PhD student Claire Griesbach receives Department of Energy Graduate Student Research Award to make nuclear fuel particles more resilient

By Meghan Chua

In the U.S., nuclear energy accounts for about half of the country’s energy generation from sources other than fossil fuels, and about 19% of all energy generation, according to the U.S. Energy Information Administration. However, many nuclear reactors currently in use are reaching the end of their lifespan, creating a need for different and more advanced nuclear reactors.

Claire Griesbach
PhD student Claire Griesbach

Researchers like Claire Griesbach, a PhD candidate in Engineering Mechanics at UW–Madison, are at the forefront of developing more advanced nuclear technologies that can continue to power the country with clean energy.

Griesbach’s PhD research aims to strengthen a fuel particle used in advanced nuclear reactors by better understanding a protective layer used to contain the harmful effects of radiation. Tristructural isotropic (TRISO) nuclear fuel particles contain a uranium kernel wrapped in protective layers that make the fuel particle inherently safe, Griesbach said. A few of these layers are made from pyrocarbon, which is commonly used in nuclear engineering because it’s highly resistant to neutron irradiation. Together, these layers prevent fission products from escaping the fuel particle, which makes a nuclear disaster less likely.

During irradiation, the uranium-containing kernel expands. This leads the first layer of porous pyrocarbon surrounding the uranium core, known as the buffer layer, to compress in on itself and become denser. Although porous pyrocarbon is designed to do this, this often causes fracture in the buffer layer, which Griesbach said can lead the fuel particle to fail and release the harmful products of irradiation. Griesbach’s research seeks to understand how the buffer layer responds to irradiation from nanoscopic to macroscopic scales to identify the best techniques to strengthen it.

This month, Griesbach received an award from the U.S. Department of Energy Office of Science Graduate Student Research Program to conduct research at the Oak Ridge National Laboratory (ORNL) on TRISO particles. At UW–Madison, she has already been able to study particles that are structured the same way as TRISO particles but don’t contain the uranium core. Griesbach used these particles to establish a baseline for how the porous pyrocarbon buffer layer looks before irradiation.

At ORNL, she’ll have access to TRISO particles containing uranium oxycarbide cores irradiated under different conditions, which she can analyze to see how the potential fracturing behavior of the material changes.

“These are materials under such extreme conditions, so the forefront of research now is trying to understand how the nanostructure and microstructure of these materials will change under such conditions because that influences how the entire particle is going to respond,” Griesbach said.

Working at ORNL will also give Griesbach access to specialized instruments to study these materials. She plans to look both at the structure of the pores within the buffer and the nanostructure of the material surrounding those pores to determine how the particle responds to irradiation.

Using a focused-ion beam scanning electron microscope, Griesbach can scan very small slices of the material and reconstruct their shape in a computer model. She can then analyze the material further, as well as share that data with collaborators who can run simulations to predict when the buffer layer might fail.

“This is very important because it hasn’t been looked at extensively yet,” Griesbach said. “I think it will inform future design iterations of these particles. If we can change the pore structure of the buffer layer, if we can change the pore size distribution or something like that based on my findings, we can make these particles even more resilient to these extreme conditions and we can extend their lifespan to make this an even more efficient source of energy.”

Griesbach first came to UW–Madison as an undergraduate and decided to continue for her master’s degree. While she was studying for her master’s, her now-advisor Ramathasan Thevamaran reached out to Griesbach to ask if she was interested in joining his newly founded lab.

“He was describing these things at such a small scale that I had never thought about before, because in all of my mechanics classes and engineering classes previously, we focused more on the macro scale,” Griesbach said. “The more and more I looked into understanding materials at the micro scale, everything started clicking. I could understand how these mechanisms at such a small scale could be contributing to the overall mechanical performance of a material.”

She joined Thevamaran’s lab and began learning the techniques she now uses in her work, including scanning electron microscopy, focused ion beam, and nanoindentation.

“It was just really, really cool to me to be able to even look at things that small,” Griesbach added.

In addition to her work studying TRISO particles, Griesbach’s PhD research has also looked at how to improve the strength and toughness of a material by exposing it to extreme conditions. She was among the authors of a paper Thevamaran published in the journal ACS Nano in December 2021 detailing a new nanofiber material that protects against high-speed projectile impacts more effectively than other solutions like steel plates and Kevlar.

Griesbach said the Wisconsin Centers for Nanoscale Technology in the College of Engineering has been a great resource for her to learn techniques and access the equipment she needs for her work. She also said she has learned a lot from her advisor Thevamaran.

“It’s really just a pleasure to work with him,” she said. “He cultivates a very friendly environment, so it’s easy to bounce ideas off of each other.”

Now, Griesbach looks forward to conducting research at ORNL, which will give her the opportunity to help develop more advanced nuclear energy technology.

“I love working on such a small scale, studying the micro- and nano-mechanics of materials, so it’s fun that I can do something to contribute to the bigger picture of clean energy,” Griesbach said.

This work was also supported by funding from the U.S. Department of Energy Nuclear Energy University Program DE-NE0008979.