Zirconium hydride

Hydrogen acts as a hardening agent, preventing dislocations in the zirconium atom crystal lattice from sliding past one another.

At room temperature, the most stable form of zirconium is the hexagonal close-packed (HCP) structure α-zirconium.

It is a fairly soft metallic material that can dissolve only a small concentration of hydrogen, no more than 0.069 wt% at 550 °C.

[6] They behave as usual metals in terms of electrical conductivity and magnetic properties (paramagnetic, unless contaminated with ferromagnetic impurities).

[4][5] The mass density behaves differently with the increasing hydrogen content: it decreases linearly from 6.52 to 5.66 g/cm3 for x = 0–1.6 and changes little for x = 1.6–2.0.

[4] At room temperature, zirconium hydrides quickly oxidize in air, and even in high vacuum.

[7] The hydrogen is anionic due to the electronegativity difference between Zr and H.[9] When prepared as thin films, the crystal structure can be improved and surface oxidation minimized.

Zircaloy is used because of its small absorption cross-section for thermal neutrons and superior mechanical and corrosion properties to those of most metals, including zirconium.

[14] In regular operation, most hydrogen is safely neutralized in the reactor systems; however, a fraction of 5-20% diffuses into the Zircaloy rods forming zirconium hydrides.

[11] This process mechanically weakens the rods because the hydrides have lower hardness and ductility than metal.

The evolving hydrogen cleans up the surrounding area, and the produced metal flows and forms a seal even at temperatures as low as 300 °C.

Powdered zirconium hydrides are flammable and can ignite and explode if exposed to heat, fire, or sparks.