Hafnium is a chemical element with the symbol Hf and atomic number 72. It is a lustrous, silvery-gray transition metal commonly used in nuclear reactors and as an alloy with titanium. Hafnium is highly resistant to corrosion and has a very high melting point, making it a great material for high-temperature applications. Hafnium is also used in semiconductor manufacturing and as a neutron absorber in nuclear power plants.