Long-held beliefs about degradation of nuclear reactors get second look

Courtesy of Wikimedia

With nuclear energy getting a second look from many governments around the world and existing nuclear plants aging, it is important to have reliable information about the facilities’ susceptibility to degradation.

New research may be of interest for those considering nuclear energy to reduce fossil fuel consumption while meeting rising energy demands.

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China, Turkey, and Bangladesh are constructing reactors that are likely to come online this year.

In the U.S., multiple states are considering building new nuclear reactors or transitioning old fossil fuel plants to nuclear. The 2022 Inflation Reduction Act provided tax incentives for existing nuclear power plants to keep producing energy, keeping the facilities competitive in the energy market.

Existing nuclear power plants – which peaked in popularity from the 1970s to 1990s – are aging. In the U.S., the average age of nuclear power reactors is about 42 years. While that might not seem very old, concrete in nuclear reactors is put through serious stresses, including radiation and high temperatures. These stresses can cause materials like quartz to expand, potentially encouraging cracks to form in the concrete.

Researchers test for concrete durability, but because the nuclear plant is running, sampling active reactor structures is not possible. Instead, testing is done in a laboratory setting. To mimic the decades of irradiation and temperature stresses, scientists speed up the rates of exposure. For example, they will bombard the cement and aggregate with intense neutron irradiation, cramming 20 years’ worth of radiation exposure into weeks or months.

While these tests can help understand durability, “the correlation between accelerated tests and real-world conditions is a crucial challenge,” said Ippei Maruyama, Dr. Eng., an engineering professor at the University of Tokyo and Nagoya University. He and his colleagues wanted to better understand if differences in neutron flux – the number of neutrons irradiated per unit area per unit time – would lead to different degrees of concrete degradation.

In their Journal of Nuclear Materials paper, Maruyama and his fellow researchers focused on how quartz behaves under these high-dose, accelerated experiments and what that might mean for predicting the integrity of aging nuclear concrete structures.

Radiation and concrete

Concrete is a robust and heterogeneous building material used throughout buildings and structures in nuclear power plants. The aggregate within the concrete is a major component of the material and can differ by geographic location. However, the rock material is often sourced from nearby rocks and often contains a good amount of quartz.

This quartz is of particular interest to researchers who evaluate concrete durability in nuclear structures.

“When it’s subjected to irradiation, quartz basically turns into glass,” noted Mathieu Bauchy, Ph.D., an assistant professor of materials science at UCLA, who was not involved in the study. “It’s amorphous – it’s kind of like a frozen liquid.” Bauchy added that this version of quartz is much more reactive than the crystalline form found in the aggregate rock.

In previous work, accelerated testing showed these quartz crystals amorphizing, expanding, and decreasing in density, said Maruyama. At the same time, irradiation can cause the cement holding the aggregate together to shrink. The mismatch of swelling and shrinking stresses can trigger cracking in the concrete.

To test the durability of concrete, including the expansion of quartz, researchers use laboratory testing to mimic the irradiation produced in a decades-old structure. This means that each sample is exposed to an accelerated rate of neutron exposure compared with the slower dose experienced in real-world conditions. But Maruyama and his colleagues wanted to test whether this accelerated exposure fully captured the behavior of irradiated quartz aggregate.

To test this, the team used three types of parent rock from Japanese concrete aggregates that contained quartz and placed them under nearly identical temperature conditions but with different neutron flux environments. After the dosing, they used powder X-ray diffraction to monitor changes within the quartz.

Density changes were particularly useful in understanding quartz behavior. “In our previous studies, we observed that neutron-irradiated aggregate specimens sometimes developed independent internal crack openings,” Maruyama said. “Therefore, we used cell volume for comparative analysis.”

The team found that after being irradiated, the quartz dimensions increased and density decreased. But the team also noted a surprising result – the higher rate of the neutron flux, the greater the expansion of quartz.

“In conventional accelerated tests, it has been shown that neutron irradiation makes aggregate expansion, and the aggregate expansion leads to concrete deterioration. However, our findings indicate that neutron flux significantly influences this expansion,” Maruyama explained. “In the actual environment of a nuclear power plant, the expansion could be extremely small. This is a remarkable and unexpected discovery.”

Courtesy of Ippei Maruyama

Bauchy agreed, noting that the main novelty of the study is that there is an effect of rate, not just the neutron fluence, or total number of neutrons per unit area – the faster the rate, the more damage to quartz.

“What they are implying is that the experimental test in accelerated conditions tends to overestimate the damage in concrete as compared to what you would experience in real conditions,” he said.

“It is good news in one way because it means that it might not be as bad as we thought,” Bauchy continued. “I think the bad news is that there is really no way to know what’s actually going on except by looking into the plant itself, which we can’t do while it’s operating.”

The future of nuclear concrete

The team used its results to create a model to predict what might happen to nuclear concrete structures in real-world conditions. Although this sort of prediction would be helpful to operators and designers, Bauchy said the current data is too limited to predict what might happen in real concrete plants. But he noted that more investigations in the laboratory and on-site would bolster modeling efforts.

“Once a concrete plant closes and is decommissioned, that's probably still the best way to really see what has happened there and understand what effect decades of irradiation has on those rocks,” Bauchy said. “I hope that this study is going to stimulate more work and maybe more funding in this area.”

Maruyama agreed, adding that the work shows the value in investing in future studies of concrete used in nuclear plants around the world. “The discovery of the existence and mechanism of the effect, along with the possibility that neutron flux at actual levels may not cause significant issues, represents a major advancement in both science and engineering,” he said.

“If we can experimentally confirm expansion behavior under more realistic conditions or directly analyze materials from decommissioned plants to demonstrate that expansion effects are negligible, concerns about neutron-induced degradation will be significantly reduced,” he said. “This would also serve as strong evidence for nuclear power plant owners to ensure the safety of long-term operation.”