Mechanistic understanding of Zircaloy corrosion in PWRs through a corrosion model

Based on theoretical calculations and the comparison of corrosion data yielded by out-of-pile loop, PWR and autoclave tests, a semi-empirical corrosion model makes the following calculations possible: The fine thermal/hydraulic conditions in the environment of the fuel rods in the core of a PWR. The localized pH coolant and over-concentration of soluble species in the coolant under boiling regime. The axial and azimuthal corrosion data (oxide thicknesses and hydriding), associated with the physico-chemical determination of the coolant/cladding system. At present the model is developed to replace the semi-empirical correlations by fundamental equations which describe the mechanisms involved in the corrosion process. This paper emphasizes the heat flux effect which acts as a diffusional driving force in the oxidation process. An approximated solution of the diffusion problem accounts for gradients in concentration, stress and temperature. Theory is in good agreement with the corrosion data collected from autoclave (isothermal) and from out-of-pile loop (heat flux). (author). 5 refs, 9 figs