Measuring only a few hundred millionths of a centimetre across, the atom consists mainly of empty space. In the centre, or nucleus, are very tiny particles called protons and neutrons. The protons are electrically positively charged and the neutrons have no charge. Around the nucleus revolve even smaller particles called electrons, which have a negative charge. Normally an atom would have an equal number of protons and electrons, thereby cancelling each other out. If an unstable element, such as uranium 235 is bombarded with neutrons, eventually the nucleus will capture an extra neutron, become more unstable and then split. This releases massive amounts of energy, plus more neutrons. These can go on to split more atoms, and so causing a chain reaction. As well as being the basis for the atom bomb, it is also the basis for all nuclear power generated to date.
In 1956 the world's first nuclear power station opened in Great Britain, at Calder Hall, Cumberland. Although there are many different types of nuclear reactor, almost all operate on similar principles to Calder Hall. The fission takes place in the core of the reactor, this contains the fuel uranium. A moderator, (usually consisting of boron or steel rods which absorb neutrons) slows down the neutrons to enable the fission to take place more easily. A coolant circulates in through the core, absorbing the heat, and with it some radioactivity. It then circulates through a heat exchanger where it heats water which turns to steam. This steam then drives a turbine, which in turn drives a generator in the same way as a conventional steam turbine would. These reactors are widely used, although some differ in the fuel, moderator and coolant used. The advanced gas cooled reactor, or AGR, is very similar in operation but uses enriched uranium dioxide as a fuel and operates at high pressure (40 atmospheres) and high temperature (650 degrees C). Also similar is the Magnox Reactor (see video) which uses natural uranium for fuel and carbon dioxide as a coolant. The moderator is usually made of graphite. Others use water as a coolant. This can be either ordinary water, as in the PWR, [pressurised water reactor] and BWR (boiling water reactor) or heavy water, as in the case of the HWR (heavy water reactor) which is formed from a hydrogen isotope called deuterium. There is one type of reactor which differs substantially from the rest, called the Fast Breeder Reactor. This has no moderator, and therefore uses fast neutrons. As well as using uranium 235 they convert uranium 238, which is non fissionable, into fissionable plutonium. In effect this reactor can breed its own fuel, however, these are in their relative infancy and are not yet widely used.
For mainly financial reasons the number of nuclear power stations up and running is well short of what was predicted in the 1950's. Although fuel costs are extremely low, (one tonne of uranium can produce as much energy as 20,000 tonnes of coal and the fast breeder produces more fuel than it uses), they are much more expensive to build and maintain than conventional power stations. Also in many countries public opinion has been against their development. Many are not yet convinced of the safety of nuclear reactors, although safety is the number one consideration at every stage, from design, through construction to eventual operation. (Walls of the pressure vessel can be up to two metres thick, made of concrete lined with steel.)
Under intense research at present is the theory of nuclear fusion. Whereas fission is based on splitting atoms, fusion joins them together. This is the principle behind the hydrogen bomb. In the centre of the sun, the massive temperatures (about 20 million degrees C) fuse atoms together, producing the sun's energy. If these temperatures could be produced, sustained and controlled, we could have unlimited energy from water, with no pollution!
