A better way to quantify radiation damage in materials

A better way to quantify radiation damage in materials

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It was just a piece of junk sitting in the back of a lab at the MIT Nuclear Reactor facility, ready to be disposed of. But it became key to demonstrating a broader method for detecting atomic-level structural damage in materials — an approach that will aid the development of new materials and could potentially support the ongoing operations of zero-carbon nuclear power plants that would help mitigate global climate change to alleviate.

A tiny titanium nut removed from inside the reactor was just the material needed to prove that this new technique, being developed at MIT and other institutions, offers a way to study defects generated inside materials, including exposed ones, to radiation, with sensitivity five times higher than existing methods.

The new approach showed that much of the damage that takes place in reactors is at the atomic level and is therefore difficult to detect using existing methods. The technique offers a way to measure this damage directly, as it varies with temperature. And it could be used to measure samples from the fleet of nuclear reactors currently in service, potentially allowing plants to continue to operate safely well beyond their current approved lifetime.

The results are published in the journal scientific advances in a paper by MIT research specialist and recent graduate student Charles Hirst, Ph.D. ’22; MIT professors Michael Short, Scott Kemp, and Ju Li; and five others at the University of Helsinki, the Idaho National Laboratory, and the University of California at Irvine.

Rather than directly observing the physical structure of a material in question, the new approach looks at the amount of energy stored in that structure. Any disruption of the ordered structure of atoms within the material, such as that caused by exposure to radiation or by mechanical stress, actually imparts excess energy to the material. By observing and quantifying this energy difference, it is possible to calculate the total damage within the material—even when that damage is in the form of atomic-scale defects that are too small to be imaged with microscopes or other detection methods.

The principle of this method was worked out in detail by calculations and simulations. But it was actual testing on that one titanium nut from MIT’s nuclear reactor that provided the proof – opening the door to a new way of measuring damage to materials.

The method they use is called Differential Scanning Calorimetry. As Hirst explains, this is basically similar to the calorimetry experiments that many students perform in high school chemistry classes, where they measure how much energy it takes to raise the temperature of a gram of water by one degree. The system the researchers used was “basically exactly the same, measuring energetic changes. … I like to call it just a fancy oven with a thermocouple inside.”

The scan part has to do with gradually increasing the temperature and seeing how the sample reacts, and the differential part relates to the fact that two identical chambers are measured at the same time, one empty and one with the closed examining sample. The difference between the two reveals details of the sample’s energy, Hirst explains.

“We increase the temperature from room temperature to 600 degrees Celsius at a constant rate of 50 degrees per minute,” he says. Compared to the empty vessel, “Of course, your material lags behind because you need energy to heat your material. But when the energy inside the material changes, the temperature changes. In our case, there was an energy release when.” the defects recombine, and then the furnace gets a little head start… and that’s how we measure the energy in our sample.”

Hirst, who conducted the work over a five-year period as his PhD project, found that contrary to what was thought, the irradiated material showed that two different mechanisms were involved in the relaxation of defects in titanium at the temperatures studied, shown by two separate ones Peaks in calorimetry. “Instead of one process happening, we clearly saw two, and each of them corresponds to a different reaction taking place in the material,” he says.

They also found that textbook explanations of how radiation damage behaves with temperature were not accurate, as previous tests had mostly been performed at extremely low temperatures and then extrapolated to the higher temperatures of real reactor operation. “People weren’t necessarily aware that they were extrapolating, although they were totally extrapolating,” says Hirst.

“The fact is that our general knowledge of how radiation damage develops is based on extremely low-temperature electron beam radiation,” adds Short. “It just became the accepted model and that’s what all the books teach. It took us a while to realize that our general understanding was based on a very specific condition, designed to explain the science but not generally applicable to conditions in which we would actually like to use these materials.”

Now the new method can be “applied to materials taken from existing reactors to learn more about how they degrade during operation,” says Hirst.

“The biggest thing the world can do to get cheap, carbon-free electricity is to keep current reactors online. They’re already paid, they’re working,” adds Short. But to make that possible, “the only way we can keep them online is when we have more confidence that they will continue to perform well.” And that’s where this new way of assessing damage comes in.

While most nuclear power plants have been licensed to operate for 40 to 60 years, “we are now talking about operating the same plants to 100 years and that depends almost entirely on the materials being able to withstand the most severe accidents”. short says. With this new method, “we can inspect and take them out before anything unexpected happens.”

In practice, plant operators could take a tiny sample of material from critical areas of the reactor and analyze it to get a more complete picture of the condition of the entire reactor. Keeping existing reactors running is “the biggest thing we can do to keep the fraction of carbon-free energy high,” Short says. “This is one way we believe we can do that.”

The process isn’t just limited to studying metals, nor is it limited to damage from radiation, the researchers say. In principle, the method could also be used to measure other types of material defects, such as those caused by stress or shock waves, and also applied to materials such as ceramics or semiconductors.

In fact, according to Short, metals are the most difficult materials to measure with this method, and early on other researchers questioned why this team focused on damage to metals. That was partly because reactor components are usually made of metal, and also because “it’s the hardest, so if we solve this problem we’ll have a tool to crack them all!”

Measuring flaws in other materials can be up to 10,000 times easier than in metals, he says. “If we can do that with metals, we can make that extremely, ubiquitously applicable.” And all of that was made possible by a small piece of junk lying in the back of a lab.

The research team included Fredric Granberg and Kai Nordlund from the University of Helsinki in Finland; Boopathie Kombaiah and Scott Middlemas at the Idaho National Laboratory; and Penghui Cao at the University of California at Irvine.

How prolonged radiation exposure damages nuclear reactors

More information:
Charles A. Hirst et al, Revealing Hidden Defects by Stored Energy Measurements of Radiation Damage, scientific advances (2022). DOI: 10.1126/sciadv.abn2733. www.science.org/doi/10.1126/sciadv.abn2733

Provided by the Massachusetts Institute of Technology

Quote: A better way to quantify Radiation Damage in Materials (2022, August 3), retrieved August 3, 2022 from https://phys.org/news/2022-08-quantify-materials.html

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