its moisture removal) is reheated to superheated conditions by the main steam bypass flow or by steam flow extracted from the high pressure turbine, the steam cycle is called the reheating cycle. For PWR NPPs, the reheating cycle is normally applied. The use of the reheating cycle results in an increase of heat efficiency of approximately 2.5% compared to not-reheating cycle plants, as well as about a 3% reduction in low pressure turbine moisture contents, which, in turn, reduces the water erosion of low pressure turbine blades.

A typical turbine steam expansion diagram of a PWR NPP is shown in Figure 3.5.1.

3. Structures of turbines

Hie steam generated in steam generators flows to the high pressure turbine and enters it through the main steam stop valves and steam governing valves. After doing work in the high pressure turbine, the steam flows into a moisture separator which

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[Source] Mitsubishi Heavy Industries Catalog

Figure 3.5.1 Nuclear turbine steam expansion diagram

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removes moisture from the steam and a reheater which reheats it. The moisture separator and reheater are housed in a common pressure vessel. Then, the steam flows to the low pressure turbines and enters them through the reheater stop valves and interceptor valves. After doing work in the low pressure turbines, the exhausted steam flows down to a condenser which condenses the steam to water. The condensate in the condenser is returned to the turbine system cycle. A cut-away view of a typical PWR NPP turbine-generator unit is shown in Figure 3.5.2.

1. Casings

The high pressure turbine casing has a single shell structure. Steam seal surfaces of the casing including their horizontal joints are clad with stainless steel or nickel-based alloy (Inconel®) to enhance their corrosion resistance and air­tightness. The low pressure turbine casings have a double casing structure to reduce thermal stresses and thermal deformation of the casing caused by a large inlet-outlet steam temperature difference across the low pressure turbine.

2. Rotors

The high pressure turbine rotor is forged in one piece and machine-cut. However, the large diameter low pressure turbine rotor usually has a shrinkage fit structure, due to difficulties in fabricating it in one piece and separately fabricated discs are fixed to the rotor shaft by shrinkage

fitting. Since stress corrosion cracking (SCC) failures of shrinkage fitted discs have been reported to occur in PWR plants overseas, not only the selection of materials and the stress reduction measures have been reviewed in the design of the low pressure rotors, but also the technologies to apply one piece forging to the fabrication of low pressure rotor shafts and rotor discs have been developed. The application of one piece forging to the low pressure turbine rotor is expected to increase the reliability of turbine­generator units.

To prevent the turbine rotor shafts and discs from experiencing SCC failures, concentrations of dissolved oxygen in the feedwater are reduced by adding chemicals to the condensate and feedwater, minimizing the corrosive effect of the feedwater at plant startup and during plant normal operations. Maintenance techniques including heating and dehumidification of the turbine inside atmosphere during long-term plant outages, are also applied.

3. Blades

Water droplets in steam flows not only reduce the turbine efficiency, but also have the potential to erode the edge surfaces of turbine blades. In order to avoid such erosion, turbines are designed to have mechanisms to remove water droplets, separating them from the steam flow by centrifugal force generated in the rotating

© Generator

® Exciter

[Source] Mitsubishi Heavy Industries Catalog

Figure 3.5.2 Cut-away view of nuclear turbine (example)

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