Page 34 - Curriculum Visions Dynamic Book

Nuclear reactors
A nuclear reactor is a form of power unit used to generate electrical energy. It is flexible enough to be able to supply electricity for use in homes and factories, or to power submarines and warships.
Nuclear reactors make their energy from the process of fission (see page 28), using uranium and plutonium as fuels. The fission process is started when the fuel rods are bombarded with neutrons. Once the reaction is started, it develops a chain reaction that could cause a meltdown if not controlled. Many of the main features of a nuclear power plant are designed simply to control the chain reaction.
Types of reactors
Warship reactors
It has become common for nuclear power plants to be used on military vessels, especially submarines and aircraft carriers. This gives them exceptional range and removes the need for refuelling (new fuel rods have to be fitted only after more than a million km!).
In the United States fleet, some carriers such as the Enterprise have eight nuclear reactors. Most in the Nimitz class have two reactors.
These vast ships come with a
heavy price tag. The Theodore Roosevelt, for example, cost $17 billion after it had been fitted with aircraft and provided with escort vessels.
The main parts of a reactor are the same worldwide, although different cooling fluids are used in different designs. Commonly, water or gas is used as a coolant to transfer the heat energy from the reactor to the turbines of the generating station and to prevent the reactions from getting out of control.
At the heart of the reactor is the fuel, usually uranium oxide or plutonium. This is made into the form of about 200 three-metre-long fuel rods that can be lowered or raised from a surrounding fluid (water or carbon dioxide gas). Along with them may be rods made of steel alloys that absorb neutrons. Cadmium is commonly used as an alloying metal.
The control rods are raised or lowered among the fuel rods to speed up, slow down or stop the chain reaction altogether. Raising and lowering the absorbing rods is the same as stepping on or lifting your foot off the accelerator in a vehicle. In some types of reactor water is used to control the reaction as well as to carry away the heat.
The fuel rods and control rods may be packaged in a fuel cell about four metres across and four metres high. They have to be sealed inside a container that is designed to stand up to high pressures, high temperatures and explosions. It is normally made of very thick steel and may be twelve metres high. This steel vessel contains the cooling fluid that soaks up the heat from the reactor and transfers it to the turbines.
The cooling fluid becomes radioactive during use. It cannot therefore be fed directly to the turbines. Instead a set of pipes is wrapped around the cooling fluid pipes, and heat is exchanged. This fluid does not become radioactive and so is safe for use in
the generator.
All of the parts of the reactor and its cooling fluid are housed in a large concrete building designed to contain any explosion or other misadventure. This concrete building is normally shaped in the form of a large ball or a “bullet” standing on end. Thus, when you see a nuclear power plant, you can tell where the reactor is housed by looking for the round-shaped buildings.
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