Introduction:

Nuclear energy is a comparatively new source of energy. The first nuclear power plant was commissioned in June 1954 in Obninsk, Russia. Fossil fuels offer a limited source of energy, as they are non-renewable. Eventually these supplies will cease, this is predicted to be in the next few decades. An estimate based on fuel consumption in America, predicts as early as 2020 there will be no fossil fuels left.
The energy used by the whole world is approximated to be the coal equivalent to 2790 Gigatons per year. Fossil fuels reserves total for the world in 1980 had approximately 8685 Gigatons of coal and 91.2 Gigatons of oil. This is why extensive research has gone into looking for new sources of energy to keep things powered.
Energy sources currently being used are hydroelectricity, wind turbines, solar power, fossil fuels and nuclear power, and now also hydrogen fuel cells. There is much controversy over the health and safety issues of using nuclear power, especially after Three Mile Island and the Chernobyl disasters.

Where does the energy come from?
Nuclear fission
The nucleus is the centre of the atom which is normally made up of the same number of protons as it has neutrons. However, some very large nuclei in certain isotopes have an imbalance. They can often be found with too many neutrons, and this imbalance will result in the nucleus becoming unstable.
Uranium-235 is a radioactive substance which due to its large size and unstable state can undergo induced fission. Its nucleus can be split into smaller atoms when induced by a neutron. This process will release two or three neutrons, depending on how the atom splits. These new neutrons can then initiate the decomposition of the nuclei of other atoms of Uranium. Propagation by the chain reaction releases more neutrons and causes further nuclear splits.
Under controlled conditions, the rate of this chain reaction can be kept at a constant rate. This produces high temperatures but is not allowed to react out of control as in a nuclear bomb. The heat produced is used to turn water into steam, the steam then turns a turbine and generator, creating electricity.

Nuclear Reactors
In a reactor the uranium source required is 3-4% Uranium-235. Therefore it is necessary to enrich natural Uranium to use for nuclear power. This is done by converting uranium oxide extracted from ore into gaseous form, uranium hexafluoride. From this form it can be enriched from its natural proportion of 0.7% uranium-235 to 3-4%, this is done by separation of isotopes. A higher enrichment means better efficiency, and ordinary water can then be used as a moderator.
The form of uranium usually used is pellet form, these are arranged into rods and then to bundles. These bundles are surrounded by a moderator such as water, graphite or heavy water. The moderator slows down the emitted neutrons by reducing their energy as they collide with the nuclei of the moderator. Control rods are placed in the bundles which control the rate of the nuclear reaction. These can also be used to shut down the reactor completely when something goes wrong.
These control rods are materials which absorb neutrons, such as Cadmium and Boron. They work by reducing the number of neutrons in the reactor and therefore slowing down the reaction and consequently reducing the heat. To reduce heat, the rods are put further into the bundles where they absorb more neutrons. To raise the heat the opposite is done, and the heat level rises.
As the atoms are split the energy is released as heat. This is used to heat water and turns it into steam. The steam drives a steam turbine, which spins a generator to produce electricity. This is what happens in a basic reactor, others include the use of intermediate heat exchangers or gaseous coolant fluid. The set up of a nuclear power plant is basically the same as that of a coal power plant. The main difference is how the water is heated to produce steam, from then on