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Tubular samples of Zircaloy-4 clad, of the standard 17 × 17 PWR design, in the cold worked, stress relieved condition, with a mean wall thickness ξ = 580 μm, and samples machined to reduce their wall thickness to 200 and 80 μm, respectively, have been oxidized in air at one bar pressure for exposure periods up to 753, 195 and 152 days, at temperatures of 623, 673 and 723 K, respectively.
Oxidation produces tube lengthening that increases with exposure time and temperature and varies inversely with clad wall thickness. The rate kinetics of oxidation induced straining mirror those of the weight gain measurements. Axial strains up to εz = 0.0265, 0.0185 and 0.0016 in the thinnest clad, and 0.0047, 0.0014 and 0.0003 in standard clad, are observed following exposure at 723, 673 and 623 K, respectively.
A model is developed in which the observed length variation, εz, is expressed in terms of the weight gain owing to oxidation, Δw, the exposure temperature, T, and clad thickness, ξ, according to
Zircaloy-4, cladding tubes, wall thickness, oxidation, weight gain, length changes
Research Officer, Nuclear Electric plc, Berkeley, Gloucestershire