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Oxide nodules form on Zircaloy nuclear components under irradiation. Similar nodules were observed on Zircaloy coupons in cold-rolled or extruded conditions after autoclave treatments at 475 and 500 °C in steam at 1500 to 1700 psi. The stages of nodular corrosion in the autoclave were: nodule nucleation, growth, coalescence, propagation to accelerated uniform corrosion, and complete specimen oxidation. Observations on boiling water reactor (BWR) fuel rods suggest that a similar progressive attack has occurred; however, in no case has the in-reactor attack appeared to progress to the stage of complete component failure.
Recent autoclave tests confirmed the nodular character of the attack on cold-worked materials. Alpha anneals (up to 790 °C) did not suppress the nodular attack consistently. However, alpha + beta (840 °C) and beta (1010 and 1040 °C) anneals did suppress the attack if they were followed by a fast cool. The efficacy of the anneals applied similarly to Zircaloy-2 and Zircaloy-4. Stresses associated with U-bend specimens and heavy (86 percent) cold work did not enhance the nodular attack before stress relief occurred.
Additional work is required to confirm preliminary evidence that materials in the reactor and autoclave environments respond similarly to factors such as metallurgical conditions. Availability of an accurate ont-of-reactor prediction of in-reactor performance would be valuable for materials screening, though not necessarily for mechanism definition.
The nodular attack on reactor components appears to depend on nuclear flux, and develops in oxygenated reactor coolants, principally in the vicinity of fuel rod spacers. Experience with irradiated specimens in reactor loops suggests that uniform concentrations of dissolved oxygen alone do not cause the large nodules which frequently develop on BWR fuel rods.
Localized water chemistry associated with flow disturbances or, in some cases, dissimilar metals in fuel spacers, may be factors in the nodular attack in-reactor.
zirconium alloys, corrosion, nuclear fuel claddings
Staff scientist, Battelle Northwest, Richland, Wash
Engineering specialist, Materials Technology Branch, Idaho National Engineering Laboratory, Idaho Falls, Idaho