With the potential to triple globally by 2050, nuclear energy capacity has become a growing priority for countries around the world. However, while nuclear power could significantly help address global energy demands, a substantial challenge remains: developing safe disposal systems for spent nuclear waste, which can be radioactive and hazardous for thousands of years.
EESA scientists have developed first-of-its-kind simulation tools to demonstrate how dissolved charged species, referred to as ions, move through the barrier systems designed to enclose radioactive waste permanently deep within Earth’s subsurface. This breakthrough was achieved through a study published last week in the Proceedings of the National Academy of Sciences (PNAS) that utilized high-performance computing software developed at Berkeley Lab and data from an ongoing real-world experiment conducted at a Swiss underground rock laboratory.
Using high-performance computing to validate barrier system performance
The project, funded by the U.S. Department of Energy’s Spent Fuel Program, was led by Berkeley Lab scientist Carl Steefel and Christophe Tournassat, a visiting faculty affiliate from the University of Orléans in France. They worked with Dauren Sarsenbayev, a first-author of the paper and graduate student at Massachusetts Institute of Technology (MIT) supervised by Assistant Professor Haruko Wainwright. The work also was made possible through funding from the Department of Nuclear Energy Ingenuity Program, an internship program at Berkeley Lab developed to train the next generation of experts helping to improve our understanding and predictive capabilities for geological disposal of nuclear waste.
The research successfully replicated the behavior of selected charged and uncharged species intended to represent the radionuclides expected to be present within the complex natural and engineered barrier systems used in underground nuclear waste repositories. These barriers typically consist of multiple engineered and natural materials, such as concrete and the clay-rich geological formations that are present nearly a mile below ground.
Although the barrier systems are designed to contain the radionuclides disposed of underground for hundreds and even thousands of years, the gradual failure of the packages can lead to dangerous radionuclides beginning to migrate out from the disposal sites and into the biosphere. The findings published here highlight the crucial role of innovative computer modeling to accurately predict the transport of radionuclide species at a sufficiently high resolution. Such analyses are essential for developing a defensible safety case for the underground disposal of radioactive waste.
Barrier systems that store nuclear waste can be either natural or engineered. Natural barrier systems take advantage of the properties of natural rock formations like clay-rich shale to dispose of the waste (typically “spent nuclear fuel”), relying on their low permeability and high adsorption, allowing ions to adhere to the surface. Engineered barrier systems allow the disposal of nuclear waste in human-made containers or structures that are designed to limit fluid flow and molecular diffusion, and thus migration of the radionuclides out to the biosphere. The team validated their simulation capability with high-performance computing (HPC) software CrunchODiTi developed at Berkeley Lab by Tournassat and Steefel, applying it to a 13-year long experiment at the Mont Terri Rock Underground Laboratory in Switzerland.
Combating challenges in nuclear waste disposal
“Each of the materials within the natural and engineered portions of an underground nuclear waste repository has its own set of chemical and physical properties, and this leads to potentially very different behavior for the dissolved radionuclides in solution expected in any underground repository,” explained Tournassat. “In this simulation, we accounted for specific properties of clay that affect the diffusion rate of charged nuclei.”
The experiment investigated the diffusion of substances, particularly radionuclides like tritiated water (HTO) and chloride-36 (36Cl-), through the interface between concrete and Opalinus Clay. Clays are negatively charged, which leads to a nanoscale phenomenon known as “anion exclusion” wherein negatively charged ions (anions, for example the radionuclide 36Cl-) migrate at different rates than either positively charged ions (cations) or uncharged species. An accurate prediction of migration rates requires that these nanoscale processes affected by the electrostatics of the clay be considered explicitly at the geological formation scale.
Steefel identifies an additional challenge. “The problem of designing a safe geological nuclear waste repository and predicting its behavior into the future, often over geological time scales, is made additionally challenging by the fact that the physical and chemical properties of interfaces between materials evolve over time.
“These changes over time are often focused at reactive interfaces, for example between high pH concrete and clay-rich rock, so addressing these effects requires that the computer simulation tools can achieve high spatial resolution, often on the order of centimeters.”
In commenting on the broader use of computer simulation modeling, Sarsenbayev said in a statement issued by MIT, “If the U.S. eventually decides to dispose of nuclear waste in a geological repository, then these models could dictate the most appropriate materials to use. These models allow us to see the fate of radionuclides over millennia. We can use them to understand interactions at timespans that vary from months to years to many millions of years.”
This study demonstrates the importance of simulations, grounded in real-world data, in validating the effectiveness of barrier systems and developing public trust in geologic nuclear waste disposal. As nuclear energy capacity and investments grow, simulations like these can help inform and accelerate solutions to addressing the critical issue of safe, reliable geologic nuclear waste disposal.
