Research points way to hydrogen-resistant fuel cladding

30 March 2016

A way of making zirconium alloys used in the cladding of nuclear assemblies more resistant to hydrogen, which can lead to embrittlement, has been investigated by researchers at the Massachusetts Institute of Technology (MIT).

Hydrogen, which is released when water molecules from a reactor's coolant break apart, can enter the zirconium alloy and react with it. This leads to a reduction in the metals' ductility, or its ability to sustain a mechanical load before fracturing. That in turn can lead to premature cracking and failure.

MIT associate professor Bilge Yildiz, together with postdoc student Mostafa Youssef and graduate student Ming Yang, has looked at ways that could lead to the development of hydrogen-resistant zirconium alloys. Their findings were published in the journal Physical Review Applied on 26 January.

A layer of zirconium oxide naturally forms on the surface of the zirconium in high-temperature water which, the researchers say, "acts as a kind of protective barrier". The team found that the initial entry of the hydrogen atoms into the metal depends on the characteristics of the zirconium oxide layer.

"If we know it enters or how it can be discharged or ejected from the surface, that gives us the ability to predict surface modifications that can reduce the rate of entry," said Yildiz.

According to the researchers, "If carefully engineered, this layer of oxide could inhibit hydrogen from getting into the crystal structure of the metal. Or, under other conditions, it could emit the hydrogen in gas form."

The team found that hydrogen's dissolution in the oxide layer can be controlled by doping the layer by introducing atoms of another element or elements into it. They found the amount of hydrogen solubility in the oxide varies according to the doping element's ability to introduce electrons into the oxide layer.

"There is a certain type of doping element that minimizes hydrogen's ability to penetrate, whereas other doping elements can introduce a maximum amount of electrons in the oxide and facilitate the ejection of hydrogen gas right at the surface of the oxide," said Mostafa.

The team's findings suggest two potential strategies: one aimed at minimizing hydrogen penetration into the oxide layer and one at maximizing the ejection of hydrogen atoms that do get into it. They claim the first strategy could be achieved by incorporating the right amount of an element, such as chromium, while the second could be achieved by using a number of elements, including niobium.

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

The research was supported by the Consortium for Advanced Simulation of Light Water reactors, funded by the US Department of Energy, with computational support provided by the US National Science Foundation.

Although the team's analysis focused on zirconium alloys, the basic principles they found could apply to many metallic alloys used in other energy systems and infrastructure applications that form oxidation layers on their surfaces. Yildiz said this could be "any place you have metals exposed to high temperatures and water".

Researched and written
by World Nuclear News