Enhanced corrosion of zirconium-based alloys in proximity to other metals : the "shadow effect"

Abstract

Fuel cladding for water-cooled power reactors must meet certain requirements for optimal performance. To function in the extreme conditions typical of a nuclear reactor core the material used must be corrosion resistant, have low thermal neutron cross section, and high strength. Corrosion resistance is one of the most important parameters for reactor materials. From the beginning of the use of reactors, engineers have been faced with the problem of excessive corrosion in several different forms. In recent years, a peculiar corrosion phenomenon has increased in significance. Several occurrences of local corrosion enhancement of zirconium-base alloys in proximity to other components have been observed. This corrosion enhancement talcs the form of a "shadow" of a metal component in proximity, hence its name, "shadow effect." Although much recent attention has been given to the shadow effect, it has been known since the sixties, but has only lately been considered a possible threat to material integrity. Today the interest in local corrosion enhancement due to the shadow effect and its implications for in-core performance of cladding and structural material is increasing worldwide. International experience has shown that the phenomenon has occasionally resulted in serious corrosion problems threatening material integrity. In order to prevent future obstruction from the phenomenon an understanding of the shadow effect needs to be developed. This becomes important in today's rapid expansion of aggressive reactor environments with higher burn-up and the need for longer fuel residence times for more economical runs. This project was conducted at MIT, funded by ABB Atom, which had the goal of identifying the basic mechanisms of the shadow effect. The MIT research reactor MITR-11 was used to simulate BWR core coolant conditions. The sample train included Zr-2- alloy with various surface treatments. Different counter electrodes surrounded each cladding piece. They were high and low beta emitters, inert material and Zircaloy-2 in contact and non-contact at various separation distances. Post-irradiation examination of the cladding pieces showed: * Beta-radiation is not the main mechanism for the shadow effect. * Shadow corrosion is partly dominated by an electrochemical mechanism. * Radiolysis plays an important role for the formation of shadow corrosion.