir-tree root attachment for easier access during inspection, is well adapted to the long inspection intervals targeted by the present generation of nuclear power plants.
Thanks to the firm vibration control provided by the snubber connection, this type of blade has the benefit of weighing less than free-standing blades of the same length. With these relatively light blades it is possible to define a bearing structure able to withstand the postulated inbalance created by a LSB failure.
Its aerodynamics have also been optimised and the last stages benefit from the 3D profiles developed by advanced calculations and now made possible by modern manufacturing techniques. In particular, the last two diaphragms feature bowed profiles and not the straight profiles used in the previous generation of machines.
Welded rotor technology is another key feature of Alstom’s steam turbines and gas turbines. For very large rotors, it permits the best control of the material inner properties. Because of the reduced stress levels in welded rotors compared to shrunk-on disks rotor design, steel with lower yield strength can be selected for better resistance to stress corrosion cracking while maintaining the required properties for the disks supporting the last stage blades.
Achieving overall cycle efficiency
While the steam turbine design is important to achieve high overall cycle efficiency, all elements of the steam and water cycle need to be analysed and the efficiency losses minimised.
In most of the older operating units, heating steam is taken from the live steam and the steam reheat is achieved in a single-stage reheater. With modern units, however, the thermodynamic cycle is improved by the addition in the MSR of an extra heating stage, fed from a HP cylinder steam path extraction point. Final reheat temperature is kept the same using only half the live steam flow used for the single-stage reheater. This replacement of main steam by partly expanded HP steam improves cycle efficiency by 0.3 to 0.6 per cent depending on cycle conditions. MSR terminal temperature difference and pressure drop have a direct impact on unit efficiency.
The feedwater heaters configuration has also been optimised. The steam turbine will be adapted to the reactor by selecting the pressure of the HP extraction feeding the last HP heater to match the desired reactor feed water temperature. Experience shows that for Arabelle units, the steam is best extracted after the HP fourth stage and that the desired feedwater temperature can be reached by simple steam path adaptation.
To raise the feedwater temperature to this level, a series of regenerative feedwater heaters using extraction steam from the turbine will be used. The recommended configuration includes seven extraction levels with four LP heaters, one deaerator and two HP heaters, i.e. one heater more than in previous generations. In addition, forward recovery pumping of condensates from the LP heaters is used to increase the plant net output. All those improvements have a measurable impact on the plant power output, so any additional cost will be recovered.
In order to meet the specific requirements of the Baltic project, the steam turbine will be designed to accommodate the steam needs of a nearby district heating network. The Arabelle original turbine architecture, with distinct HP and IP steam paths, is particularly well suited to extract relatively large amounts of steam at two distinct pressure levels: at around 10 bars (the steam extraction is then located at the HP steam path exhaust) and at around 3 bars (corresponding to the pressure level at the IP steam path exhaust).
In the particular case of the Baltic project, up to 300 MWth will be extracted from the steam turbine to feed a district heating heat exchanger located in the plant.
The half-speed Arabelle turbine technology is currently the most powerful on the market, capable of delivering up to 1900 MW and offering excellent resistance to corrosion and stress corrosion cracking, together with a long service life – 60 years, extended inspection intervals and facilitated maintenance.
Serving the Russian and International marketplace
Arabelle turbines are now part of a growing list of units installed around the world. Russia aims to triple its share of nuclear power in its energy mix (currently at 11 per cent), replace older plants and export new ones, and the Baltic nuclear project is part of that strategy. The current timetable aims to bring the first unit onstream in 2016 and the second in 2018.
In late 2008, AAEM conducted a feasibility study to design the turbine-generator package for Russia’s Seversk nuclear power station in the Tomsk region of Western Siberia. It was originally slated for completion by 2015 and 2017 but was deferred, partly because of a fall in power demand caused by the global recession.
In May 2011, AAEM received a €3 million engineering contract – the VVER-TOI technical project – to develop the design of a turbine hall incorporating the Arabelle turbine and associated equipment with the latest generation of VVER reactors called VVER TOI (technological, optimised, innovative design).
This valuable experience positions Alstom as an important partner ready to serve the Russian market, as well as the international market, with a 1000–1900 MW turbine technology that is unique in the marketplace.
Philippe Anglaret is vice president of Business Development Nuclear and Olivier Mandement is director of Product Promotion Nuclear at Alstom.
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