Aging reactors: For the first time, researchers verify the effect of radiation on concrete expansion

It’s been known for some time that radiation impacts the structural integrity of concrete. However, until now the details of this were unknown. Researchers, including those from the University of Tokyo, can finally demonstrate what properties of concrete affect its structural characteristics under different neutron radiation loads.

Their findings raise some concerns while reducing others; for example, quartz crystals in concrete can heal themselves, potentially allowing some reactors to run for longer than initially thought possible. The research is published in the Journal of Nuclear Materials.

Some high-profile incidents involving nuclear power stations naturally raise fear in people. But many believe nuclear power to be one of the cornerstones for achieving a carbon-neutral world. This places an emphasis on finding ways to improve safety, reliability, cost-effectiveness and other things, to reduce fears and increase receptiveness to this technology.

One aspect of nuclear power stations that relates to safety and also longevity lies in the materials used in their construction; in particular, the concrete used throughout the buildings. It’s known to be a very robust material and has been studied for a long time to better understand its material characteristics.

But only now have researchers been able to explore in detail the way that neutron radiation from nuclear reactors can impact concrete’s longevity.

“Concrete is a composite material made up of multiple compounds. These can vary depending on various factors, including local geography, especially the rock aggregate which is a major component in concrete. But rock will often contain quartz. So, understanding how quartz changes under different radiation loads can help us predict how concrete should also behave in general,” said Professor Ippei Maruyama from the Department of Architecture.

“Neutron radiation-induced degradation is a particularly costly area of study, making extensive research difficult. Our research team has been addressing this issue since 2008, formulating strategies to solve the problem by consulting a wide range of literature and conducting interviews with experts. This culminated in our recent experiments using X-ray diffraction to look at irradiated quartz crystals.”

Among other things, Maruyama and his team looked at two properties of neutron radiation: the total dose the samples receive and the rates at which they receive it, or flux.

What they found was a little surprising at first. For a given total dosage of neutron radiation, the amount of expansion in a quartz crystal was far higher when the dose rate was higher, and vice versa.

As an analogy, you could think about the impact of the sun on your skin—it’s commonly advised not to spend too much time exposed to direct sunlight without protection, while it’s less of a concern to receive the same exposure spread out over a longer stretch of time.

“The discovery of the flux effect indicates not only that neutron radiation distorts the crystal structure, causing amorphization and expansion, but that there is also a phenomenon where the distorted crystals recover and the expansion diminishes, hence a lower rate affords more time to heal,” said Maruyama.

“We also saw this phenomenon depends on the size of the mineral crystals within concrete. Larger crystal grains exhibited less expansion, suggesting a size-dependent effect. Considering these findings, the degradation of concrete due to neutrons, which is currently a concern, may involve less expansion than previously thought.

“Consequently, degradation may be less severe than anticipated, potentially allowing nuclear power plants to operate more safely over longer periods.”

The team now aims to address several challenges in understanding the expansion behavior of different rock-forming minerals, further clarifying the mechanisms of expansion and developing the ability to predict the expansion of aggregates based on their material properties and environmental conditions.

The team also seeks to predict the way cracks form based on mineral expansion. This research could contribute to the selection of materials and design of concrete for future nuclear power plants. Additionally, it may provide valuable insights into the durability and stability of inorganic materials used in space-based structures for extraterrestrial construction in orbit of Earth, and beyond.