nuclear steam turbines
Building on experience: 1750 MW Arabelle ST for Flamanville 3 EPR1 September 2008
Work is now well underway on manufacturing the Arabelle steam turbine for EDF’s Flamanville 3 EPR nuclear unit. This is the largest steam turbine ever constructed, but builds on the large body of experience amassed with the French nuclear fleet, notably the 1550 MW Arabelle steam turbines at Chooz B and Civaux. A major contributor to the high efficiency and compactness of the new unit is the combined HP–IP (“HIP”) casing, which makes maximum use of single flow steam expansion – a notable feature of the Arabelle architecture.
EDF’s 4500 MWt Flamanville 3 pressurised water reactor, which is currently under construction, is being equipped with the most powerful steam turbine generator ever built, rated at 1750 MWe (gross, at the generator terminals). The turbine island supplier is Alstom whose scope of supply includes engineering, manufacturing and procurement, construction and commissioning of the steam turbine, generator, condenser, moisture separator reheaters and auxiliary equipment. The contract was signed on 22 September 2006 and engineering, procurement and manufacturing activities are currently underway.
Manufacture of long lead time components such as large castings and forgings started during the first half of 2008, with first deliveries to site scheduled for mid-2009 and start of commissioning one year later.
The Flamanville 3 steam turbine is of the Arabelle design. Four 1550 MWe (gross) Arabelle steam turbine generators are already in operation in France, two each at Chooz B and Civaux respectively, which entered commercial operation in 2000, and two more are being installed at the Ling Ao extension currently under construction in China. Orders have also been placed for two 1750 MWe Arabelle machines for the Taishin EPR units in China (see panel below).
The Arabelle design was developed towards the end of the 1980s and built on the experience amassed with the previous generations of nuclear steam turbines designed and built by Alstom.
The Arabelle, which has a design life of 60 years, was conceived at a time when most turbine manufacturers were limiting their ambitions in the face of the then very bleak outlook for nuclear power. But with the present nuclear renaissance the Arabelle design is finding favour with those planning a new generation of nuclear plants and who are seeking higher efficiency, reduced maintenance costs, and maximum reliability. It is available in two frames for 50 Hz, the Arabelle 1700, as at Flamanville 3 and Taishan, and its “little sister” the Arabelle 1000, suitable for 1000 MWe units such as those currently under construction in China. Two equivalent frames are also available for 60 Hz applications. The Arabelle can be used with both PWRs and BWRs.
The much higher efficiency achieved by Arabelle’s architecture combined with a range of advanced technologies translates into a gain of over 60 MWe in electrical output compared with a P4/P’4 turbine operating with a 4500 MWt reactor.
The use of very large exhaust sections, made possible by the “half-speed” rotation (1500 rpm), allows the very highest electrical outputs to be achieved combined with lower condenser back-pressures. The Flamanville 3 steam turbine will have a total final exhaust area of 155 m2, with a last stage blade length of about 1.74 m.
The Arabelle design has a number of features that aim to improve efficiency and reduce installation and maintenance costs.
In particular the architecture features a combined high pressure (HP)/intermediate pressure (IP) casing – an HIP casing – and the whole machine employs impulse type blading.
Highly efficient single-flow steam expansion is used between the HP inlet and the IP exhaust, producing more than 1000 MW on a 13-wheel rotor, which is only 120 tonnes in weight.
The Arabelle is also shorter and lighter than previous-generation nuclear steam turbines per MW of output.
In addition, it can be assembled more easily as its low pressure modules have independent structures; the LP inner casing is independent of the LP exhaust hood and both are connected by flexible annular rubber joints. This allows the LP turbines to be assembled independently of the condenser and facilitates shaft-line alignment, which can be carried out without any need for upper LP exhaust hood dismantling.
The positioning of the two moisture separator reheaters, between the HP and IP steam expansions, reduces the number of reheat valves (four instead of six).
Also, the vertical MSRs, located on either side of the HIP casing, free up storage areas on both sides of the LP casings, making maintenance operations easier.
In addition, the IP–LP connecting ducts are of the “cross-under” type, running under the pedestal. This means there is direct access to the LP modules as there is no need to dismantle the ducts to open the LP casings, with the result that the erection and maintenance times are reduced by 15% and 20% respectively.
The Arabelle steam turbine configuration results in fewer components (notably inlet valves and expansion bleeding wheels) and improved mechanical behaviour.
Benefits of single flow
Nuclear turbines of previous generations typically feature one double flow high pressure casing in which the main inlet steam flow is divided into two symmetrical flows. After expansion the steam is led to the moisture separator reheaters, 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 flows (depending on whether there are two or three LP modules) for final expansion down to the condenser pressure.
In an Arabelle machine, by contrast, the steam expands in a single flow HP path. The expanded steam is fed to the two MSRs, where it is superheated in two stages, by a derivation of the main steam and by an HP extraction, then expanded in a single flow IP part. This intermediate pressure section is unique, and the Arabelle is the only saturated nuclear steam turbine to date with an IP expansion not integrated into the LP modules. In the Arabelle machine the final split to feed two or three double flow LP casings is done at a relatively low pressure level – around three times lower than for turbines of the previous generation.
The most striking feature of the Arabelle turbine is thus an architecture that makes optimum use of high efficiency single flow steam expansion. This ensures higher efficiency by reducing the secondary losses that develop at the root and at the ceiling of the steam path.
With the Arabelle architecture, single flow steam expansion is maintained typically from the inlet pressure of 75 bar down to 3 bar, meaning that more than 60% of the expansion is done at best efficiency. The overall gain in efficiency contributed by this single flow architecture as compared with the previous generation (P4/P’4)?configuration is put at 1%.
The combined HP/IP casing used in the Arabelle turbine is in fact similar in principle to those used in fossil fired applications, with the aim of reducing the overall turbine length. But the dimensions are much bigger than those found in fossil stations. A saturated nuclear steam turbine needs to accommodate an inlet volume flow roughly five times greater than a fossil-fired unit of the same nameplate rating due to the much lower steam pressures and temperatures.
Why welded rotors?
Welded rotor technology (rather than shrunk on discs or monobloc rotors) is a key feature of Alstom steam and gas turbines. In particular, for very large rotors, it allows better control of material properties and initial defect sizes as smaller pieces are easier to forge and inspect than bigger ones. Because of the reduced stress level compared with the shrunk-on disc design, steel with lower yield strength can be used. This gives better resistance to stress corrosion cracking, while at the same time providing the required properties for the discs supporting the last stage blades.
The huge forgings required for monobloc solid rotors are in very short supply, with an extremely limited number of suppliers able to cater for a market that looks likely to experience a major revival. This shortage is an issue when it comes to planning future power plants and potentially a nightmare for plant construction should problems occur during manufacture.
With welded rotor technology, the forgings are relatively small and can be provided by a number of qualified vendors around the world, providing much more flexibility and margin in terms of planning and plant construction. Alstom owns several welding facilities that have been adapted to cater for these rotors and is also developing a new one in its Chattanooga factory so as to be able to provide additional capacity to serve the US market in particular.
To match the Arabelle steam turbine at Flamanville 3, a Gigatop four-pole hydrogen and water-cooled generator has been selected. This builds on generator technology experience acquired over many years with the French nuclear fleet and aims to achieve low maintenance requirements coupled with extended lifetime.
The rating of the Flamanville 3 machine will be 2000 MVA, making it the biggest generator in the world.
Reliability and economies of scale
All studies by nuclear operators worldwide point to the benefits that arise from the economies of scale provided by larger nuclear units, which have permitting and operation and maintenance costs, as well as land use requirements not significantly different from those for smaller units.
We may believe that a unit with a 2000 MVA generator is close to the maximum which can be reached with present technology, but frontiers tend to be breached in time.
Because of the size, it is of utmost importance that spurious trips are reduced to a minimum (ie reliability is maximised), and that in case of a grid breakdown the unit can operate in isolated mode feeding its own electrical auxiliaries (“house load” mode), permitting fast reconnection once the grid problem is corrected.
House load tests were a particular feature of commissioning trials for the 1550 MWe Chooz B and Civaux units (where, incidentally, less than one month elapsed between initial criticality and grid connection).
And it is interesting to note that during the major European blackout of November 2006 where a large part of the grid went down following the untimely switch off of a high voltage line in Germany, both the Chooz units successfully went into house load mode operation and then back on line, greatly facilitating recovery after the incident.
The Arabelle steam turbine generator for Flamanville 3 will employ the proven Chooz B/Civaux design, leveraging the exceptionally high reliability, over 99.9%, demonstrated there.
No significant design changes were necessary and the Flamanville 3 machine can be seen as a relatively modest scale up of the Chooz B/Civaux steam turbine generators, with the added benefit of experience feedback from the earlier units as well as from recent steam turbine retrofit and upgrade projects.