An artist’s rendering of nuclear fuel rods in front of a computational valley predicted for alloying compositions.
Photo credit: Mostafa Youssef and Lixin Sun

High-tech metal alloys are used in important materials such as the cladding that protects the fuel inside a nuclear reactor. But even the best alloys degrade over time, victims of a reactor’s high temperatures, radiation, and hydrogen-rich environment. A team of Massachusetts Institute of Technology (MIT) researchers found a way of greatly reducing the damaging effects these metals suffer from exposure to hydrogen.

The team’s analysis focused on zirconium alloys, which are widely used in the nuclear industry, but the basic principles they found could apply to many metallic alloys used in other energy systems and infrastructure applications. The findings appear in the journal Physical Review Applied, in a paper by MIT Associate Professor Bilge Yildiz, postdoc Mostafa Youssef, and graduate student Ming Yang.

Hydrogen, released when water molecules from a reactor’s coolant break apart, can enter the metal and react with it, reducing in the metal’s ductility. That, in turn, can lead to premature cracking and failure.

In nuclear power plants, “the mechanical integrity of that cladding is extremely important,” Yildiz says, so finding ways to improve its longevity is a high priority. However, it turns out that the initial entry of the hydrogen atoms into the metal depends crucially on the characteristics of a layer that forms on the metal’s surface.

A coating of zirconium oxide naturally forms on the zirconium surface in high-temperature water and acts as a protective barrier. If carefully engineered, this oxide layer could inhibit hydrogen from getting into the crystal structure of the metal. Or it could emit the hydrogen in gas form.

While researchers have been studying hydrogen embrittlement for decades, “Almost all of the work has been on what happens to hydrogen inside the metal: What are the consequences, where does it go, how does it lead to embrittlement? We learned a lot from those studies,” Yildiz says. But there had been very little work on how hydrogen gets inside in the first place.

The hydrogen has to first dissolve in the oxide layer before penetrating into the metal beneath. But the hydrogen’s dissolution can be controlled by doping that layer — with atoms of another element or elements. The team found that the amount of hydrogen solubility in the oxide follows a valley-shaped curve, depending on the doping element’s ability to introduce electrons into the oxide layer.

So being able to predict the dopants that belong to each type is essential to making an effective barrier. The team’s findings suggest two potential strategies, one aimed at minimizing hydrogen penetration and one at maximizing the ejection of hydrogen atoms that do get in.

The blocking strategy is “to target the bottom of the valley” by incorporating the right amount of an element, such as chromium, that produces this effect. The other strategy is based on different elements, including niobium, that propel hydrogen out of the oxide surface and protect the underlying zirconium alloy.

The doping could be accomplished by incorporating a small amount of the dopant metal into the initial zirconium alloy matrix, so this in turn gets incorporated into the oxidation layer that naturally forms on the metal.

The team’s findings are likely to be a general approach that can be applied to different alloys that form oxidation layers on their surfaces. This could lead to improvements in longevity for alloys used in fossil fuel plants, bridges, pipelines, and fuel cells.

The work was supported by the Consortium for Advanced Simulation of Light Water Reactors, funded by the U.S. Department of Energy, and the U.S. National Science Foundation.

Massachusetts Institute of Technology

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