What is Uranium? How Does it Work - World Nuclear Association
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What is Uranium? How Does it Work?
(Updated January 2017)
Uranium is a very heavy metal which has been used as an abundant source of concentrated energy for 60 years. 
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 occurs in seawater, and can be recovered from the oceans. 
Uranium 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 apparently formed in supernovas 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.
Uranium has a melting point of 1132°C. The chemical symbol for uranium is U.
The Uranium Atom
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). 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 uncharged 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'.
Meanwhile, like all radioactive isotopes, they 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
The nucleus of the U-235 atom comprises 92 protons and 143 neutrons (92 + 143 = 235). 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. 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 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.
It is  this process , in effect "burning" uranium, which occurs in a nuclear reactor. The heat is used to make steam to produce electricity.
Inside the reactor
Nuclear power stations and fossil-fuelled power stations of similar capacity have many features in common. Both require heat to produce steam to drive turbines and generators. In a nuclear power station, however, the fissioning of uranium atoms replaces the burning of coal or gas. In a nuclear reactor the uranium fuel is assembled in such a way that a controlled fission chain reaction can be achieved. The heat created by splitting the U-235 atoms is then used to make steam which spins a turbine to drive a generator, producing electricity.
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.
Because of the kind of fuel used (ie the concentration of U-235, see below), if there is a major uncorrected malfunction in a reactor the fuel may overheat and melt, but it cannot explode like a bomb.
A typical 1000 megawatt (MWe) reactor can provide enough electricity for a modern city of up to one million people.
 
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 yields a similar amount 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.
But sometimes a Pu-239 atom simply captures a neutron without splitting, and it becomes Pu-240. Because the Pu-239 is either progressively "burned" or becomes Pu-240, the longer the fuel stays in the reactor the more Pu-240 is in it. (The significance of this is that when the spent fuel is removed after about three years, the plutonium in it is not suitable for making weapons but can be recycled as fuel.)
From uranium ore to reactor fuel 
Uranium ore can be mined by underground or open-cut methods, depending on its depth. After mining, the ore is crushed and ground up. Then it is treated with acid to dissolve the uranium, which is recovered from solution. 
Uranium may also be mined by in situ leaching (ISL), where it is dissolved from a porous underground ore body in situ and pumped to the surface.
The end product of the mining and milling stages, or of ISL, is uranium oxide concentrate (U3O8). This is the form in which uranium is sold.
Before it can be used in a reactor for electricity generation, however, it must undergo a series of processes to produce a useable fuel.
For most of the world's reactors, the next step in making the fuel is to convert the uranium oxide into a gas, uranium hexafluoride (UF6), which enables it to be enriched. Enrichment increases the proportion of the uranium-235 isotope from its natural level of 0.7% to 4 - 5%. This enables greater technical efficiency in reactor design and operation, particularly in larger reactors, and allows the use of ordinary water as a moderator.
After enrichment, the UF6 gas is converted to uranium dioxide (UO2) which is formed into fuel pellets. These fuel pellets are placed inside thin metal tubes, then known as fuel rods, which are assembled in bundles to become the fuel elements or assemblies for the core of the reactor. In a typical large power reactor there might be 51,000 fuel rods with over 18 million pellets.
A worker holds up a newly made fuel pellet (KazAtomProm)
For reactors which use natural uranium as their fuel (and hence which require graphite or heavy water as a moderator) the U3O8 concentrate simply needs to be refined and converted directly to uranium dioxide.
When the uranium fuel has been in the reactor for about three years, the used fuel is removed, stored, and then either reprocessed or disposed of underground (see  Nuclear Fuel Cycle  or  Radioactive Waste Management  in this series).
Who uses nuclear power?
About 11% of the world's electricity is generated from uranium in nuclear reactors. This amounts to over 2500 billion kWh each year, as much as from all sources of electricity worldwide in 1960.
It comes from over 440 nuclear reactors with a total output capacity of about 390,000 megawatts (MWe) operating in 31 countries. Over 60 more reactors are under construction and another 160 are planned.
Belgium, Bulgaria, Czech Republic, Finland, France, Hungary, South Korea, Slovakia, Slovenia, Sweden, Switzerland and Ukraine all get 30% or more of their electricity from nuclear reactors. The USA has about one hundred reactors operating, supplying 20% of its electricity. France gets three quarters of its electricity from uranium.
Over the 60 years that the world has enjoyed the benefits of cleanly-generated electricity from nuclear power, there have been 17,000 reactor-years of operational experience.