Zirconium Hydride - Applications

Applications

Formation of zirconium hydrides is an important factor in the operation of several types of nuclear reactors, such as boiling water reactors Fukushima I and II, which suffered from a series of explosions caused by the 2011 Tōhoku earthquake and tsunami. Their uranium fuel pellets are enclosed in metal rods made from Zircaloy – an alloy of typically about 98.25% zirconium with 1.5% tin and minor amounts of other metals. 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. The rods are cooled by streaming water which gradually oxidizes zirconium, liberating hydrogen. In Fukushima reactors, the reactor cooling system failed because of the tsunami. The resulting temperature increase accelerated chemical reactions and caused accumulation of significant amounts of hydrogen, which exploded upon reaction with oxygen when the gas was released to the atmosphere.

Under 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. This process mechanically weakens the rods because the hydrides have lower hardness and ductility than metal. More important though is that the solubility of hydrogen in zirconium is limited to a few percent maximum. Thus the excess hydrogen forms voids which contribute more to the weakening of the alloy. Among Zircaloys, Zircaloy-4 is the least susceptible to this hydrogen blistering.

As a pure powder, zirconium hydrides are used as hydrogenation catalysts, in powder metallurgy, and as getters in the vacuum tube industry. In vacuum system, zirconium hydrides help establish a seal between a metal and ceramic. In this method, a hydride powder (particularly ZrH₄) is mixed with the sealing metal; heating the mixture results in decomposition of the hydride. 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.

ZrH₂ is used in powder metallurgy, as a hydrogenation catalyst, and as a reducing agent, vacuum tube getter, and a foaming agent in production of metal foams. It is also used as a neutron moderator in thermal-spectrum nuclear reactors such as the TRIGA research reactor developed by General Atomics or the Soviet TOPAZ nuclear reactors. Other uses include acting as a fuel in pyrotechnic compositions, namely pyrotechnic initiators.