A longstanding issue in boiling water reactors—shadow corrosion on zirconium alloy fuel rods and fuel channels—has been reproduced in the Michigan Ion Beam Laboratory as part of an effort to understand and prevent the phenomenon. Research led by Peng Wang, a University of Michigan assistant research scientist in nuclear engineering and radiological sciences, was published in the January 2022 issue of the Journal of Nuclear Materials and described in a recent university news article.
The shadow effect: Shadow corrosion damages the surface of zirconium alloy (Zircaloy) fuel rods and fuel channels by creating a thicker layer of zirconium oxide that reproduces the outline of nearby structural parts, as if the shadow of the neighboring part had been imprinted on the Zircaloy. It can result in pinholes in the cladding, forcing replacement of the fuel.
“It can also warp the channels between fuel assemblies, potentially preventing control blades from regulating the reactor power,” said Gary Was, professor emeritus of nuclear engineering and radiological sciences and senior author of the study.
Contact between the Zircaloy fuel rods and the nickel-based alloy or stainless steel of the supporting structure creates a voltage that drives the corrosion reaction. Hydrogen peroxide molecules that form at the nickel alloy surface when radiation splits water molecules can diffuse to the Zircaloy surface and accelerate the corrosion.
In the lab: Wang and his team combined intense radiation from ion beams with a water-filled cell that recreated the high-temperature, high-pressure environment of a reactor core. “This is a very unique setup,” Wang said. “We’re the first to successfully reproduce shadow corrosion outside of a reactor.”
Within the water-filled corrosion cell, a flat nickel alloy sample is positioned in parallel to a Zircaloy sample. A curved nickel alloy sample varies in its distance from the Zircaloy. The researchers found that the Zircaloy was more heavily oxidized where it was closer to the nickel alloy, and the level of oxidation decreased with distance.
Wang and Was worked with Karsten Nowotka, a group leader in fuel materials engineering at Framatome, which funded the study. According to the University of Michigan, methods developed to prevent shadow corrosion will be announced this year.