Plutonium

Plutonium was the by-product of uranium. Like Uranium, it is a radioactive and dangerous element.

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 not only mined uranium but also to process it into plutonium. It would then be used to power the Nimons' transport capsules. Sebastian also gave King Edward and Queen Edith exquisitively crafted necklaces which were claimed to have been made of a brand new metal that was warm to touch. In reality, the metal was actually plutonium in order to kill the king and queen. Sebastian then planned to assault the castle with this dangerous element.

In 1055 Bishop Leofric of Exeter presented the King and Queen with exquisitely crafted necklaces, made of a new metal that was warm to the touch. The Doctor arrived shortly afterward, to Edward's pleasure... and Edith's chagrin. The Doctor's interest (and Edith's suspicions) made the two of them contrive a ruse to get Leofric to expose his plan: kill the King and Queen with necklaces made of plutonium, and assault the castle with tons of the stuff in an attempt to summon the Nimon to Earth. (BFA: Seasons of Fear)