How is nuclear power generated?
Nuclear power plants generate electricity in much the same way other thermal powered plants do:
- the fuel creates heat
- the heat creates steam
- the steam turns a turbine
- the turbine turns a generator
- the generator makes electricity
How does the fuel create heat?
Uranium fuel creates heat through nuclear fission. Uranium is a relatively common element, occurring in most rocks in very small concentrations. Uranium has 16 isotopes, or forms of the same element containing the same number of protons but different numbers of neutrons, of which uranium-238 (U-238) makes up approximately 99.3 per cent of naturally occurring uranium and U-235 makes up about 0.7 per cent.
U-238 is slightly radioactive, with a half-life of 4.5 billion years. U-235 is less stable and can be easily split, consequently, it is the primary fuel used to generate heat in a nuclear power plant. It consists of 92 protons and 143 neutrons. U-235 decays naturally through alpha radiation (emitting helium ions) and spontaneous fission. However, U-235 can also undergo induced fission. This occurs when a free neutron collides with a U-235 nucleus; the nucleus will absorb the neutron, which causes the nucleus to become unstable, which causes it to split (fission) and release further neutrons. The time from the absorption of the neutron until the splitting of the nucleus is on the order of 10-12 seconds. The splitting action releases energy in the form of heat. If the released neutrons go on to collide with other U-235 nuclei, causing more fission and the release of additional neutrons, a chain reaction occurs, resulting in the generation of a large amount of heat.
How is the heat energy converted into electricity?
Nuclear reactors control the chain reaction and harness the heat. Two main types of nuclear reactor are used to generate electricity:
- heavy water reactors
- light water reactors
Heavy water reactors, such as the CANDU reactor, use "heavy" water as coolant and moderator. This is necessary because heavy water reactors use natural uranium for fuel, uranium that emits fewer neutrons than the enriched variety. Natural uranium contains about 0.7 per cent uranium-235.
Heavy water is water enriched with deuterium oxide. It does not absorb as many neutrons as light water. Heavy water also slows down neutrons far more than does light water. This means that neutrons are retained much longer within the reactor before being absorbed and hence are more likely to collide with a U-235 nucleus, producing fission.
Light water reactors use purified ordinary water for both moderator and coolant. The fuel is enriched uranium, containing approximately three per cent uranium-235.
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