Zirconium

Zirconium's neutron absorption cross-section is about 0.18 barns, roughly a thirtieth of iron's, so neutrons pass through it almost unimpeded. That single property made Zircaloy the cladding for uranium fuel rods in Hyman Rickover's naval reactors and in nearly every power reactor since. There is a catch that shaped the industry: hafnium always occurs with zirconium and absorbs neutrons six hundred times more strongly, so reactor-grade zirconium must be scrubbed of it — and the separated hafnium then goes into control rods, where absorption is the point. Above about 1200 °C zirconium strips oxygen from steam and liberates hydrogen, which is what exploded at Three Mile Island and Fukushima. Zircon crystals from the Jack Hills in Western Australia have been dated at 4.4 billion years, the oldest known terrestrial material.