HKNIC - Main types of nuclear power reactors
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Main types of nuclear power reactors
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Nuclear power generating units are classified according to their nuclear reactors. There are six main types in operation:

Image Credit: DNMC

They fall into two main groups, according to the nature of the nuclear fission that takes place. In the first group, known as thermal-neutron reactors or simply thermal reactors, the neutrons are slowed down by a material known as moderator to enable nuclear fission to take place. In the second group, known as fast-neutron reactors or fast reactors, a moderator is not required.

Of the six main types listed above, the first five types are thermal reactors and only the last is a fast-neutron reactor.

Pressurised Water Reactor (PWR)

This is the most common types of commercial reactor and was initially developed in the US for submarine propulsion. In a PWR, water at 150 times atmospheric pressure is used as a coolant for the slightly-enriched nuclear fuel and as the moderator for the nuclear reaction. The high pressure keeps the water from boiling and at a heat exchanging device known as a steam generator, a separate stream of water is heated in a secondary closed circuit at about 70 times atmospheric pressure and 280°C to produce steam to drive a turbine-generator which produces electricity.

The reactor is sheltered within a concrete structure which helps shield the outside from radiation and is further contained completely in a containment building.

The VVER is the Russian design of the Pressurised Water Reactor.

A PWR nuclear power generating unit has a typical electrical output in the range of 300MW – 1,600 MW. Daya Bay is a twin-unit PWR nuclear power station.