Two types of reactors are in use today, the pressurized water reactor and the boiling water reactor. In both, water is circulated through the reactor core to extract the heat generated by the fission process. In a pressurized water reactor (Fig. 4.2), the water is kept under very high pressure, about 2,000 psi or 15 Megapascal (MPa).
The pressure is high enough that the water does not boil. Pumps move the heated water to a device called a steam generator that acts as a boiler. In the steam generator, the heated water is passed through tubes where it heats water on the other side of the tube wall. This water is allowed to boil, generating steam. The steam then drives a turbine that is connected to an electrical generator. The advantage of this system is that the water used to generate steam is not exposed to neutrons in the reactor. The water that passes through the reactor core is exposed to neutrons and, as a result, picks up radioactivity.
In a boiling water reactor (see Fig. 4.3), the water that passes through the reactor core is kept under a lower pressure of about 900 psi or 7 MPa.
The water boils, becoming steam which is sent directly to the turbine. Since the water is exposed to the neutrons in the reactor, the steam is radioactive and the areas around the turbine and steam supply system are high radiation areas. The disadvantages of having such levels is somewhat offset by the simplicity of the system and its lower operating pressure. Today, about one-third of the commercial power reactors are boiling water reactors and two-thirds are pressurized water reactors.
Other types of reactors are also used, including one that uses natural uranium but has heavy water instead of light water4, reactors that use gas as a coolant and also reactors that use liquid metals such as sodium to cool the reactor .
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