Out-of-Pile Nickel Alloy-Induced Accelerated Hydriding of Zircaloy Fasteners

Mechanical joints between Zircaloy and nickel-bearing alloys, mainly the Zircaloy-4/Inconel-600 combination, were exposed to water at 232 and 271°C to study hydriding of Zircaloy in contact with a dissimilar metal. Accelerated hydriding of the Zircaloy occurred at both temperatures. At 232°C the dissolved hydrogen level of the water was over ten times that at 271°C. At 271°C the initially high hydrogen ingress rate decreased rapidly as exposure time increased and was effectively shut off in about 25 days. Severely hydrided Zircaloy components successfully withstood thermal cycling and mechanical testing. Chromium plating of the nickel-bearing parts was found to be an effective and practical barrier in preventing nickel alloy smearing and accelerated hydriding of Zircaloy.