Zirconium, like titanium and its sister metal hafnium, is classified as a reactive metal (a metal that readily combines with oxygen at elevated temperatures to form very stable oxides). Zirconium is similar in many respects chemically and mechanically to titanium, but it has both a higher density and melting temperature (6.51 g/cm3 and 1852° C, or 3365° F, as compared to titanium at 4.51 g/cm3 and 1668° C, or 3034° F). Zirconium has excellent resistance to many corrosive media, including superheated water, and it is almost transparent to thermal energy neutrons. Today, a high proportion of zirconium is used in water-cooled nuclear reactors; the next largest use is in chemical-processing equipment. See also Titanium and Titanium Alloys.