The effects of thermo-mechanical processing of cladding on the corrosion of Zircaloy-4 in commercial PWRs have been investigated. Visual observations and nondestructive measurements at poolside, augmented by observations in the hot cell, indicate that the initial black oxide transforms into a grey or tan and later white oxide layer at a thickness of 10 to 15 μm independent of the thermal processing history of the tubing. At an oxide layer thickness of 60 to 80 μm, the oxide may spall depending somewhat on the particular oxide morphology formed and possibly on the frequency of power and temperature changes of the fuel rods. Because spalling of oxide lowers the metal-to-oxide interface temperature of fuel rods, it reduces the corrosion rate and is beneficial from that point of view.
To determine the effect of thermo-mechanicai processing on in-reactor corrosion of Zircaloy-4, oxide thickness measurements at poolside and in the hot cell have been analyzed with the MATPRO corrosion model. A calibrated corrosion parameter in this model provides a measure of the corrosion susceptibility of the Zircaloy-4 cladding. It was found necessary to modify the MATPRO equations with a burnup dependent term to obtain a near constant value of the corrosion parameter over a burnup range of approximately 10 to 45 MWd/kgU. Different calculational tests were performed to confirm that the modified model accurately predicts the corrosion behavior of fuel rods.
For Zircaloy-4 cladding with a chemical composition near the mid-range of the ASTM specification, it is shown that the in-reactor corrosion performance can be correlated with the thermo-mechanical treatment of the cladding. The evaluation of the in-reactor corrosion data indicates that the corrosion performance of Zircaloy-4 cladding is improved by longer and/or higher temperature anneals in the alpha range after the material has been beta quenched. The best correlation between corrosion performance and thermo-mechanical treatment is obtained with the second order cumulative annealing parameter. This parameter is a measure of the size of intermetallic precipitates of Zr, Fe, and Cr in Zircaloy-4. However, a causal relationship between Zr (Cr, Fe)2 precipitate size and corrosion behavior has not been proven.