A radioactive element found in the actinide series on the periodic table. It has the chemical symbol of U, standard atomic weight of 238. 02891 and 92 protons. This element is useful producing energy needed to power cities and regions of countries. The fissionable isotope of uranium is uranium-235. Scientist discovered that this isotope could be induced to release energy when a thermal (slow) neutron strikes this isotope's nucleus producing smaller mass fragments, additional neutrons, and energy that stemmed from mass from those reactants converted to energy, the so-called mass defect. Nuclear plants were built to house this process beginning in the early 1950s. This process of splitting atoms is called fission.