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Diagram of a PWR-nuclear power station | |
I. | Primary circuit | |
II. | Secondary circuit | |
III. | Tertiary circuit | |
A. | Reactor | |
B. | Steam generator | |
C. | Turbine | |
D. | Condenser |
Three separate units |
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A nuclear power plant differs from a conventional power plant by the way is generated. While steam is produced in a conventional thermal power plant by means of a combustion process in a steam boiler, in a nuclear power plant, a fission process takes place in a reactor. The production of steam in a PWR (pressurized water reactor) is achieved in several steps: 1. In the reactor, fuel rods transfer their heat to a primary circuit containing water. This water becomes extremely hot - approximately 300°C - but does not boil as it is kept pressurized at approximately 155 bar. Hence the name "pressurized water reactor". 2. The heated water is routed to a heat exchanger, i.e. the steam generator. The heat from reactor coolant water is transferred to a separate secondary circuit (the water-steam circuit). Due to a lower pressure in this circuit the water is converted into steam; the steam is then used to drive a turbine connected to a generator. 3. Finally, the steam leaving the turbine is converted back into water by cooling it. This cooling process occurs in the condenser via a separate tertiary cooling circuit, using water from an external source. |
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PWR nuclear power plants therefore have three separate circuits: • the primary circuit which transfers the heat from the reactor via the steam generator to the secondary circuit • the secondary circuit (water-steam circuit) • the tertiary cooling circuit which converts the expanded steam back into water. |
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Such a design prevents radioactive releases into the environment. |