Power plant design

Turbines for Taishan

17 May 2010

Construction of the twin-EPR Taishan nuclear power station in China has now begun. The plants will drive the largest steam turbines in the world. By Jean-François Gautier

Up to 80% of China’s electricity is today produced from coal. Such a heavy reliance on coal results in high levels of pollution and CO2 emissions, which the country is tackling by increasing the share of nuclear and renewables in its energy mix. With plans to increase the share of nuclear from around 2% currently to 5% (70 GW) by 2020, China has one of the largest effective nuclear power expansion programmes in the world.

Late in 2009, construction began on Taishan units 1 and 2 located at Chixi Town, Taishan, Guangdong province. Each unit will comprise one EPR third generation reactor – the first in Asia – for the nuclear island coupled to a conventional island that will use Alstom’s 1755MW Arabelle steam turbine.

Alstom participates to the power island under construction under two contracts – one for the supply of the turbine generator package (TGP) and the second for the engineering and procurement of the conventional island excluding the TGP (CIEP), by which all components within the turbine island will be matched together for an optimal plant installation and operation.

In February and July 2008, TNPJVC, the owner of the Project, a joint venture of CGNPC (China Guangdong Nuclear Power Company) and EDF (Electricité de France), awarded these two contracts to two separate consortiums.

The TGP contract is being executed by a consortium between Alstom and its long-standing partner Dong Fang Electric Company. The turbine generator package supply contract includes two Arabelle steam turbines, the generators, the condensers with duplex heaters, the moisture separator reheaters (MSR) and auxiliary equipment needed to convert steam produced at the nuclear reactor into electrical power.

The conventional island contract is being executed by a consortium including Alstom Power Turbomachines group, AWEC (Alstom Wuhan Engineering Company) and two other companies – CNPEC (China Nuclear Power Engineering Company) and CNDPC (China Nuclear Design Power Company) – which are subsidiaries of CGNPC. The conventional island E & P contract covers the turbine hall and the equipment inside and the pumping station. Alstom will supply in particular key equipment such as feedwater heaters, deaerator, circulating water pumps, condenser extraction pumps in the turbine hall and the pumping station.

Alstom has been CGNPC’s and Dong Fang’s industrial partner since the construction of Daya Bay the first large Chinese nuclear power plant built in 1984. It also achieved the construction of Ling Ao Phase 1 in 2003. AWEC was created to serve as an engineering centre for nuclear products and for project development, management and procurement. In 2004, Alstom created Alstom Beizhong Power ABP, which makes and sells steam turbines and turbogenerators of 600 MW and above. This factory is already providing parts for ongoing nuclear projects in China.

Each steam turbine will have an output of 1755 MW, currently the highest in world. This is made possible by Alstom’s unique Arabelle technology.

In the primary circuit of a PWR reactor, pressurized water from the reactor flows to the steam generators, which generate steam for the secondary circuit. The EPR NSSS generates saturated steam at about 7.5 MPa and 290°C. These steam conditions are much lower than in a conventional plant, where steam pressure is generally between 17 MPa and 30 MPa and temperature in the 540-620°C range. These factors, combined with the very large unit output, explain why the nuclear steam turbines are so much bigger than the turbines used in coal-fired power plants.

The Taishan conventional island will develop around a four-casing Arabelle 1700 turbine, which features one combined high pressure (HP) and intermediate pressure cylinder and three low pressure (LP) cylinders.

Steam first flows to the HP turbine. As the steam expands, its pressure and temperature decreases, and its moisture content increases. To improve cycle efficiency and to reduce the erosion risks, this water has to be removed and the steam reheated before it can expand in the IP turbine. This is done in two parallel moisture separator reheaters (MSR). Taishan MSR’s are large heat exchangers that recover the wet steam from the HP exhaust, separate the free water, which is recycled in the feedwater line and then reheat the dried steam up to about 270°C using steam extracted from the HP turbine and the main steam line. This two-stage reheating process is a significant improvement compared to previous single stage reheat encountered in all presently-operating nuclear plants in China. The superheated steam then flows from the MSR to the IP turbine admission.

After expanding in the IP section, the steam is then fed to the three LP turbines in parallel, i.e. a third of the steam is fed to each LP section. Steam exhausted from the LP turbines then flows to the condenser.

The condenser is a large heat exchanger cooled by seawater. At the Taishan site, coastal waters are quite shallow, and the huge seawater flow necessary to cool the condenser requires the construction of a large undersea tunnel from the plant site to an island that is 4 km from the coast. Seawater is taken in deeper water from beyond this island and fed to a reservoir that can store more than 1 million m3 of seawater. Four concrete volute pumps supplied by Alstom are used to pump the seawater from this reservoir to the condenser and then back to the sea.

The steam condenses in the condenser. The condensate extraction pumps and the feedwater pumps then send back this water via seven stages of feedwater reheating to the steam generators, thus closing the cycle.

The most significant feature of the Arabelle turbine is an architecture that makes the best use of single-flow steam expansion. The single-flow arrangement improves efficiency by reducing secondary losses that develop at the root and at the tip of the steam path.

With this arrangement, the single-flow steam expansion is maintained typically from the inlet pressure of about 7.5 MPa down to around 0.35 MPa. More than 60 per cent of the expansion is thus 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 1 per cent.

In an Arabelle turbine, the steam expands in a single flow HP path. It is then divided to feed the two MSRs, and the two superheated steam flows are again joined and expanded in a single flow IP part. This IP section is unique, and Arabelle is the only large saturated nuclear steam turbine to date with an IP expansion not integrated in the LP modules. The final split to feed the three double flow LP cylinders is done at a relatively low-pressure level – about three times lower than for turbines of the previous generation.

In order to reduce overall turbine length, the HP and IP expansions are grouped in a combined HP/IP cylinder, similar to those sometimes used in fossil fired applications except for their much larger size.

Early progress

Although construction at Taishan only started in 2009, fabrication of major components for both EPR units has begun. On the conventional island, more than 75 per cent of the engineering has been performed for the TGP and conventional island since the authorisation to proceed was given on October 31, 2008.

For the TGP, Alstom is performing the functional engineering (general arrangement, design specifications for the equipment etc) and part of the detailed design, which are the fine adjustments needed to functional designs when the equipment is purchased. For the conventional island contract, Alstom is undertaking the functional design.

Most of the conventional island equipment under Alstom’s scope – including pumps and associated motors and gearboxes, separator panels for the MSRs – has been ordered. Procurement and manufacturing are proceeding on time. Civil works at the site is also underway, with first concrete for both the nuclear and conventional islands of Unit 1 being poured on 1 September 2009. Alstom has already delivered the formworks for concrete suction elbows and volute of the main cooling water pumps, and will deliver the turbine-generator foundation spring boxes in March 2010.

Condenser erection is scheduled to start in June of next year, which will then be followed by the start of turbine erection in October 2011. The first unit should be completed at the end of 2013, with first connection to the grid in September that year. Full commercial operation of Unit 1 is scheduled for December 31, 2013. Unit 2 will begin commercial operation in 2014.

In line with the Chinese self-reliance policy with international collaboration, Alstom continues to develop its localisation in China. Alstom and Dong Fang are providing the TGP for CGNPC’s twin CPR1000 LingAo II NPP. Commercial operation of the two units is planned in 2010 and 2011, respectively.

Alstom is also delivering the TGP for the Hongyanhe power plant located close to Dalian in Liaoning province. This plant will feature four 1000 MW units, again using CPR1000 reactors. The first concrete was poured on August 18, 2007. The commercial operation of Unit 1 is planned for 2012.

In addition, six Arabelle units are ordered for the NingDe, FuQing and FangJiaShan projects in the provinces of Fujian and Zhejiang. These are based on CPR 1000 reactor technology.

These projects, with Taishan, are helping China to meet the targets of its nuclear power programme in the coming decades.

Author Info:

Jean-François Gautier, Alstom Power.

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