CAMBRIDGE, MA — Graphite, a seemingly simple form of carbon, stands as a pillar of nuclear reactor technology, integral both to the world’s earliest nuclear reactors and the advanced systems being designed today. Its role as a neutron moderator is critical: graphite slows the high-energy neutrons released during fission reactions, increasing the likelihood of sustained chain reactions. Yet behind its apparent simplicity lies a labyrinth of structural complexities that respond unpredictably under the assault of radiation. These responses—specifically its tendency to contract and later swell—have long challenged scientists seeking to unravel the mechanisms underpinning material degradation in nuclear environments.
Recent work by MIT scientists and their collaborators marks a significant leap forward in deciphering the enigmatic behavior of irradiated graphite. Using cutting-edge X-ray scattering techniques coupled with fractal analysis, they’ve illuminated how the microscopic architecture of graphite relates directly to its macroscopic dimensional changes under neutron irradiation. Their findings, published in the journal Interdisciplinary Materials, not only deepen our fundamental understanding but also promise more precise and less invasive methods to gauge the lifespan of graphite components critical to nuclear reactor safety and performance.
According to Boris Khaykovich, the study’s senior author and MIT Research Scientist, the research focused on uncovering the root causes of graphite swelling and failure. “Historically, assessing degradation has required destructive testing of numerous irradiated samples,” he explains. “Our study proposes a paradigm shift: by mapping the fractal nature of graphite’s pore structures, it might be possible to predict failure thresholds without exhaustive sample destruction, streamlining industry practices significantly.”
Graphite’s internal structure reveals a fascinating composite landscape despite its pure carbon composition. It comprises relatively crystalline filler particles embedded in a less ordered binder matrix, interspersed with pores that range from nanometers to several microns in size. This distribution of porosity exhibits fractal characteristics—meaning patterns repeat at various scales—a feature that poses both a challenge and an opportunity for materials scientists attempting to model irradiation effects across multiple length scales.
Over decades, it has been well-documented that irradiated graphite experiences an initial densification phase during which its volume contracts by as much as 10%, followed by swelling and cracking. However, connecting these volume changes directly to underlying pore morphology has remained elusive. The MIT-led team approached this by applying X-ray scattering to graphite samples irradiated two decades prior at Oak Ridge National Laboratory. This approach enabled them to quantify distributions of pore sizes and surface areas—parameters crucial for understanding mechanical behavior and swelling.
Sean Fayfar, one of the co-authors and an MIT Research Scientist, elaborates, “Our X-ray scattering data showed a remarkable self-similarity in porosity across a broad range of scales. Leveraging fractal models allowed us to relate how minute nanopores and larger micron-scale voids collectively evolve during prolonged irradiation, something never before demonstrated with such clarity in nuclear graphite.” This fractal perspective is pivotal, as it moves beyond simplified pore concepts towards a comprehensive multiscale framework that better reflects real graphite microstructures.
A particularly striking discovery was the non-monotonic evolution of graphite’s pore size distribution during irradiation. Initially, pores diminish as the graphite densifies, presumably through the collapse or infilling of larger pores. But after sustained radiation exposure, the trend reverses—a phase described as recovery or annealing—where new pores form and existing pores become smoother yet slightly larger. This unexpected observation challenges conventional wisdom and suggests that graphite undergoes a complex cyclic transformation rather than a straightforward degradation.
This cyclic pore behavior mirrors the measured volume changes of the material and points toward nuanced mechanisms of strain accumulation and release within the graphite lattice. Khaykovich draws an analogy to granular materials like sand or sugar, where crushing particles leads to densification. Here, the bombardment by neutrons fractures graphite’s internal structure, filling large voids with fine debris, but additional energy eventually creates fresh porosity, causing swelling to resume. While not a perfect analogy, it frames graphite irradiation as a dynamic balance of pore collapse and creation driven by radiation-induced energy flux.
From a practical standpoint, understanding the correlation between fractal-like pore distribution and lattice strain is revolutionary. It can inform predictive models that forecast graphite’s mechanical failure probability over time under neutron exposure, a critical step for reactor maintenance and safety protocols. The research team speculates that mathematical tools such as the Weibull Distribution—already utilized in ceramics and porous metals to describe failure probabilities—might soon be adapted for nuclear graphite, enabling non-destructive lifetime assessments of reactor components.
This breakthrough carries profound implications for next-generation reactor designs, including molten-salt and high-temperature gas reactors, which will rely heavily on graphite’s moderation properties. As reactor lifespans extend and operational conditions intensify, precise knowledge of how graphite behaves under extreme irradiation will be paramount for ensuring structural integrity, thermal performance, and overall safety. The MIT study moves the field closer to providing those critical quantitative metrics.
Though graphite has been foundational to nuclear reactors since the Chicago Pile-1’s construction in 1942, the nuances of its response to irradiation at the microstructural level remained difficult to capture. This research integrates advanced materials characterization, fractal mathematics, and decades-old irradiation experiments into a cohesive narrative that bridges the gap between fundamental science and engineering applications within the nuclear energy sector.
Looking forward, the researchers plan to examine additional graphite grades and explore how variations in fractal porosity influence mechanical resilience and failure modes under irradiation. Such work is essential to tailor graphite formulations optimized for longevity and stability in different reactor environments. The interplay between pore evolution and lattice strain highlighted by this study offers a roadmap for more intelligent material design and quality control in nuclear engineering.
Khaykovich underscores the broader vision: “Graphite is deceptively simple yet incredibly complex. Our research provides industry with not just qualitative intuition but the quantitative tools necessary to predict and manage graphite degradation more effectively. In nuclear reactor construction, where precision and reliability are non-negotiable, even subtle improvements in material understanding can translate into safer and more economical energy production.” As nuclear energy continues to play a vital role in global decarbonization efforts, such advances in materials science are indispensable.
This work, supported in part by the U.S. Department of Energy, exemplifies interdisciplinary collaboration spanning universities and national laboratories. By bridging experimental radiation physics, materials science, and applied mathematics, the team’s innovation paves the way for next-generation nuclear technologies to harness graphite’s capabilities with unprecedented confidence and control.
Subject of Research: Investigation of the relationship between pore structure, lattice strain, and volume changes in irradiated nuclear graphite
Article Title: Linking Lattice Strain and Fractal Dimensions to Non-monotonic Volume Changes in Irradiated Nuclear Graphite
News Publication Date: 12-Aug-2025
Web References:
https://onlinelibrary.wiley.com/doi/10.1002/idm2.70008
http://dx.doi.org/10.1002/idm2.70008
Keywords
Nuclear engineering, Nuclear reactors, Carbon, Chemical elements, Energy, Materials science
