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Graphite is a key structural component in some of the world’s oldest nuclear reactors and many of the next-generation designs that are being constructed today. However, it also condenses and swells in response to radiation and the mechanism behind those changes has proven difficult to study.
MIT researchers and collaborators have just uncovered a link between properties of graphite and how the material behaves in response to radiation. The findings could yield more accurate, less destructive ways of predicting the lifespan of graphite materials used in reactors around the world.
Research Scientist Boris Khaykovich is the senior author of the new study. He said, “We did some basic science to understand what leads to swelling and, eventually, failure in graphite structures. More research will be needed to put this into practice, but the paper proposes an attractive idea for industry: that you might not need to break hundreds of irradiated samples to understand their failure point.”
Specifically, the study indicates a connection between the size of the pores within graphite and the way the material swells and shrinks in volume, leading to degradation.
Lance Snead is the co-author and a MIT Research Scientist. He said, “The lifetime of nuclear graphite is limited by irradiation-induced swelling. Porosity is a controlling factor in this swelling, and while graphite has been extensively studied for nuclear applications since the Manhattan Project, we still do not have a clear understanding of the porosity in both mechanical properties and swelling. This work addresses that.”
The open-access paper was published this week in the journal Interdisciplinary Materials. It was co-authored by Khaykovich, Snead, MIT Research Scientist Sean Fayfar, former MIT research fellow Durgesh Rai, Stony Brook University Assistant Professor David Sprouster, Oak Ridge National Laboratory Staff Scientist Anne Campbell, and Argonne National Laboratory Physicist Jan Ilavsky.
Graphite has played a central role in the generation of nuclear energy ever since 1942, when physicists and engineers built the world’s first nuclear reactor on a converted squash court at the University of Chicago. That first reactor, called the Chicago Pile, was constructed from about forty thousand graphite blocks, many of which contained nuggets of uranium.
Today graphite is an important component of many operating nuclear reactors and is expected to play a central role in next-generation reactor designs like molten-salt and high-temperature gas reactors. Graphite is an excellent neutron moderator, slowing down the neutrons released by nuclear fission so that they are more likely to create fissions themselves and sustain a chain reaction.
Khaykovich explained, “The simplicity of graphite makes it valuable. It’s made of carbon, and it’s relatively well-known how to make it cleanly. Graphite is a very mature technology. It’s simple, stable, and we know it works.”
Khaykovich continued, “We call graphite a composite even though it’s made up of only carbon atoms. It includes “filler particles” that are more crystalline, then there is a matrix called a “binder” that is less crystalline, then there are pores that span in length from nanometers to many microns.”
Each grade of graphite has its own composite structure, but they all contain fractals, or shapes that look the same at different scales. Those complexities have made it difficult to predict how graphite will respond to radiation in microscopic detail. It’s been known for decades that when graphite is irradiated, it first densifies, reducing its volume by up to ten percent, before swelling and cracking. The fluctuation of volume is caused by changes to graphite’s porosity and lattice stress.
MIT Nuclear Research Laboratory
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