Oxidation of Zircaloy Fuel Cladding in Water-Cooled Nuclear Reactors

Abstract

Our work involves the continued development of the theory of passivity and passivity breakdown, in the form of the Point Defect Model, with the emphasis on zirconium and zirconium alloys in reactor coolant environments, the measurement of critically-important parameters, and the development of a code that can be used by reactor operators to actively manage the accumulation of corrosion damage to the fuel cladding in both BWRs and PWRs. In addition, the modified boiling crevice model has been further developed to describe the concentration of solutes in porous deposits (CRUD) on fuel under boiling (BWRs) and nucleate boiling (PWRs) conditions, in order that we can accurately describe the environment that is contact with the Zircaloy cladding.In the current report, we have derived the total steady-state current density and the partial anodic and cathodic current densities to establish a deterministic basis for describing the oxidation. The models are "deterministic" because the relevant natural laws are satisfied explicitly, most importantly the conversation of mass and charge and the equivalence of mass and charge (Faraday's law). Cathodic reactions (oxygen reduction and hydrogen evolution) are also included in the models, because there is some evidence that they control the rate of the overall film formation process. Under open circuit conditions, the cathodic reactions, which must occur at the same rate as the zirconium oxidation reaction, are instrumental in determining the corrosion potential and hence the thickness of the barrier and outer layers of the passive film.Controlled hydrodynamic methods have been used to measure important parameters in the Point Defect Model, which is now being used to describe the growth and breakdown of the passive film on zirconium and on Zircaloy fuel sheathing alloy in water-cooled nuclear reactors. Firstly, polarization studies have been carried to determine the relevant potential ranges for BWR and PWR coolant systems. Impedance and Mott-Schottky analyses have been carried out on steady state passive films at different formation potentials in 0.1 M B(OH) 3 + 0.001 M LiOH solution of pH = 6.94 at 250 o C. The pressure was 62 bar (900 psi) to maintain the electrolyte in the liquid state. The relationships between current density and film thickness and the film formation potential satisfy the diagnostic criteria of the PDM for an n-type passive film. The kinetic parameters are being extracted by optimization of the PDM on the experimental impedance data, which are validated as conforming to the constraints of linear system theory by using the Kramers-Kronig (K-K) transforms. Mott-Schottky analysis has shown that the passive film is n-type in electronic character corresponding to a preponderance of oxygen/hydrogen vacancies or metal interstitials in the barrier layer. The donor density, N D , calculated from the slope of Mott-Schottky plots is in the range of 10 17~1 0 18 cm -3 , which decreases with increasing formation potential, demonstrating that the film is only lo...