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Nuclear Materials Under Extreme Conditions

By Lesley Kriewald

Designing advanced materials for next-generation nuclear reactors

To build safer, longer-lasting nuclear reactors, researchers need advanced materials that can withstand the harsh environment inside a reactor. 

Lin Shao, an associate professor in the Department of Nuclear Engineering, is a materials physicist on the hunt for these advanced materials. Shao's research focuses on understanding why materials fail, particularly in the harsh environments inside a reactor, such as high temperature, stress, corrosion and neutron damage. 

The aim of the research is not only reactor safety but also reactor economics and nuclear waste management. Since 2007, Shao's research group has received $8.3 million in funding, primarily from the U.S. Department of Energy and the National Science Foundation. 

Shao and his collaborators study a variety of materials, from stainless steel, which was used as cladding material (the hollow tube used to contain nuclear fuels), to ceramics and even concrete. 

"We need to test these materials to get an idea of how long the materials will last," Shao says, "and we need to understand what the mechanism is behind these failures so we can come up with a strategy to have better material development so the materials can have a longer lifetime." 

Aiding in this study is the country's largest university radiation facility, Shao's Ion Beam Laboratory, with five accelerators providing the particles at various energies that are needed to simulate what happens inside a nuclear reactor—in a relatively short time.

Unique capabilities

Aiding in this study is the country's largest university radiation facility, Shao's Ion Beam Laboratory, with five accelerators providing the particles at various energies that are needed to simulate what happens inside a nuclear reactor—in a relatively short time. 

"Previously, if someone said, 'This material is good and can be used as a structural component inside a reactor,' we can test to see whether that's true," Shao says. "We could put this material inside a reactor and irradiate it for a long time—probably five or 10 years—and then evaluate the damage. 

"But we just can't afford to wait such a long time," he continues. "Therefore, we use our accelerator to create those high-energy, strong beams to simulate radiation damage and get a very quick answer. Our machines are very capable. For example, one day of exposure to radiation from an accelerator can represent a few years of the radiation inside a reactor."

Shao says the capabilities of the five accelerators can be combined with those of the Texas A&M Cyclotron Institute, which produces very high-energy particles, and the university's Nuclear Science Center, which can supply gamma and neutron particles. 

"Combine these facilities, and we have unique capability to carry out systematic research," Shao says.

The unique capabilities of Shao's lab have led to several subcontracts from various national laboratories that lack the kind of radiation testing facilities needed to test material behavior. 

Different types of damage

Shao uses the example of stainless steel cladding. An energetic particle of radiation can rupture the atoms that make up the steel tube, causing damage that sets off a chain reaction that can lead to failure. 

 "You have this perfect stainless steel inside, but the neutron will cause damage and that damage will cause trouble," he says. "That is the fundamental reason that we have so many nasty conditions inside a reactor." 

Swelling is one example of the damage that can occur. The atoms of a particular material are arranged in a certain shape. But left inside a reactor for a few years, the shape changes and grows. 

"If you're able to see what's inside, you'll find it's like Swiss cheese," Shao says. "The material begins to have a cavity inside due to this damage. It used to be okay to have 5 percent swelling, but in the next generation of nuclear reactors, even 1 percent swelling will cause a safety issue. So therefore, the whole field is hungry to find what is the best material that can suppress this kind of swelling enough to keep the structural integrity."

Materials inside a nuclear reactor may also experience cracking. That can happen when stainless steel that contains nuclear fuel comes into contact with coolant, usually water at about 400–500 degrees Celsius in current reactors but will be about 1,000 degrees Celsius in next-generation reactors. After a few years, that water will begin to interact with the stainless steel and cause cracking. If this cracking cannot be stopped, then the nuclear fuel is exposed to coolant, contaminating the whole loop and causing accidents. 

Gas atoms that accumulate inside a metal can form bubbles, similar to the bubbles seen around the edge of a pancake while it's being cooked. Neutrons interacting with the material create these gas atoms. 

Shao says it's fair to blame all accelerated materials failures on neutrons, in fact. Neutrons destroy the original material structures through two types of interactions: nuclear reactions that create energetic fission fragments and direct neutron scattering in which the neutron knocks one atom out of its original location. A given atom can be pushed around by either fission fragments or directly by neutrons and get relocated up to hundreds of times. 

"So no matter how good a material is at the beginning, everything will get messed up at the end," Shao says. 

Neutron damage also changes the water chemistry inside the reactor so that the water behaves like an acid. Therefore, corrosion damage is also of concern to nuclear engineers.

We need to test these materials to get an idea of how long the materials will last, and we need to understand what the mechanism is behind these failures so we can develop materials with a longer lifetime.

Cracking, swelling, bubble formation and corrosion are just several typical ways that materials inside a reactor can fail. In reality, these failure mechanisms are correlated into each other and the combined effect is far more severe. 

The complexity of these materials issues has driven the nuclear materials research from large-scale descriptive studies into atomic- scale fundamental studies, which requires collaborative researches across the fields of engineering, physics and chemistry. 

"Thanks to the excellent collaborative research environments at Texas A&M, we are lucky to find many partners having common interests in these areas," Shao says.   

Understanding damage on the atomic level

Shao says that in addition to testing, his research group seeks to understand the mechanism behind the behaviors observed. 

To do that, the researchers have established a strong modeling program to "see" what experiments can't see. One collaboration with Tahir Cagin, from the Artie McFerrin Department of Chemical Engineering, has allowed the researchers to understand how some metals and ceramics react when bombarded with radiation. 

"We are able to catch those atom interactions at a very short time scale, to go beyond the experimental capability there. We combine experiments and modeling to get an understanding of the mechanism behind material failure." 

Shao says the need for more safety in next-generation reactors drives this push for better material design, and that means studying nanostructured materials at the atomic scale. 

If you put two different materials together, the boundary (or interface) between these two different kinds of materials has a unique effect of trapping defects caused by radiation damage. These boundaries between materials, if they can be stabilized under neutron damage, behave as trash cans, collecting the garbage, or damage, created by radiation.

"Our key interest is trying to maximize this interface region so the material will have a self-annealing effect: The material will repair the damage itself."

This garbage-collecting property leads Shao to expect that these nanoengineered materials will have a significant impact on material design, with longer lifetime inside a nuclear reactor. This particular area of study has led to several research grants, including Shao's National Science Foundation CAREER award in 2009.

"We want to match modeling and simulations, to actually see individual atoms," Shao says. "So we use transmission electron microscopy to see what happens at the metal–metal interface when we put two metals together and bombard with particles." 

Most recently, Shao proposed to bond metals with ceramic glasses to create a new type of interface. The work has been funded by the U.S. Department of Energy as a collaborative project with the University of Nebraska at Lincoln and the Massachusetts Institute of Technology.   

Benefits of nanostructured materials

An energy particle is like a bullet, bombarding the material and causing damage inside. With bulk material—say, a lump of silicon—that energy bullet after one picosecond causes cascading damage by displacing all the atoms in the matrix. But when the same energy bullet is fired at a small nanowire, such as a fiber tube, the same radiation causes very few defects inside. 

"That's the interface effect we are talking about," Shao says. "Our key interest is trying to maximize this interface region so the material will have a self-annealing effect: The material will repair the damage itself." 

One way to increase this boundary, or interface region, is to order the atoms in a polycrystal, a large crystal made up of many smaller crystals. 

"Our desire here is trying to maximize those boundaries so each material inside will have a domain, a crystal as small as possible so we can maximize those regional interfaces. If we can do that, we can trap more defects there to stop damage from spreading."

Major focus 

A major focus of the research team is cladding material. That hollow tube serves as the first buffer layer between the coolant and the nuclear fuel—a critical function, Shao says. 

Shao cites the Fukushima nuclear reactor accident in Japan as an example of just how critical cladding material can be.

"Everybody talks about that reactor accident. That accident came from the fact that the fuel cladding, that hollow tube, was made of zircaloy, a zirconium-based alloy. That alloy works very well at normal operating temperature, 500 degrees Celsius. But after the tsunami, due to the coolant being shut off, the water became very hot. That hot water interacted with the zirconium and released a huge amount of hydrogen. That's what caused the explosion there. 

"So you can see that material selection for that cladding material is critical. So that is the trend in this country, trying to replace zirconium."

"We do whatever we can do to find the best material that best performs under that normal operation temperature, but who knows what will happen if we have an accident? Anything can happen, so we try to say, 'Are we able to structure this so we find a better material, which is safer even in extreme conditions?'"

Shao is investigating using a silicon carbide composite to replace zirconium in cladding material in order to avoid the situation in the Japan reactor. Because silicon carbide is a ceramic, no hydrogen will be released when (or if) the cladding interacts with hot water. And using fiber bundles of silicon carbide rather than bulk silicon carbide helps to strengthen the material. 

"If you had a material made of just bulk silicon carbide, and you tried to bend it, it would crack," Shao says. "But silicon carbide fiber material can be bent to minimize cracking. When you push or displace a fiber, it pulls out of the matrix and then can go back into its shape. So you have a way to release the stress. 

Therefore, we do whatever we can do to find the best material that best performs under that normal operation temperature, but who knows what will happen if we have an accident? Anything can happen, so we try to say, 'Are we able to structure this so we find a better material, which is safer even in extreme conditions?'

Dr. Lin Shao
Dr. Lin Shao
Associate Professor
Nuclear Engineering
979.845.4107