Retarding effect of hydrogen on Zircaloy oxidation

The potential zirconium-steam oxidation reaction in water reactor cores during overheated conditions must be understood because it can release large amounts of heat that will add to the nuclear decay heat a time when the core is already undercooled. The reaction also creates large quantities of hydrogen which will be released into the containment building and which pose a hydrogen burn threat. The rates at which the core heats up and which hydrogen is generated depends upon the zirconium-steam reaction rate. This report describes a combined experimental and analytic effort to identify and quantify zirconium-steam reaction rates in a core that is overheating due to decay heat. Experimental evidence developed in this project indicates that just-released hydrogen at the fuel rod surface interferes with the oxidation process and can account for a factor of 10 or greater reduction in the zirconium-steam oxidation rate. An additional large reduction in the reaction rate should occur because the quantity of steam near the fuel rod is depleted by the reaction and just-released hydrogen interferes with additional steam reaching the fuel rod surface