Hafnium is the 72nd element in the periodic table, chemically extremely similar to zirconium. Out of all the elements, hafnium and zirconium are among the most difficult to tell apart, although the density of hafnium is about twice that of zirconium. Hafnium is a rare, silvery, ductile, corrosion-resistant metal, which makes up only 0.00058% of the Earth's upper crust by weight.
Hafnium is well-known among chemists and physicists for several reasons. One is because of its neutron-absorbing properties. Hafnium is used to make the control rods for nuclear reactors. When a hafnium control rod is pushed into a reactor, it absorbs stray neutrons released by uranium or plutonium-fueled nuclear reactions, cooling the reactor down. This is essential to keep the reactor under control and prevent meltdown. Because even tiny impurities of zirconium can radically lower hafnium's nuclear-absorbing capacity, a difficult separation process is necessary to produce hafnium of the purity required to serve as a control rod.
Another reason for hafnium's fame is an alloy it can produce, hafnium carbide (HfC), which has the highest melting point out of any binary compound (3890 °C, 7034 °F). Although it has not been used extensively in construction or aerospace, it has been suggested as a building material for structures exposed to intense heat.
Perhaps the most exotic properties associated with hafnium are those associated with its nuclear isomer, Hf-178-m2. A nuclear isomer is a special version of an element which contains excited protons and/or neutrons in its nucleus, putting it above the ground state. This gives it the potential to store and release tremendous amounts of energy, in the form of gamma rays. Hafnium isomer has more potential to store energy than any other similarly long-lived isomer. (Most isomers decay in a fraction of a second.) One kilogram of pure Hf-178-m2 would have a calculated energy of 1330 gigajoules, the equivalent of exploding about 317 metric tons of TNT. For better or for worse, Hf-178-m2 is quite rare, and it commonly thought to the most expensive substance in the world, costing millions per gram. DARPA and the Pentagon have looked into the use of hafnium isomer to create gamma ray bombs that circumvent nuclear treaties, with no known success as of yet.