Plutonium

Plutonium was a dangerously radioactive substance that was the by-product of processing uranium, another dangerous and radioactive element. (BFA: Seasons of Fear)

In 1002 CE, The First Doctor, with Susan, Ian and Barbara convinced Eric the Red to take the time travellers to America in order to obtain some plutonic ore to refuel the Tardis batteries. (DWM: Who Discovered America?)

On 20th Century Earth, a shady L.A.-based conglomerate known as InterCom aroused the suspicion of UNIT after being linked to illegal sales of plutonium. (PDA: The King of Terror)

Exxilon Fuel pods, which resembled opaque spheres, contained plutonium. (NA: The Left-Handed Hummingbird)

Sebastian Grayle, under the guise of "Leofric of Exeter" in 1055, was aided by the Nimon to mine uranium and to to process it into plutonium, which would then be used to power the Nimons' transport capsule. Sebastian also used this the metal to give to King Edward and Queen Edith in the form of an exquisitely crafted necklaces that was claimed to have been made from a brand new metal that was warm to touch, in an attempt to kill the king and queen. Sebastian then planned to assault the castle with this dangerous element. (BFA: Seasons of Fear)