Hafnium, a rare, silvery, ductile, corrosion-resistant metal, is chemically similar to zirconium. Its neutron absorption properties make it useful for control rods in nuclear reactors, while hafnium carbide has the highest melting point of any binary compound. Its nuclear isomer, Hf-178-m2, has the potential to store and release huge amounts of energy, making it a potential component of gamma-ray bombs.
Hafnium is the 72nd element of the periodic table, chemically extremely similar to zirconium. Of all the elements, hafnium and zirconium are among the most difficult to distinguish, although the density of hafnium is about twice that of zirconium. Hafnium is a rare, silvery, ductile, corrosion-resistant metal that 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 absorption properties. Hafnium is used to make control rods for nuclear reactors. When a hafnium control rod is driven into a reactor, it absorbs stray neutrons released by nuclear-fueled reactions to uranium or plutonium, cooling the reactor. This is essential to keep the reactor under control and prevent meltdown. Because even minute impurities of zirconium can radically reduce the nuclear absorbency of hafnium, a difficult separation process is required 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 of any binary compound (3890°C, 7034°F). While 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 that contains excited protons and/or neutrons in its nucleus, placing it above the ground state. This gives it the potential to store and release huge amounts of energy, in the form of gamma rays. The hafnium isomer has more potential to store energy than any other isomer of similar lifetime. (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 the explosion of about 317 tons of TNT. For better or for worse, Hf-178-m2 is quite rare, and is commonly considered the most expensive substance in the world, costing millions per gram. DARPA and the Pentagon have looked into using the hafnium isomer to create gamma-ray bombs that circumvent nuclear treaties, with no known success yet.
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