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. 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. This process mechanically weakens the rods because the hydrides have lower hardness and ductility than metal. Only a few percent of hydrogen can dissolve in zirconium. Excess hydrogen forms voids that weaken Zircalloy. Among Zircaloys, Zircaloy-4 is the least susceptible to hydrogen blistering. 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. At neutron energies above 0.14 eV it is as effective at moderating a nuclear reactor as elemental hydrogen (the best known material), but far more dense, and therefore permits compact reactors with high power per unit volume. It has neutron resonances that prevent almost all moderation at energies below 0.14 eV. Zirconium deuteride is superior, because it has a lower neutron absorption cross-section than aneutronic hydrogen, decreasing neutron absorption in a reactor. 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. Other uses include acting as a fuel in pyrotechnic compositions, namely pyrotechnic initiators. Safety Powdered zirconium hydrides are flammable and can ignite and explode if exposed to heat, fire, or sparks. When heated above 300 °C, they decompose releasing hydrogen gas, which is also flammable. References External links