Uranium 101

What is Uranium? How Does it Work?

  • 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 formed in a supernova 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 is 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%.


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 (4.5 billion years). This means that it is barely radioactive, less so than many other isotopes in rocks and sand. Nevertheless it generates 0.1 watts/ton 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.


World Nuclear Association, 2012. What Is Uranium? How Does It Work?