This page contains information regarding Star Trek: Discovery, and thus may contain spoilers.
Yurium was a chemical compound with the chemical formula YU2. This compound was capable of storing large amounts of energy and was usually used as an explosive. A radioisotope byproduct, Jakmanite 632, was created artificially from the decay of high-energy yurium. (TNG: "Hollow Pursuits")
During his scheme to commandeer the USS Discovery using temporal loops, Harry Mudd destroyed the ship in one of the loops using an anicium-yurium reaction. (DIS: "Magic to Make the Sanest Man Go Mad")