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the fission of Uranium
atoms in the reactor provides the heat to produce steam for generating
electricity.

heavy steel pressure
vessel surrounding a reactor core.
The reactor core contains the Uranium fuel.
The fuel is formed into cylindrical ceramic pellets about one-half inch
in diameter, which are sealed in long metal tubes called fuel tubes. The
pins are arranged in groups to make a fuel assembly. A group of fuel assemblies
forms the core of the reactor.

Heat is produced in a nuclear reactor when neutrons
strike Uranium atoms causing them to fission in a continuous chain
reaction. Control elements, which
are made of materials that absorb neutrons, are placed among the fuel assemblies.
When the control elements, or control rods as they are often called, are
pulled out of the core, more neutrons are available and the chain reaction
speeds up, producing more heat. When they are inserted into the core, more
neutrons are absorbed, and the chain reaction slows or stops, reducing
the heat.

  Most commercial nuclear reactors use ordinary water to remove
the heat created by the fission process. These are called light water reactors.
The water also serves to slow down, or "moderate" the neutrons. In this
type of reactor, the chain reaction will not occur without the water to
serve as a moderator. In the United States, two different light-water reactor
designs are currently in use, the Pressurized
Water Reactor (PWR) and the Boiling
Water Reactor (BWR).

 

In a PWR, the heat is removed from the reactor by water flowing
in a closed pressurized loop. The heat is transferred to a second water
loop through a heat exchanger. The second
loop is kept at a lower pressure, allowing the water to boil and create
steam, which is used to turn the turbine-generator and produce electricity.
Afterward, the steam is condensed into water and returned to the heat exchanger.

In a BWR, water boils inside the reactor itself, and the steam goes
directly to the turbine-generator to produce electricity. Here, too, the
steam is condensed and reused.