Uranium (U) is a radioactive chemical element with atomic number 92. It is named after the planet Uranus. More than 99% of the uranium found in nature is of the 238U isotope, with 92 protons and 146 neutrons.
Uranium is probably the element that is most associated with nuclear fission, the energy-releasing atom-splitting chain reaction that underpins both nuclear power plants and nuclear weapons. Uranium-235, rather than uranium-238, is needed for fission to occur, so natural uranium must be "enriched" before it's useful, meaning the proportion of the uranium-235 isotope must be increased.
Uranium was used in the Manhattan Project and the Hiroshima bomb in 1945, and also in the first nuclear power plant, the Obninsk Nuclear Power Plant outside Moscow, in 1954.