Hafnium:

Because of its neuronic, mechanical, and physical properties, hafnium is an excellent control material for water-cooled, water-moderated reactors. It is searched together along with zirconium, and the  process  which  generates  pure  zirconium  generates  hafnium as  a  through-product. A Hafnium is resistant to corrosion through high-temperature water, has adequate mechanical strength, and could be readily fabricated. Hafnium contains of four isotopes, each of that has appreciable neutron absorption cross sections. The capture of neutrons through the isotope hafnium-177 leads to the formation of hafnium-178; the latter forms hafnium-179, which leads to hafnium-180. The first three have big resonance-capture cross sections, and hafnium-180 has a moderately huge cross section.  Therefore, the element hafnium in its natural form has a long, meaningful lifetime as a neutron absorber. Since of the limited availability and high cost of hafnium then its use as a control material in civilian power reactors has been restricted.