Uranium-235
From RationalWiki
Uranium-235 is the most awesome of natural nuclides. It is the only naturally occurring fissile material. Although it makes up only 0.7% of natural uranium, it can be used to unlock the vast amounts of energy available from uranium-238 and thorium via a technology called breeder reactors. The half life of uranium-235 is 700 million years. It is slightly[citation needed] more radioactive than uranium-238.
Fission of uranium-235 is the source of around 15% of the world's production of electricity, and around 6% of total energy production.