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The nuclear power plant and the fossil thermal power plant both use steam to convert the heat or thermal energy to mechanical rotation to rotate the generator to produce electricity. Only the heat source is different. In a nuclear plant, the heat source is from the nuclear reaction whereas in a thermal power plant it is from the combustion of coal.
The difference is in the inlet steam parameters to the turbine in a nuclear plant. Thermal power plants use steam at superheated conditions. In nuclear plants, the steam is at saturated conditions and at a lower pressure. This is due to the inherent design limitations in the nuclear reactors.
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In fossil power plants the inlet steam parameters are typically temperatures of 540 º C to 580 º C and pressures of 170 bar or even higher. In addition, there is additional heating in the form of re-heating. In a nuclear plant, the ratings are typically saturated steam at 78 bar, which is steam temperature of 298 º C. The nuclear plant uses a 'wet steam turbine'.
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Increased Steam Flow.
The reduced inlet steam parameters in the nuclear plant results in lower thermal efficiencies. Nuclear plants operate at lower thermal efficiencies , lower by more than 10 %. The energy of steam per unit mass entering the turbine is also less.
This results in a very high steam flow for the same MW output, almost double that of fossil power plants.
The configurations of the turbines change due to this. The economics of scale requires the nuclear plants to be in the range of 600 MW to 1000 MW resulting in very big Turbines.
Fossil plants turbines normally have one High Pressure one Intermediate Pressure and a double flow Low Pressure (LP) cylinder.
The LP turbine exhausts to very low pressure. The volume of steam leaving the LP turbine is very high. The higher flows require very long last stage blades to keep the exit velocities and exit loss very low. This results in very high stresses in the blades. The sizing of the LP turbine is limited due to the size of the Last Stage Blades. Therefore, each LP turbine has a flow limitation.
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One way to overcome this limitation is to increase the number of LP turbines. Large Nuclear plants have apart from an HP /IP stage two or three double flow LP turbines connected in tandem.
Another way is to reduce the speed. Since the stress on the Last Stage Blades is speed dependent reducing the turbine speed by half reduces the stress resulting in bigger sized LP turbines. In such cases the turbine runs at 1500 or 1800 rpm. This requires specially designed four pole generator rotors. This results in a lower number of LP turbines. This is more helpful in 60 Hz countries.
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Nuclear steam turbines are 'wet steam turbines'.
Since the steam is at saturated conditions, after each stage expansion the steam gets wetter. The water particles result in lower efficiency of the turbine. This results in erosion damage to the blades. In addition, this results in vibrations and stress in the last stage. To overcome this nuclear steam turbines use special design of blades and flow paths.
Moisture separators located in the steam path at exit of HP / IP and in the cross under pipes reduce the undesirable effects of the moisture in the steam. Moisture Separator Reheaters also are used. These use extracted steam to aid in moisture removal.
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Nuclear steam cycles do not have Reheating as in fossil units. This also reduces the cycle efficiency.
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Even with the much lower thermal efficiency, Nuclear power is feasible due to low unit cost of fuel.