Hydrogen Embrittlement of Zirconium Alloys

Hydrogen embrittlement is a problem with zirconium and zirconium alloys, which often are used as cladding materials for nuclear reactors. Zirconium reacts with water as follows.

Part of the hydrogen produced by the corrosion of zirconium in water combines with the zirconium to form a separate phase of zirconium hydride (ZrH1.5) platelets. The metal then becomes embrittled (ductility decreases) and it fractures easily. Cracks begin to form in the zirconium hydride platelets and are propagated through the metal. Zircaloy-2 (a zirconium alloy), which has been used as a fuel rod cladding, may absorb as much as 50% of the corrosionproduced hydrogen and is subject to hydrogen embrittlement, especially in the vicinity of the surface. Studies at Westinghouse, Batelle, and elsewhere have revealed that the nickel in the zircaloy-2 was responsible for the hydrogen pickup. This has led to the development of zircaloy- 4, which has significantly less nickel than zircaloy-2 and is less susceptible to embrittlement. In addition, the introduction of niobium into zircaloy-4 further reduces the amount of hydrogen absorption.

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