Nuclear “phantom corrosion” reproduced in the laboratory, paving the way for longer fuel life

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The first diagram shows the experimental setup, with how the nickel alloy bends towards the zirconium alloy in the water-filled corrosion cell. Zirconium oxide is thickest where the nickel alloy comes closest. The second diagram shows the band of the circular sample of zirconium alloy which is affected by the band of nickel alloy and radiation. Finally, the electronic image shows the oxidation band on the zirconium alloy sample. Image credit: Peng Wang, Michigan Ion Beam Laboratory

Solutions to a 55-year-old problem in boiling water reactors – which make up a third of nuclear reactors in the United States – are underway now that the problem has been mimicked with ion beams.

“Shadow Corrosion” affects fuel rods and zirconium alloy (Zircaloy) fuel channels, creating images of neighboring parts on their surface. The damage is a thicker layer of zirconium oxide – like rust on the steel – almost as if the shadow of the neighboring part has been imprinted on the Zircaloy. It can create pinholes in the Zircaloy coating around the fuel rods, requiring early replacement.

“It can also distort the channels between fuel assemblies, potentially preventing the control vanes from regulating reactor power,” said Gary was, professor emeritus of nuclear engineering and radiological sciences and lead author of a new study in the Journal of Nuclear Materials.

While no meltdown has occurred due to shadow corrosion, it does increase the cost of nuclear power as operators have to shut down reactors and waste fuel.

“The benefits of longer fuel life and reduced risk of fuel failure include lower fuel cost, fewer breakdowns, less radiation exposure for workers, and lower maintenance costs,” which lowers the operating costs of the reactor, “said Mr.” Outages, including downtime for refueling, cost about $ 1 million per day. “

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

“Until now, shadow corrosion has never been reproduced in laboratory autoclave experiments, because the simultaneous effect of irradiation was required. What the University of Michigan experiment showed was simulating the actual plant situation, ”said Raul Rebak, corrosion engineer at GE Research in Schenectady, New York, who was not not involved in research. GE is the world’s leading manufacturer of boiling water reactors.

Ion beams can test nuclear material about a thousand times faster – and at a thousandth the cost – compared to research reactors. The ion beams produce more intense radiation, accelerating the aging of nuclear material. However, most ion beam laboratories cannot reproduce all of the conditions necessary for shadow corrosion.

A special high temperature and high pressure water cell that recreates the environment of a reactor core in the Michigan Ion Beam Laboratory has been developed to enable this achievement. They call it a corrosion cell.

“It’s a very unique setup,” said Peng wang, assistant researcher at UM in nuclear engineering and radiological sciences, principal researcher and first author of the article. “We are the first to succeed in reproducing phantom corrosion on the outside of a reactor.

The contact between the Zircaloy fuel rods and the nickel alloy of the support structure creates a stress which causes the corrosion reaction. But to complete the circuit, it takes radiation to separate the water molecules, producing more reactive entities such as hydrogen peroxide. These form on the surface of the nickel alloy and then diffuse towards the surface of the Zircaloy, accelerating its corrosion.

Wang demonstrated this with a flat nickel alloy sample parallel to the Zircaloy sample in the corrosion cell and with a curved sample varying in distance from the Zircaloy. The curved sample showed that Zircaloy was more strongly oxidized where it was closest to the nickel alloy, with the level of oxidation decreasing with distance.

“This result highlights the versatility and high degree of control that accelerators and ions offer to create experiments under very well-controlled conditions that mimic the reactor environment,” he said. “You can study problems to the point of understanding the processes and then developing solutions.

Wang and Was worked with Framatome, a nuclear equipment company headquartered in Courbevoie, France, on phantom corrosion solutions, which are expected to be announced next year. Karsten Nowotka, Group Leader in Combustible Materials Engineering at Framatome, contributed to this study, and Framatome funded the work.

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