[Image above] The core of a nuclear reactor vessel. Credit: Nuclear Regulatory Commission; Flickr CC BY-NC-ND 2.0
Currently, more than 440 commercial nuclear power reactors operate in 31 countries, supplying a total energy capacity of 390,000 MWe—and providing more than 11% of the world’s electricity, according to the World Nuclear Association.
While that power comes completely sans carbon emissions, it also generates a significant amount of nuclear waste.
According to the Nuclear Energy Institute, the nuclear industry generates ~2,000 metric tons of used nuclear fuel every year. “Over the past four decades, the entire industry has produced 76,430 metric tons of used nuclear fuel. If used fuel assemblies were stacked end-to-end and side-by-side, this would cover a football field about eight yards deep,” according to the website.
That’s a lot of irradiated materials. Besides the obvious containment and storage issues for this quantity of hazardous wastes, one of the most challenging problems is that we really don’t understand a lot about how materials themselves change in response to irradiation—and that information is crucial for ensuring that storage techniques keep this potentially dangerous waste safely contained for years to come.
This is a critical issue because absorbing the energy of radiation rearranges the atoms in a material, creating structural disorder, and thus changing the properties of the material. And because irradiated materials are disordered, they have long been assumed to atomically resemble the amorphous structure of glasses.
But are those assumptions valid?
New models, new insights
New atomistic simulations by researchers at the University of California, Los Angeles and Oak Ridge National Lab (Oak Ridge, Tenn.) are revealing that assumption—which has been used to model, design, and predict irradiated materials—may be entirely incorrect. Instead, their results indicate that irradiated materials are even more disordered that previously thought.
Because it’s really difficult to track how individual atoms in a material move, adjust, and rearrange over time, the team used a simulation technique called reactive molecular dynamics to model how atoms within a sample of quartz—a well-studied and often-used material—interact with one another upon irradiation.
“The molecular dynamics technique is based on numerically solving Newton’s laws of motion for a group of interacting atoms,” Mathieu Bauchy, assistant professor in Civil and Environmental Engineering at UCLA and senior author of the research, says in AIP press release. “All atoms apply a force on each other that can be used to calculate the acceleration of each atom over time.”
Using these simulations, the team could predict how quartz’s atomic structure changes when the material is irradiated. The scientists used similar simulations to model the structure of vitrified quartz, and compared differences in the atomic structure between the two simulated samples.
The comparisons show that irradiated quartz and vitrified quartz are rather different from one another. “This is quite surprising because glasses and heavily irradiated materials typically exhibit the same density, so that glasses are often used as models to simulate the effect of the exposure to radiation on materials,” Bauchy says in the release.
The simulations show that irradiated quartz is actually more disordered than vitrified quartz, with an atomic structure that is actually more akin to a frozen liquid than glass.
Credit: Mathieu Bauchy; YouTube
But these are simulations, not experiments with actual samples of material—so how can the researchers be confident that the models accurately predict what’s going to happen in the real world?
“The reliability of such simulations entirely depends on accurate knowledge of the forces applied by atoms on each other,” Bauchy explains via email. The team modeled those forces, and then tested the model to see how well it predicted the known structures of crystalline quartz and glassy silica. “Good agreement, both in trend and absolute values, makes us confident in the ability of this technique to accurately model irradiation-induced damage in quartz.”
Those models show that the differences aren’t minor. According to Bauchy, the structural difference between irradiated and vitrified quartz is quite remarkable.
“In the short-range atomic order (<3 Å), irradiated quartz exhibits increased disorder that is associated with the presence of mis-coordinated atomic species, such as 3-fold coordinated oxygen, 5- and 6-fold coordinated silicon atoms, and silicon polyhedra shearing edges,” he explains via email. “All of these defects are virtually absent from the structure of glassy silica. The medium-range atomic order (>3 Å and <10 Å) also reveals some significant differences.”
Simulated atomic structure of partially irradiated quartz. Credit: N.M. Anoop Krishnan; UCLA
Are nuclear storage methods safe?
In addition to a better understanding of the structural disorder of irradiated materials, however, the research also has important implications for current methods of storing irradiated materials.
While low- and medium-level nuclear waste is primarily contained using cement-based materials, high-level nuclear waste is preferentially vitrified, or encased in glass, for storage.
As Bauchy explains, vitrified materials are considered to “self-heal” under irradiation because of their amorphous structure, which was previously assumed to be similar to the structure of irradiated materials.
But the structural differences identified by the team’s simulations suggest that the structure of glasses themselves might be altered by irradiation—which then questions the safety of vitrified nuclear waste.
“…Present models might be underestimating the extent of the damage exhibited by materials subjected to irradiation, which raises obvious safety concerns,” N.M. Anoop Krishnan, postdoctoral researcher at UCLA and first author of the new work, says in the AIP release.
Bauchy adds that the team is next working to develop simulations to predict how irradiation affects the structure of glasses to try to understand how vitrified waste is affected. In addition to ensuring the safety of existent storage techniques, he adds, the research might also identify new new radiation-resistant glass formulations.
Although glass is a preferred storage method for hazardous waste, however, it’s also an expensive option.
According to a recent report from the U.S. Government Accountability Office, cement-based storage methods can adequately sequester nuclear waste at a much lower cost. And based on these new questions about the safety of vitrified waste, cement may offer a more attractive alternative in that regard as well.
“Immobilizing high-level nuclear waste within cementitious wasteforms would offer a faster, cheaper, and easier alternative to conventional vitrification,” Bauchy writes via email. “However, establishing cementitious wasteforms as a realistic alternative to glasses requires an accurate knowledge regarding the long-term resistance to cracking and corrosion of cementitious materials subjected to radiation.”
So it’s not surprising that Bauchy and his team are also using their atomistic simulations to model how irradiation affects cement’s structural disorder as well.
“We intend to carry out an extensive study to understand the effect of irradiation on calcium–silicate–hydrate (C–S–H, the ‘glue’ of concrete) and on typical aggregates used in concrete. Such a study would help to elucidate the ‘genome’ of concrete—that is, to understand how composition and structure control their properties.”
The open-access paper, published in The Journal of Chemical Physics, is “Irradiation- vs. vitrification-induced disordering: The case of a-quartz and glassy silica” (DOI: 10.1063/1.4982944).