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Over 16% of the world's electricity is produced from nuclear energy, more than from all sources worldwide in 1960.
A nuclear reactor produces and controls the release of energy from splitting the atoms of elements such as uranium and plutonium. In a nuclear power reactor, the energy released from continuous fission of the atoms in the fuel as heat is used to make steam. The steam is used to drive the turbines which produce electricity (as in most fossil fuel plants, but without the combustion of fossil fuels and resultant greenhouse gas emissions).
There are several components common to most types of reactors:
Fuel. Usually pellets of uranium oxide (UO2) arranged in tubes to form fuel rods. The rods are arranged into fuel assemblies in the reactor core.
Moderator. This is material which slows down the neutrons released from fission so that they cause more fission. It is usually water, but may be heavy water or graphite.
Control rods. These are made with neutron-absorbing material such as cadmium, hafnium or boron, and are inserted or withdrawn from the core to control the rate of reaction, or to halt it. (Secondary shutdown systems involve adding other neutron absorbers, usually in the primary cooling system.)
Coolant. A liquid or gas circulating through the core so as to transfer the heat from it. In light water reactors the moderator functions also as coolant.
Pressure vessel or pressure tubes. Usually a robust steel vessel containing the reactor core and moderator/coolant, but it may be a series of tubes holding the fuel and conveying the coolant through the moderator.
Steam generator. Part of the cooling system where the heat from the reactor is used to make steam for the turbine.
Containment. The structure around the reactor core which is designed to protect it from outside intrusion and to protect those outside from the effects of radiation in case of any major malfunction inside. It is typically a metre-thick concrete and steel structure.
Most reactors need to be shut down for refueling, so that the pressure vessel can be opened up. In this case refueling is at intervals of 1-2 years, when a quarter to a third of the fuel assemblies are replaced with fresh ones. The CANDU and RBMK types have pressure tubes (rather than a pressure vessel enclosing the reactor core) and can be refueled while still generating electricity by disconnecting individual pressure tubes.
If graphite or heavy water is used as moderator, it is possible to run a power reactor on natural instead of enriched uranium. Natural uranium has the same elemental composition as when it was mined (0.7% U-235, over 99.2% U-238), enriched uranium has had the proportion of the fissile isotope (U-235) increased by a process called enrichment, commonly to 3.5 - 5.0%. In this case the moderator can be ordinary water, and such reactors are collectively called light water reactors. Because the light water absorbs neutrons as well as slowing them, it is less efficient as a moderator than heavy water or graphite.
Practically all fuel is ceramic uranium oxide (UO2 with a melting point of 2800°C) and most is enriched. The fuel pellets (usually about 1 cm diameter and 1.5 cm long) are typically arranged in a long zirconium alloy (zircaloy) tube to form a fuel rod, the zirconium being hard, corrosion-resistant and permeable to neutrons. Up to 264 rods form a fuel assembly, which is an open lattice and can be lifted into and out of the reactor core. In the most common reactors these are about 3.5-4.0 metres long.
Further Reading:World Nuclear Power Reactors 2005-06 and Uranium RequirementsNuclear Power in the World TodayNuclear share figures, 1995-2005Nuclear Power ReactorsPlans For New Reactors WorldwideSafety of Nuclear Power ReactorsNuclear Power Plants and EarthquakesAdvanced Nuclear Power ReactorsGeneration IV Nuclear ReactorsSmall Nuclear Power ReactorsNuclear Reactors for Space
Sources:Wilson, P.D., 1996, The Nuclear Fuel Cycle, OUP.