Corrosion electrochemistry of fuel element materials in pond storage conditions

Magnox clad nuclear fuel elements are subject to aqueous storage following discharge from reactor. The integrity of the clad is imperative in avoiding undesirable release of soluble fission products. The corrosion performance of the material is important and pond chemistry is closely controlled to maintain passivity and prevent clad penetration. In order to allow optimization of operating chemistry and hence minimize corrosion, the aqueous corrosion behaviour of commercial magnesium alloy Magnox A180 has been studied with the use of modern electrochemical techniques. The conventional methods employed in historical corrosion investigations, such as weight loss and hydrogen evolution, are often insensitive to certain mechanisms and require significant time-scales. Here the use of electrochemical noise and impedance spectroscopy alongside DC polarization has allowed the study of general and localized corrosion, notably clad pitting, with greatly enhanced sensitivity and reduced time-scales. Magnox corrosion has proved to be a complex and dynamic system and the formation of a passive layer and propensity to subsequent breakdown are dictated by various environmental factors, including pH, anion concentration and inhibition. This has demanded the development and application of novel techniques and real time observations of pitting initiation and propagation have been made. Empirical models have been formulated from experimental data allowing prediction of the onset of localized corrosion and the application of finite element techniques for deterministic prediction is proposed.