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Examination of specimens from one batch of Zircaloy fuel cladding that had been partially cracked in iodine vapor at 300°C has shown small fractured hydrides at the initiation sites of the transgranular stress corrosion cracking (SCC) process. Investigation showed that small radial hydrides were present at the inside surface of the original tubing and that these fractured at the start of the SCC test. In other environments, (for example, a solution of iodine in methanol), where extensive intergranular corrosion occurs at the initiation site, these cracked hydrides were ignored as initiators for the SCC process. A survey of other batches of tubing found a number of batches also showing radial hydrides at the inside. The occurrence of these hydrides did not correlate well with susceptibility to SCC in iodine vapor at 300°C. It has proved impossible to render a resistant batch of tubing susceptible to SCC by deliberately precipitating radial hydrides at the inside surface. However, by “decorating” cracked hydrides with thick (2–5-μm) oxide films, it has been found that a few radial hydrides crack at the start of the SCC test in those batches of tubing susceptible to SCC in a split-ring test, but no hydrides crack in resistant batches of tubing. Thus, the cracking of a radial hydride at the inside surface is an important initiation step in batches of tubing containing such hydrides. There must also be other initiation processes available in order to account for failures in susceptible batches of tubing that do not contain radial hydrides at the inside surface.
Zircaloy, stress corrosion cracking, hydride cracking, iodine vapor, crack initiation, fuel cladding
Branch head, Materials Science Branch, Atomic Energy of Canada Limited, Chalk River Nuclear Laboratories, Chalk River, Ontario