uranium - a heavy toxic silvery-white radioactive metallic element; occurs in many isotopes; used for nuclear fuels and nuclear weaponsmetal, metallic element - any of several chemical elements that are usually shiny solids that conduct heat or electricity and can be formed into sheets etc. uranium 235 - a uranium isotope with mass number 235; capable of sustaining chain reactions uranium 238 - the commonest isotope of uranium; it is not fissionable but when irradiated with neutrons it produces fissionable plutonium 239 pitchblende, uraninite - a mineral consisting of uranium oxide and trace amounts of radium and thorium and polonium and lead and helium; uraninite in massive form is called pitchblende which is the chief uranium ore |