Hafnium 178m2, Hf, atomic number 72. This nuclear isomer has the potential to unleash enormous quantities of gamma radiation, making it an excellent ingredient for use in devastating weapons of mass destruction. As a result Hafnium 178 is a highly controlled substance that is universally illegal to own without special dispensation.
— In-Game Description
It can't be found on markets. The only way to find it is by attacking Federal and Imperial convoys that may hold them as cargo. These convoys usually carry only one ton of hafnium and are heavily protected.