Nuclear “shadow corrosion” reproduced in the laboratory paves the way for longer fuel life

The first diagram shows the test setup with the bending of the nickel alloy to the zirconium alloy in the water-filled corrosion cell. The zirconia is thickest where the nickel alloy comes closest. The second graph shows the streak of the circular zirconium alloy sample affected by the band of nickel alloy and radiation. Finally, the electron image shows the oxidation band of the zirconium alloy sample. Photo credit: Peng Wang, Michigan Ion Beam Laboratory

Solutions to a 55-year-old problem in boiling water reactors – which comprise a third of all nuclear power plants in the United States – are on the way after the problem is recreated with ion beams.

“Shadow corrosion” affects fuel rods and fuel channels made of a zirconium alloy (Zircaloy) and creates images of nearby parts on their surface. The damage is a thicker layer of zirconium oxide – like rust on steel – almost as if the shadow of the neighboring part were imprinted on the zirconium. It can lead to pinholes in the zircaloy cladding around the fuel rods that require early replacement.

“It can also deform the channels between fuel assemblies and potentially prevent control blades from regulating reactor performance,” said Gary Was, Professor Emeritus of Nuclear Engineering and Radiological Sciences and lead author of a new study in the Journal of Nuclear Materials.

Although core meltdowns due to shadow corrosion have not occurred, it drives up the cost of nuclear energy as operators shut down reactors and waste fuel.

“The benefits of longer fuel life and less risk of fuel failure include lower fuel costs, fewer failures, less radiation exposure for workers, and lower maintenance costs – all of which lower the operating costs of the reactor,” Was said. “Outages, including downtime to refuel, cost about $ 1 million a day.”

Research reactors can detect shadow corrosion, but this method is expensive and time consuming to study the problem and its solutions. The shadow corrosion could not be repeated even without radiation, only with a heated chamber filled with water.

“So far, shadow corrosion has never been reproduced in laboratory autoclave experiments because the simultaneous exposure to radiation was necessary. What the University of Michigan experiment showed was the simulation of the actual plant situation, ”said Raul Rebak, a corrosion engineer at GE Research in Schenectady, New York, who was not involved in the research. GE is the world’s leading manufacturer of boiling water reactors.

Ion beams can test nuclear material around a thousand times faster than research reactors – and at a thousandth the cost. Ion beams generate more intense radiation and accelerate the aging of nuclear materials. However, most ion beam laboratories cannot reproduce all of the conditions required for shadow corrosion.

To enable this performance, a special high-temperature, high-pressure water cell was developed that simulates the environment of a reactor core in the Michigan Ion Beam Laboratory. They call it a corrosion cell.

“This is a very unique set-up,” said Peng Wang, UM research assistant in nuclear engineering and radiological sciences, lead researcher and first author of the paper. “We are the first to successfully reproduce shadow corrosion outside of a reactor.”

The contact between Zircaloy fuel rods and the nickel alloy of the support structure creates a tension that drives the corrosion reaction. Closing the cycle, however, requires radiation to break down water molecules, creating more reactive entities like hydrogen peroxide. These form on the nickel alloy surface and then diffuse to the Zircaloy surface, accelerating their corrosion.

Wang demonstrated this with a flat nickel alloy sample that ran parallel to the Zircaloy sample in the corrosion cell – and with a curved sample that varied in its distance from the Zircaloy. The curved sample showed that the Zircaloy was more oxidized where it was closer to the nickel alloy, with the degree of oxidation decreasing with distance.

“This result underscores the versatility and high level of control that accelerators and ions offer to create experiments with very well-controlled conditions that mimick the reactor environment.” “You can study problems to the point where you understand the processes and then develop solutions.

Wang and Was have worked with Framatome, a nuclear equipment company headquartered in Courbevoie, France, on shadow corrosion solutions that are expected to be announced next year. Karsten Nowotka, group leader in fuel technology at Framatome, contributed to this study, and Framatome funded the work.

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