Uranium

Uranium was a dangerously radioactive element that could be used in nuclear power plants as well as in deadly and devastating weapons. Uranium, when processed, would convert to plutonium.

This material was found on the the planet Desrault, where the Sontarans were negotiating with the Time Lords and the Order of the Black Sun for the element. (DWM: Black Sun Rising)

Sebastian Grayle, under the guise of "Leofric of Exeter" and with the aid of the Nimon, mined uranium and converted it to plutonium in order to power the transport capsules for the Nimon. The Nimon taught Sebastian where and how to mine the uranium. (BFA: Seasons of Fear)