![]() The heat is used to make steam to produce electricity.Įxamples of nuclear fissioning of uranium-235 Inside the reactor It is this process, in effect 'burning' uranium, which occurs in a nuclear reactor. 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. If enough of these expelled neutrons cause the nuclei of other U-235 atoms to split, releasing further neutrons, a fission 'chain reaction' can be achieved. 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, also two or three additional neutrons are thrown off. The nucleus of the U-235 atom comprises 92 protons and 143 neutrons (92 + 143 = 235). Nevertheless it generates 0.1 watts/tonne as decay heat and this is enough to warm the Earth's core. This means that it is barely radioactive, less so than many other isotopes in rocks and sand. U-238 decays very slowly, its half-life being about the same as the age of the Earth (4500 million years). Meanwhile, like all radioactive isotopes, they decay. It is therefore said to be 'fissile' and we use the expression 'nuclear fission'. The isotope U-235 is important because under certain conditions it can readily be split, yielding a lot of energy. 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%. These isotopes differ from each other in the number of uncharged particles (neutrons) in the nucleus. Like other elements, uranium occurs in several slightly differing forms known as 'isotopes'. On a scale arranged according to the increasing mass of their nuclei, uranium is one of the heaviest of all the naturally-occurring elements (hydrogen is the lightest). ![]() 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.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. Uranium was apparently formed in supernovae about 6.6 billion years ago.It was named after the planet Uranus, which had been discovered eight years earlier. Uranium was discovered in 1789 by Martin Klaproth, a German chemist, in the mineral called pitchblende.Uranium occurs in seawater, and can be recovered from the oceans. Uranium occurs in most rocks in concentrations of 2 to 4 parts per million and is as common in the Earth's crust as tin, tungsten and molybdenum.Uranium is a heavy metal which has been used as an abundant source of concentrated energy for over 60 years.This is why fusion is still in the research and development phase – and fission is already making electricity. The reasons that have made fusion so difficult to achieve to date are the same ones that make it safe: it is a finely balanced reaction which is very sensitive to the conditions – the reaction will die if the plasma is too cold or too hot, or if there is too much fuel or not enough, or too many contaminants, or if the magnetic fields are not set up just right to control the turbulence of the hot plasma. Unlike nuclear fission, the nuclear fusion reaction in a tokamak is an inherently safe reaction. In conventional nuclear power stations today, there are systems in place to moderate the chain reactions to prevent accident scenarios and stringent security measures to deal with proliferation issues. This chain reaction is the key to fission reactions, but it can lead to a runaway process resulting in nuclear accidents. ![]() The result of the instability is the nucleus breaking up, in any one of many different ways, and producing more neutrons, which in turn hit more uranium atoms and make them unstable and so on. It is triggered by uranium absorbing a neutron, which renders the nucleus unstable. ![]() Fission and chain reactionsįission is the nuclear process that is currently run in nuclear power plants. Both reactions release energy which, in a power plant, would be used to boil water to drive a steam generator, thus producing electricity. However, fusion is combining light atoms, for example two hydrogen isotopes, deuterium and tritium, to form the heavier helium. In fission, energy is gained by splitting heavy atoms, for example uranium, into smaller atoms such as iodine, caesium, strontium, xenon and barium, to name just a few. ![]()
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