Operating A Nuclear Power Station
How the Energy in Uranium Atoms is Converted into Electricity
Sep 2, 2008
If a neutron strikes the nuclei of some atoms, they split into fragments, releasing some of the energy. Each time such an atom is split, more neutrons are produced, which move on to split further atoms, so a chain reaction occurs.
Generating Electricity
In an electric power station, a fuel is burned to produce heat, which boils water. The steam turns turbines, which are connected to wire coils. As these coils revolve between the poles of a magnet, an electric current is generated in the wires and fed into the national grid. In a fossil fuel station, the heat is produced by the burning of coal, oil or gas. A nuclear power station, on the other hand, uses the energy released by splitting atomic nuclei, usually of uranium, but sometimes plutonium.
Uranium
Uranium atoms are the heaviest of any natural element. A sample of uranium, however, contains different types of atom, known as isotopes, the most important being uranium-235 (U-235) and the heavier U-238. The nucleus of a U-235 atom contains 92 protons and 143 neutrons. That of U-238 has 92 protons and 146 neutrons.
Atoms of U-235 can be split to produce energy. U-238 atoms do not split, but capture neutrons and change into plutonium, an element that does not occur naturally, but can also act as a nuclear fuel.
The earliest nuclear power stations used natural uranium as their fuel. This contains less than 1% U-235. The more modern reactors use enriched uranium, in which the U-235 component has been increased to around 3%. An atomic bomb, by comparison, is made using almost 100% U-235.
The Nuclear Reactor
The fuel is made into rods or pellets before being encased in cans made of steel, or an alloy of magnesium or zirconium, and assembled in the reactor. The fuel rods are surrounded by a moderator made of graphite, or sometimes water, which slows down the neutrons produced during the reaction. A single reactor may contain more than 100 tonnes of fuel, which can generate as much energy as two million tonnes of coal.
Controlling the Nuclear Reaction
Because the number of neutrons produced when each atom is split is two or more, the neutron flux can build up so rapidly that the reaction could run out of control. The temperature would rise sufficiently high to melt the uranium. To prevent this, control rods, made of boron steel, are raised or lowered to absorb some of the neutrons and so maintain the level within certain boundaries. In the event of an accident, or malfunction, the control rods can be dropped into the core to shut down the reactor very quickly. The rods can also be withdrawn rapidly to increase energy production during a sudden surge in demand for electricity. A nuclear power station can respond to fluctuations in demand much more readily than other power stations.
Circulating around the reactor core is a coolant gas, usually carbon dioxide, which absorbs the heat from the reaction. The gas temperature may rise to anything between 300 and 600 degrees Centigrade, depending on the reactor. The gas is then passed around pipes containing water, which quickly boils, the steam being used to turn the turbines. The cooled gas is re-circulated through the reactor.
Although the reaction that occurs in a nuclear pile is exactly the same as that in an atomic weapon, the concentration of U-235 in the former is so low that a nuclear explosion cannot occur. It was a chemical explosion, not a nuclear one, which caused the accident at Chernobyl, in 1986.
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