Zirconium (Zr)

Neutrons pass through zirconium as though it were barely there. Its thermal neutron absorption cross-section is about 0.18 barns, which is very low for a structural metal, and it combines that with something no other cheap material offers: it does not corrode in 300 °C pressurised water. A reactor needs a tube that holds uranium pellets, contains fission products, survives years in hot water under pressure, and gets out of the neutrons' way. Zirconium is the only affordable material that does all four, and essentially every water-cooled power reactor in the world is built out of it.

The twin that eats neutrons

There is a catch, and it very nearly stopped the nuclear industry before it started. Zirconium ore always contains hafnium, typically one to three per cent of the zirconium content, and the two elements are chemically almost indistinguishable. The lanthanide contraction leaves hafnium's ion essentially the same size as zirconium's, so they share the same charge, the same radius, the same coordination chemistry and the same behaviour in nearly every separation anyone had devised.

Their nuclear behaviour could hardly be more different. Hafnium's thermal neutron cross-section is around 600 times zirconium's — hafnium is used deliberately in control rods precisely because it swallows neutrons. Zirconium containing its natural hafnium is useless as cladding.

Separating them at industrial scale was one of the harder chemical engineering problems of the 1940s, and it was solved under pressure from Hyman Rickover's naval reactor programme. The classic route pushes the two through liquid–liquid extraction between water and methyl isobutyl ketone with thiocyanate present, exploiting a small difference in how the two thiocyanate complexes partition; extractive distillation of the chlorides is the other main approach. Both are expensive, and both exist entirely because of a fraction of a barn.

Hafnium, incidentally, has no other significant source. Almost all of the world's hafnium is a byproduct of making zirconium clean enough for reactors.

Steam, at twelve hundred degrees

Zirconium's corrosion resistance depends on a thin, adherent oxide film. Above roughly 1200 °C that protection fails, and the metal reacts directly with steam, producing zirconium dioxide and hydrogen gas, releasing heat as it goes.

This is the mechanism behind the hydrogen explosions that destroyed the reactor buildings at Fukushima Daiichi in March 2011. Once cooling was lost and the fuel overheated, the cladding itself became a chemical fuel, generating hydrogen that accumulated under the roofs. The same reaction produced the hydrogen bubble that alarmed engineers at Three Mile Island in 1979. It is the standing trade-off of water-cooled reactor design, and it is the main driver behind development of accident-tolerant claddings — coated zirconium, silicon carbide composites, iron chromium aluminum alloys — that oxidise far more slowly when things go wrong.

Jargon from Ceylon, and a bar grown on a wire

The gemstone came first. Zircon has been cut and worn for millennia; the colourless variety was called jargon, and the name traces back through Arabic to Persian zargun, "gold-coloured". Martin Heinrich Klaproth heated a jargon from Ceylon in 1789 and identified a new earth in it, which he named zirconia.

Getting the metal took much longer. Berzelius produced a black impure powder in 1824 by reducing potassium zirconium fluoride, and for a century zirconium was assumed to be inherently brittle. In 1925 Anton Eduard van Arkel and Jan Hendrik de Boer in Eindhoven showed it was not: they reacted crude zirconium with iodine to form a volatile iodide, let the vapour decompose on an electrically heated tungsten filament, and grew a bar of pure, ductile, shining metal. The crystal bar process was slow and never became the bulk route — that is the magnesium reduction Kroll process — but it proved that the brittleness had always been dissolved oxygen and nitrogen rather than the element itself.

A crystal that remembers 4.4 billion years

Zircon, zirconium silicate, is the single most valuable mineral in geochronology, and the reason is a matter of ionic fit. When zircon crystallises it accepts uranium into the zirconium site readily and rejects lead almost completely. Any lead found inside a zircon grain therefore got there by radioactive decay, and the ratio of lead to uranium gives an age with no assumption about starting composition. Better still, two separate uranium isotopes decay to two separate lead isotopes at different rates, so each grain carries an internal consistency check.

Zircon is also nearly indestructible. It survives weathering, transport, burial and even partial melting of its host rock, which means grains can outlive the rocks they formed in. Detrital zircons from the Jack Hills of Western Australia have been dated to about 4.4 billion years, making them the oldest known fragments of the Earth, older than any surviving rock, and their oxygen isotopes suggest liquid water existed on the surface within roughly 150 million years of the planet's formation. An entire chapter of Earth history rests on a mineral that is otherwise a nuisance in a foundry.

Most zirconium is never metal

For all the nuclear drama, only a small percentage of zirconium production ends up as metal. The bulk of it is used as the mineral, zircon sand, mined from heavy mineral sand deposits — mostly in Australia and South Africa — as a companion to the titanium minerals that are the main target.

Its principal markets are unglamorous and enormous:

• Ceramic opacifier. Milled zircon in a glaze scatters light and makes it opaque white. This is what gives bathroom tiles and sanitaryware their finish, and it is the largest single use of zircon by tonnage.
• Foundry sand and refractories. Zircon's high melting point, low thermal expansion and resistance to molten metal make it a mould facing for demanding castings and a lining for glass furnaces.
• Chemical plant. Zirconium metal resists hot sulfuric, hydrochloric and organic acids well enough to be used for heat exchangers and reaction vessels where stainless steel fails. Zirconium alloys also appear in orthopaedic implants, where an oxidised surface provides a hard ceramic bearing on a tough metal substrate.

Finely divided zirconium powder ignites easily, which is why it appears in flash compositions, percussion primers and photographic flashbulbs of the older kind — a property of the powder's surface area rather than of the bulk metal, which is entirely inert in ordinary use.

Grown in a skull

Cubic zirconia is zirconium dioxide held in its high-temperature cubic form at room temperature by a stabilising oxide. The obstacle to making it was that no crucible survives the melt. The solution, developed in the Soviet Union in the 1970s, is skull melting: radio-frequency heating melts the interior of a mass of zirconia while a water-cooled shell keeps the outer layer solid, so the material is contained by itself.

The result is hard, colourless, refracts light strongly and costs almost nothing, and it dominated the imitation-diamond market for three decades until moissanite and then laboratory diamond arrived.

Isotopes, and the awkward heaviest one

Zirconium has five isotopes in nature, of which zirconium-90 is the most common. The heaviest, zirconium-96, is not genuinely stable: it undergoes double beta decay with a half-life around 2 × 10^19 years, so strictly speaking zirconium has four stable isotopes and one that is merely extremely patient.

Fission produces a good deal of zirconium in its own right, including zirconium-93, which has a half-life of about 1.5 million years and is one of the long-lived nuclides that dominates repository safety calculations far into the future. There is a certain symmetry in an element chosen to hold fission products in also being one of the fission products it holds.