Transuranic element

A transuranic element was any element with an atomic number higher than Uranium. These included Neptunium and Plutonium. (AUDIO: Power Play)

The First Doctor suggested that the people of Arkhaven may have mastered the use of stable transuranic elements, which would have allowed them to launch a spacecraft from the center of their city without worrying about the pollution it would cause. (PROSE: City at World's End)