Observations of Accelerated Hydriding in Zirconium Alloys

During aqueous corrosion of zirconium alloys, a fraction of the hydrogen generated by the metal-water reaction is absorbed by the metal. This hydriding can result in a loss of ductility in zirconium alloy components if hydrogen absorption is appreciable. Tests have shown that hydriding can be accelerated in zirconium alloys when they are coupled to other metals. The degree of acceleration was found to be influenced by the nature of the dissimilar metal, the amount of hydrogen dissolved in the water, the composition of the zirconium alloy, and the presence of neutron radiation. Tests in light water (H2O) and heavy water (D2O) have shown that hydrogen is preferentially absorbed over deuterium during aqueous corrosion in these media. In neutral water at 360°C, hydrogen absorption by zirconium alloys in H2O was up to twice as great as the equivalent hydrogen absorbed in reactor-grade D2O. In tests with D2O where alkalinity was maintained with lithium hydroxide, hydrogen rather than deuterium was the predominant species absorbed. Results and observations from these tests are discussed in terms of the current understanding of hydrogen absorption mechanisms.