“Shadow corrosion” affects the zirconium alloy (Zircaloy) used to clad nuclear fuel rods. This creates images of nearby parts on their surface. The damage is a thicker layer of zirconium oxide similar to a layer of rust on steel. It looks like the shadow of the neighboring part had been imprinted on the Zircaloy. It can also create pinholes in the Zircaloy cladding layer on the fuel rods. This can lead to the need for early replacement.
       Gary Was is a professor emeritus of nuclear engineering and radiological sciences at the University of Michigan and senior author of a new study in the Journal of Nuclear Materials. He said that the shadow corrosion “can also warp the channels between fuel assemblies, potentially preventing control blades that regulate the reactor power.” He adds, “The advantages of longer fuel life and a reduced risk of fuel failure include lower fuel cost, fewer outages, less radiation exposure for workers and lower maintenance cost—all of which lower the operating cost for the reactor. Outages, including down time for refueling, cost about $1 million per day.”
     No meltdowns have been caused by shadow corrosion, but it does drive up the cost of nuclear power because operators have to shut down reactors and waste fuel.
     Raul Rebak is a corrosion engineer at GE Research in Schenectady, New York, who was not involved with the research. He said, “Until now, shadow corrosion was never reproduced in laboratory autoclave experiments because the simultaneous effect of irradiation was needed. What the University of Michigan experiment has shown was the simulation of the actual plant situation.” GE is the leading manufacturer of boiling water reactors.
     Ion beams can be used to test nuclear materials about a thousand times faster and a thousand times cheaper when compared to research reactors used to test materials. Ion beams can produce more intense radiation to accelerate the aging of nuclear materials. However, most of the labs with ion beam equipment cannot reproduce all the conditions necessary for shadow corrosion. A special high-temperature, high-pressure water cell that creates the environment of a reactor core in the Michigan Ion Beam Laboratory was specially developed to enable this procedure. It is called a corrosion cell.
     Peng Wang is a U-M assistant research scientist in nuclear engineering and radiological sciences as well as the lead researcher and first author of the paper. “This is a very unique setup. We’re the first to successfully reproduce shadow corrosion outside of a reactor.”
     Contact between Zircaloy fuel rods and the nickel alloy of the supporting structure creates a voltage. This voltage drives the corrosion reaction. It is necessary for radiation to split the water molecules to complete the circuit. This produces more reactive entities such as hydrogen peroxide. These reactive entities form at the nickel alloy surface and then diffuse to the Zircaloy surface. This accelerates its corrosion.
     This process was demonstrated in the lab with a flat nickel alloy sample running in parallel to the Zircaloy sample in the corrosion cell. A curved sample that varied in its distance from the Zircaloy was also included in the demonstration. The curved sample showed that the Zircaloy was more heavily oxidized where it was closer to the nickel alloy. The level of oxidation decreased with the distance between the nickel and the Zircaloy.
     Was said, “This result highlights the versatility and the high degree of control that accelerators and ions offer to create experiments with very well-controlled conditions that mimic the reactor environment. You can study problems to the point where you understand the processes and then develop solutions.”
     Wang and Was have been working in collaboration with the French nuclear equipment company Framatome. The results of their work on solving shadow corrosion will be announced next year. Karsten Nowotka is a group leader in fuel materials engineering at Framatome. She contributed to this study, and Framatome funded the work.