Test Reactor Studies of the Shadow Corrosion Phenomenon
Authors: Andersson B , Limbäck M , Wikmark G , Hauso E , Johnsen T , Ballinger RG , Nystrand AC 
Published: September 2009

The shadow effect, resulting in enhanced corrosion on zirconium-base alloys in proximity to other metal, has been observed since the 1960s. In 1997, hot-cell examinations revealed thick oxide layers on fuel cladding surfaces that during irradiation had been located in the shadow of Inconel spacer grids. The acronym ESSC (Enhanced spacer shadow corrosion) was used to describe the phenomenon. Following the observation of ESSC, several investigations were initiated to develop an explanation of the shadow corrosion phenomenon. In this paper, results are presented and compared from in-reactor experiments performed in three different test reactors, the R2 reactor in Studsvik, the MITR-II reactor at MIT, and the Halden test reactor.

The first study, performed in Studsvik, was initiated to assess the feasibility of studying the shadow corrosion phenomenon during relatively short irradiation periods in a test reactor. The results clearly showed that a shadow was formed on Zircaloy cladding in contact with Inconel within 34 days of exposure in the center of the core, while no shadow was observed on specimens situated outside and upstream from the core.

The test at MIT was initiated to identify the basic mechanisms of the shadow effect. The MIT research reactor MITR-II was used to simulate BWR core coolant conditions. A sample train included Zircaloy-2 claddings with different counter materials surrounding each clad specimen. The counter materials were intended to serve as the materials tentatively producing shadow corrosion when located in close proximity to the clad specimens. The counter materials chosen were strong or weak beta emitters, platinum, inert material (zirconia), Inconel X-750, coated Inconel X-750, and Zircaloy-2 at various separation distances. The results showed that beta radiation from the counter material is not the main mechanism for the shadow effect, that a coating on the counter material inhibits the formation of shadow corrosion, and that shadow corrosion is dominated by a mechanism of electrochemical character. Shadow corrosion was observed on samples situated outside and downstream from the core. This observation in combination with the results from the test in the R2 reactor imply that radiolysis has an important role for the formation of shadow corrosion.

Two separate tests were performed in the Halden test reactor. The first test was focused on evaluating the effect of variations in clad chemical composition and heat treatments as well as type of counter material. The considered clad materials included Zircaloy-2 with varied average secondary phase particle size as well as a Zircaloy-2 base material. The Zircaloy-2 base material has a chemical composition similar to but outside the range specified for Zircaloy-2. The counter materials included Inconel as well as two different variants of zirconium-based alloys. The second test in Halden was dedicated to study the effect of the clad surface treatment, covering a matrix of ground and pickled outer surfaces in combination with treatments resulting in oxide layers of three different thicknesses, from as-fabricated to 2 μm.