Zirconium Alloy - Oxidation of Zirconium By Steam

Oxidation of Zirconium By Steam

One disadvantage of metallic zirconium is that in the case of a loss of coolant accident (LOCA) in a nuclear reactor, zirconium 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 exothermic reaction, although only occurring at high temperature, is similar to that of alkaline metals (such as sodium or potassium) with water. It also closely resembles the anaerobic oxidation of iron by water (reaction used at high temperature by Antoine Lavoisier to produce hydrogen for his experiments).

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 that did not damage the containment building. This same reaction occurred in boiling water reactors 1, 2 and 3 of the Fukushima I Nuclear Power Plant (Japan) after reactor cooling was interrupted by related earthquake and tsunami events during the disaster of March 11, 2011, leading to the Fukushima I nuclear accidents. Hydrogen gas was vented into the reactor maintenance halls and the resulting explosive mixture of hydrogen with air oxygen detonated. The explosions severely damaged external buildings and at least one containment building. Many pressurized water reactor (PWR) containment buildings have catalyst-based recombinator units installed to rapidly convert hydrogen and oxygen into water at room temperature before the explosive limit is reached.