LP Turbine - Low-pressure Steam Turbine

Most nuclear power plants operate a single-shaft turbine-generator that consists of one multi-stage HP turbine and three parallel multi-stage LP turbines, the main generator and an exciter.

Each LP Turbine (low-pressure turbine) is usually a double-flow reaction turbine with about 5-8 stages (with shrouded blades and with free-standing blades of the last 3 stages). LP turbines produce approximately 60-70% of the gross power output of the power plant unit. Each turbine rotor is mounted on two bearings, i.e., there are double bearings between each turbine element.

LP turbine is equipped usually with 3 or 4 self-regulating extraction lines, which are used to provide steam for:

the low-pressure feedwater heaters

In the LP turbine, the low-pressure stage receives steam (this steam is usually superheated steam – point E at the figure; ~1.15 MPa; 250°C) from a moisture separator-reheater (MSR). The steam from the HP turbine must be reheated to avoid damages that could be caused to the blades of the steam turbine by low-quality steam. High content of water droplets can cause rapid impingement and erosion of the blades, which occurs when condensed water is blasted onto the blades. The moisture-free steam is superheated by extraction steam from the high-pressure stage of the turbine and by steam directly from the main steam lines.

In the low-pressure turbine the steam continuously expands from point E to F. The exhausted steam then condenses in the condenser and it is at a pressure well below atmospheric (absolute pressure of 0.008 MPa). The steam is in a partially condensed state (point F), typically of a quality near 90%. High pressure and low pressure stages of the turbine are usually on the same shaft to drive a common generator, but they have separate cases. The main generator produces electrical power, which is supplied to the electrical grid.

Rankine Cycle - Ts diagram — Rankine cycle – Ts diagram

Advantages and Disadvantages of Steam Turbines

Advantages

Since the steam turbine is a rotary heat engine, it is particularly suited to be used to drive an electrical generator.
The thermal efficiency of a steam turbine is usually higher than that of a reciprocating engine.
The very high power-to-weight ratio, compared to reciprocating engines.
Fewer moving parts than reciprocating engines.
Steam turbines are suitable for large thermal power plants. They are made in various sizes up to 1.5 GW (2,000,000 hp) turbines used to generate electricity.
In general, steam contains a high amount of enthalpy (especially in the form of heat of vaporization). This implies lower mass flow rates compared to gas turbines.
In general, the turbine moves in one direction only, with far less vibration than a reciprocating engine.
Steam turbines have greater reliability, particularly in applications where sustained high power output is required.

Disadvantages

Although approximately 90% of all electricity generation in the world uses steam turbines, they also have some disadvantages.

Relatively high overnight cost.
Steam turbines are less efficient than reciprocating engines at part load operation.
They have longer startup than gas turbines and surely than reciprocating engines.
Less responsive to changes in power demand compared with gas turbines and with reciprocating engines.

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Steam turbine of typical 3000MWth PWR — Schema of a steam turbine of a typical 3000MWth PWR.

References:

Nuclear and Reactor Physics:

J. R. Lamarsh, Introduction to Nuclear Reactor Theory, 2nd ed., Addison-Wesley, Reading, MA (1983).
J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.
W. M. Stacey, Nuclear Reactor Physics, John Wiley & Sons, 2001, ISBN: 0- 471-39127-1.
Glasstone, Sesonske. Nuclear Reactor Engineering: Reactor Systems Engineering, Springer; 4th edition, 1994, ISBN: 978-0412985317
W.S.C. Williams. Nuclear and Particle Physics. Clarendon Press; 1 edition, 1991, ISBN: 978-0198520467
Kenneth S. Krane. Introductory Nuclear Physics, 3rd Edition, Wiley, 1987, ISBN: 978-0471805533
G.R.Keepin. Physics of Nuclear Kinetics. Addison-Wesley Pub. Co; 1st edition, 1965
Robert Reed Burn, Introduction to Nuclear Reactor Operation, 1988.
U.S. Department of Energy, Nuclear Physics and Reactor Theory. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.

Advanced Reactor Physics:

K. O. Ott, W. A. Bezella, Introductory Nuclear Reactor Statics, American Nuclear Society, Revised edition (1989), 1989, ISBN: 0-894-48033-2.
K. O. Ott, R. J. Neuhold, Introductory Nuclear Reactor Dynamics, American Nuclear Society, 1985, ISBN: 0-894-48029-4.
D. L. Hetrick, Dynamics of Nuclear Reactors, American Nuclear Society, 1993, ISBN: 0-894-48453-2.
E. E. Lewis, W. F. Miller, Computational Methods of Neutron Transport, American Nuclear Society, 1993, ISBN: 0-894-48452-4.

See above:

Steam Turbine