Uranium, periodic table U, is a metal. Its most common isotope is 238U with 146 neutrons and an abundance of 99% of all Uranium. It is unstable and slightly radioactive, with a half life of 4.468×109 years (4.5 billion hears). The most important commercial isotope of uranium is 235U, that is more radioactive with a half life of 7.04×108 years. 235U represents 0,72% of all Uranium.
Uranium used in nuclear reactors is usually enriched in 235U to increase its radioactivity to the point it can sustain a nuclear chain reaction. Commercial nuclear power plants use fuel that is typically enriched to around 3% 235U.
Uranium's average concentration in the Earth's crust is (depending on the reference) 2 to 4 parts per million, or about 40 times as abundant as silver. If Uranium can be mined and enriched on Mars, it would represent an excellent energy source for a Martian settlement. One kilogram of uranium-235 can theoretically produce about 20 terajoules of energy (2×1013 joules), assuming complete fission; as much energy as 1.5 million kilograms (1,500 tonnes) of coal. Breeder reactors can convert 238U into Plutonium, than can in turn produce energy through added fission reactions, increasing the overall burn rate (or Burnup) of the nuclear fuel.