The chemical symbol of uranium is U; its atomic number (number of protons in its nucleus) is 92. Its average atomic mass (protons plus neutrons) is 238, and is generally written uranium-238. An element is identified by its atomic number, which never changes. The atomic mass of some elements, however, may not be constant due to variations in the number of its neutrons. Atoms possessing such characteristics are called isotopes. The major isotopes of natural uranium are uranium-238 (99.3%) and uranium-235, the most active of the two, at 0.7%.
Uranium is a silvery-white element of the actinide series, roughly 20% denser than lead and is the only naturally occurring fissile element on earth. It occurs in many minerals and is used chiefly as a source of nuclear energy by fission of the radioisotope uranium – 235.
Uranium is found in many areas of the earth’s crust. Uranium is more common than tin, about 40 times more common than silver and 500 times more common than gold. It is found in most rocks and sediments, seawater, aquifers, and hot springs. Normally the amount of uranium in a given area is very small, but where certain geological conditions exist, uranium can be more concentrated and may be economically recovered.
Some isotopes such as those of uranium are unstable and release atomic particles as they disintegrate into less complex forms. This process is called radioactivity. When neutrons from the atomic particles strike other uranium-235 atoms, each atom splits apart, releasing more neutrons and heat. This activity, called nuclear fission, is the force that drives all present nuclear energy. When there is sufficient concentration of uranium-235 in the mix, the fission process can sustain itself, producing a chain reaction, and releasing tremendous amounts of energy.
While undergoing fission, the enriched fuel becomes increasingly contaminated with by-products from the activity, reducing the efficiency of the reaction process. After some period of time, this so-called ‘spent’ fuel must be replaced with fresh, enriched material. Nuclear electrical generation facilities typically require new fuel about every four years.
Natural uranium does not contain enough uranium-235 to produce fission. Nuclear power plant fuel therefore, must be enriched from its natural 0.7% to about 4.0%. Because of the low levels of enrichment, this fuel cannot explode like an atom bomb (weapons grade enrichment approaches 100%. Nuclear fuel, the most common use of uranium, is an astonishingly efficient source of clean energy for generating electricity throughout the world. Through fission, one atom of uranium – 235 can release 50 million times more energy - about 200 million electron volts – compared to the combustion of a single carbon atom – about 4 electron volts. As shown in the chart below, nuclear energy, fueled with enriched uranium, and hydroelectric energy are base load, low carbon options to provide electricity.