Graphite serves as a crucial structural material in some of the world's oldest nuclear reactors and many next-generation reactors under construction. However, its contraction and expansion behavior under radiation has long been difficult to decipher. Recently, a research team from the Massachusetts Institute of Technology (MIT) and collaborators revealed the correlation between graphite material properties and their radiation response behavior. This breakthrough could provide more precise, non-destructive new methods for predicting the lifespan of graphite materials used in nuclear reactors worldwide.

"We conducted some fundamental science research to understand what causes graphite structures to expand and eventually fail," said Boris Khaykovich, a research scientist at MIT and senior author of the study. "More research is needed to put this into practice, but this paper presents an idea that's quite attractive to industry: perhaps we can understand their failure points without having to destroy hundreds of irradiated samples."

The study specifically demonstrated the correlation mechanism between internal pore sizes in graphite and material volume expansion-contraction behavior, which is the root cause of material performance degradation.

"The service life of nuclear-grade graphite is limited by irradiation-induced expansion," noted co-author Lance Snead, a research scientist at MIT. "Porosity is a key parameter controlling expansion. Despite decades of nuclear graphite research since the Manhattan Project, we still lack clear understanding of porosity's role in mechanical properties and expansion behavior. This research fills that gap."

The related research was published in Interdisciplinary Materials under the title "Linking Lattice Strain and Fractal Dimensions to Non-monotonic Volume Changes in Irradiated Nuclear Graphite."

**Complex Material Studied for Decades**

Since physicists built the world's first nuclear reactor "Chicago Pile-1" in a converted squash court at the University of Chicago in 1942, graphite has played a central role in nuclear energy. This pioneering device, constructed from approximately 40,000 graphite bricks embedded with uranium fuel, ushered in a new era of human atomic energy utilization.

Today, graphite is not only a key component of many operational nuclear reactors but is also listed as a core material for next-generation reactor designs such as molten salt reactors and high-temperature gas-cooled reactors. Its value lies in its excellent neutron moderation capability—by slowing down neutrons released from nuclear fission, it significantly improves chain reaction efficiency.

"The precious aspect of graphite lies in its simplicity," Khaykovich explained. "This pure carbon material has mature and stable manufacturing processes, and we have accumulated rich experience in its purification. As a time-tested technology, it combines structural simplicity with reliable performance."

However, beneath its simple appearance lies complex nature. Despite being composed of a single carbon element, graphite is essentially a composite material containing highly crystalline filler particles, lower crystallinity binder matrix, and multi-scale pore structures spanning from nanometer to micrometer scales. Different grades of graphite have unique composite structures but all exhibit fractal characteristics—maintaining similar morphological features at different observation scales.

This complexity means that while academia has long known that graphite undergoes initial densification (volume contraction up to 10%) followed by expansion and cracking under irradiation, it has remained difficult to precisely predict its radiation response behavior from a microscopic perspective.

The root of volume expansion-contraction lies in porosity changes and lattice stress alterations. "Like all materials, graphite gradually deteriorates under radiation. This creates a paradox: we face a material we understand extremely well, yet must admit its complex behavior far exceeds current computer simulation prediction capabilities," Khaykovich stated.

**Innovative Research Methods**

To conduct this study, researchers obtained irradiated graphite samples of G347A grade from Oak Ridge National Laboratory. Co-authors Anne Campbell and Snead participated in the irradiation treatment of these samples about 20 years ago.

The research team innovatively applied X-ray scattering techniques to post-irradiation sample analysis, systematically quantifying the evolution patterns of material pore sizes and surface areas for the first time by analyzing the scattering intensity distribution of X-ray beams.

"The scattering intensity spectra revealed cross-scale pore distribution," described participant Sean Fayfar. "Graphite's pore structure has typical fractal characteristics: from nanoscale to microscale, all exhibit self-similarity, prompting us to adopt fractal models to correlate morphological features at different scales."

While fractal models had been previously used for graphite analysis, research on post-irradiation pore structure evolution was conducted for the first time. The team discovered that graphite exhibits pore filling phenomena in initial irradiation stages, but surprisingly, pore size distribution subsequently undergoes reversible changes. This recovery mechanism, which aligns with overall volume change curves, is quite peculiar.

After long-term irradiation, the material seems to initiate self-repair, similar to new pore generation and subsequent smooth expansion during annealing processes. The research ultimately established a strong correlation between pore size distribution and radiation-induced volume changes.

"Discovering the quantitative relationship between pore distribution and volume changes is a breakthrough," Khaykovich concluded. "This not only reveals the underlying mechanisms of material failure under irradiation but can also help engineers predict the evolution patterns of failure probability for graphite components in radiation stress fields."

**From Laboratory to Reactor**

The research team plans to further examine other grades of graphite, deeply exploring the correlation mechanisms between post-irradiation pore sizes and failure probability. They speculate that Weibull distribution statistical methods, commonly used for failure probability analysis of porous materials like ceramics and metal alloys, might be applicable to graphite service life prediction.

Khaykovich pointed out that this discovery might even provide new insights for deciphering material irradiation densification-expansion mechanisms: "Currently, all quantitative models for graphite irradiation densification do not consider microscale changes. This phenomenon reminds me of sugar or sand grains—when large particles are crushed into fine particles, volume contracts. For nuclear graphite, neutron-carried energy is this 'crushing force,' causing large pores to be filled by crushed fine carbon particles. But as irradiation energy continues input, new pores constantly generate, causing material to expand again. This analogy isn't perfect, but helps us understand material behavior evolution patterns."

Researchers view this paper as an important foundation for promoting future nuclear reactor graphite material research and applications. "Despite graphite research's long history and engineers' strong intuition about its performance in various environments, nuclear reactor construction must be precise to every detail," Khaykovich emphasized. "Industry needs specific numbers—thermal conductivity change ranges, crack propagation rates, volume expansion-contraction ratios. When components undergo dimensional changes, designers must obtain quantitative basis."

This research received partial funding from the U.S. Department of Energy.