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Uranium is a very heavy metal which can be used as an abundant source of concentrated energy. It is common in the Earth's crust and is found in economic concentrations in a wide variety of geological settings.
Unlike fossil fuels which are combined with oxygen in burning to release a lot of heat, uranium atoms can be made to split, or fission, to release heat. A kilogram of natural uranium produces as much heat as 20 tonnes of coal. This is harnessed to make steam and generate power.
It was discovered in 1789 by Martin Klaproth, a German chemist, in the mineral called pitchblende. It was named after the planet Uranus, which had been discovered eight years earlier.
Uranium was formed in super novae about 6.6 billion years ago. While it is not common in the solar system, today its slow radioactive decay provides the main source of heat inside the Earth, causing convection and continental drift.
The high density of uranium means that it also finds uses in the keels of yachts and as counterweights for aircraft control surfaces, as well as for radiation shielding.
The Uranium Atom
Uranium is the heaviest of all the naturally-occurring elements (hydrogen is the lightest). Uranium is 18.7 times as dense as water.
Like other elements, uranium occurs in several slightly differing forms known as 'isotopes'. These isotopes differ from each other in the number of particles (neutrons) in the nucleus. Natural uranium as found in the Earth's crust is a mixture largely of two isotopes: uranium-238 (U-238), accounting for 99.3% and uranium-235 (U-235) about 0.7%.
The isotope U-235 is important because under certain conditions it can readily be split, yielding a lot of energy. It is therefore said to be 'fissile' and we use the expression 'nuclear fission'.
Like all radioactive isotopes, uranium atoms decay. U-238 decays very slowly, its half-life being about the same as the age of the Earth (4500 million years). This means that it is barely radioactive, less so than many other isotopes in rocks and sand. Nevertheless it generates 0.1 watts/tonne as decay heat and this is enough to warm the Earth's core. U-235 decays slightly faster.
Energy from the uranium atom
When the nucleus of a U-235 atom captures a moving neutron it splits in two (fissions) and releases some energy in the form of heat. During the fission two or three neutrons are thrown off. If enough of these expelled neutrons are captured by the nuclei of other U-235 atoms to split, releasing further neutrons, a fission 'chain reaction' occurs. When this happens over and over again, many millions of times, a very large amount of heat is produced from a relatively small amount of uranium.
Inside the reactor
In a nuclear reactor the uranium fuel is assembled in such a way that a controlled fission chain reaction can be achieved.
Nuclear power stations and fossil-fuelled power stations of similar capacity have many features in common. Both use heat to produce steam to drive turbines and generators. However, the fissioning of uranium atoms replaces the burning of coal or gas. Burning coal or gas oxidises carbon to produce carbon dioxide. This is not produced in the fissioning of uranium.
The chain reaction that takes place in the core of a nuclear reactor is controlled by rods which absorb neutrons and which can be inserted or withdrawn to set the reactor at the required power level.
The fuel elements are surrounded by a substance called a moderator to slow the speed of the emitted neutrons and thus enable the chain reaction to continue. Water, graphite and heavy water are used as moderators in different types of reactors.
Uranium and Plutonium
Whereas the U-235 nucleus is 'fissile', that of U-238 is said to be 'fertile'. This means that it can capture one of the neutrons which are flying about in the core of the reactor and become (indirectly) plutonium-239, which is fissile. Pu-239 is very much like U-235, in that it fissions when hit by a neutron and this also yields a lot of energy.
Because there is so much U-238 in a reactor core (most of the fuel), these reactions occur frequently, and in fact about one third of the fuel's energy yield comes from "burning" Pu-239.