Cirenium was a mineral. Inside the Dalek Space Station the Chief Dalek sent half-Time Lord Shazar to find the cirenium. They believed the mineral could fuel TARDIS's time circuits. But the cirenium could destroy a TARDIS.
When the space station was burning after the coolant systems were damaged, the Daleks installed cirenium fuel rods to their fleet of TARDISes. However, the minerals caused the ships to explode, destroying the Daleks inside. (COMIC: Return of the Daleks)