http://en.wikipedia.org/wiki/Zirconium_alloyZirconium alloy
Zirconium alloys are solid solutions of zirconium or other metals, a common subgroup having the trade mark Zircaloy. Zirconium has very low absorption cross-section of thermal neutrons, high hardness, ductility and corrosion resistance. Therefore, one of the main uses of zirconium alloys is in nuclear technology, as cladding of fuel rods in nuclear reactors, especially water reactors. A typical composition of nuclear-grade zirconium alloys is more than 95 weight percent<1> zirconium and less than 2% of tin, niobium, iron, chromium, nickel and other metals, which are added to improve mechanical properties and corrosion resistance.
The water cooling of reactor zirconium alloys elevates requirement for their resistance to oxidation-related nodular corrosion. Furthermore, oxidative reaction of zirconium with water releases hydrogen gas, which partly diffuses into the alloy and forms zirconium hydrides. The hydrides are less dense and are weaker mechanically than the alloy; their formation results in blistering and cracking of the cladding – a phenomenon known as hydrogen embrittlement.
Oxidation of zirconium by steam
One disadvantage of metallic zirconium is that in the case of a loss of coolant accidents (LOCA) in a nuclear reactor, the Zr cladding rapidly reacts with water steam at high temperature. Oxidation of zirconium by water is accompanied by release of hydrogen gas. This oxidation is accelerated at high temperatures, e.g. inside a reactor core if the fuel assemblies are no longer completely covered by liquid water and insufficiently cooled. Metallic zirconium is then oxidized by the protons of water to form hydrogen gas according to the following redox reaction:
Zr + 2 H2O → ZrO2 + 2 H2
This reaction was responsible for a small hydrogen explosion accident first observed inside the reactor building of Three Mile Island nuclear power plant in 1979, but, the containment building was not damaged then. This same reaction occurred in the boiling water reactors 1, 2 and 3 of the Fukushima I Nuclear Power Plant (Japan) and in the spent fuel pool of reactor 4 after reactors cooling was interrupted by related earthquake and tsunami events in the disaster of March 11, 2011 leading to the Fukushima I nuclear accidents. After venting of hydrogen gas into the maintenance halls of these three reactors, Zr oxidation-water reduction related explosive mixture of hydrogen with air oxygen detonated, and resulting explosions severely damaged external installation buildings and at least one of associated containment building. To avoid explosion, many pressurized water reactor (PWR) containment buildings, have a catalyst-based recombinator installed to rapidly convert hydrogen and oxygen into water at room temperature before explosivity limit is reached.
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