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Stress corrosion cracking of Zircaloy fuel cladding by fission products is thought to be an important mechanism influencing power ramping defects of water-reactor fuels. We have used the fixed-deflection stressed slotted-ring technique to demonstrate the following:
1. Hydride layers plated electrolytically onto the surfaces of Zircaloy rings, cut from a batch of cold-drawn material, either increased the time to failure or prevented cracking in iodine vapor at 573 ± 3 K, giving rise instead to a generalized pitting attack. The presence of hydride changed the cracking mode from mainly transgranular cleavage to mainly integranular cracking.
2. In two batches of tube-reduced Zircaloy tubing, one annealed and one as-received, cracking could not be induced in iodine vapor at 573 ± 3 K even with axial scratches on the inside surface. However sharp precracks did propagate readily when their depths and the applied stresses were great enough.
3. One batch that had remained intact when tested in the as-received condition cracked readily in iodine vapor at 573 ± 3 K after stress relief. In the stress-relieved tubing, prescratching did not affect failure times, and cracks nucleated away from the notches. This contrasted with tests in methanol-iodine at room temperature, where notches shortened failure times substantially, the cracks nucleating at the notches.
The results show both the sensitivity and limitations of the stressed slotted-ring method in determining the responses of tubing to stress corrosion cracking. They are interpreted in terms of stress relaxation behavior, both on a microscopic scale for hydrogen-induced stress-relief and on a macroscopic scale for stress-time characteristics. Our analysis also takes account of nonuniform plastic deformation during loading and residual stress buildup on unloading. It is shown that when stress corrosion cracking occurred, it was in conditions where stress relaxation was sufficiently slow that the applied stress exceeded the critical stress for stress corrosion cracking, which itself is a function of exposure time and also the metallurgical condition of the tubing.
fuel cladding, hydride, iodine, residual stress, split ring test, stress corrosion cracking, stress relaxation, Zircaloy
Section manager, Research and Development, Combustion Engineering-Superheater Limited, Moncton, N.B.,
Group leader, Mechanistic Studies, Fuel Engineering Branch, Atomic Energy of Canada Limited, Chalk River Nuclear Laboratories, Chalk River, Ontario