Hafnium Bombs: WMDs of the Future?
Hafnium (Hf) is ordinarily a stable element that is used to make rods for nuclear reactors. In fact, it is through these spent rods that hafnium-178 isotope was discovered. The basic research for hafnium bombs was conducted in the 1970s and 1980s for possible applications for the Strategic Defense Initiative (a.k.a. "Star Wars").
Hafnium bombs, though, wouldn't be "bombs" in the typical sense. Instead of exploding violently, they would emit deadly gamma rays that could penetrate thorugh thick walls and deep bunkers.
Weaponizing hafnium-178 is highly controversial, as many scientists believe it's neither pratical nor possible. One major barrier is the price tag; hafnium-178 costs a cool $28 billion an ounce! However, if the power of hafnium-178 could be harnessed economically and safely, it would yield enormous peaceful benefits. One ounce of the isotope can boil 120 tons of water, giving some measure of its potential as an energy source.
Sources: Washington Post, Minding the Planet