Model predicts long-term effects of nuclear waste on underground disposal systems

As the global community witnesses a renewed interest in nuclear energy, the perennial challenge of safe and long-term nuclear waste disposal remains a critical concern. In a significant stride towards addressing this issue, a collaborative research effort involving scientists from MIT, Lawrence Berkeley National Lab, and the University of Orléans has unveiled a new high-performance computing model that accurately predicts the long-term effects of nuclear waste on underground disposal systems.

This groundbreaking study, published in the prestigious journal PNAS, demonstrates a strong alignment between sophisticated simulations of underground nuclear waste interactions and real-world experimental results from a research facility in Switzerland. The findings are poised to enhance public and policymaker confidence in the safety and efficacy of deep geological repositories.

Authored by MIT PhD student Dauren Sarsenbayev and Assistant Professor Haruko Wainwright, alongside Christophe Tournassat and Carl Steefel, the research leverages cutting-edge computational tools to model the complex behaviors of radionuclides within geological systems. “These powerful new computational tools, coupled with real-world experiments like those at the Mont Terri research site in Switzerland, help us understand how radionuclides will migrate in coupled underground systems,” explained Sarsenbayev, who is the first author of the study.

Deep underground geological formations are widely considered the safest long-term solution for high-level radioactive waste. The Mont Terri research site in northern Switzerland, operational since 1996, serves as an invaluable international test bed for studying materials like Opalinus clay – a dense, water-tight claystone identified as a key component for engineered barrier systems in nuclear waste repositories.

Sarsenbayev further elaborated on the site’s importance: “It is widely regarded as one of the most valuable real-world experiment sites because it provides us with decades of datasets around the interactions of cement and clay, and those are the key materials proposed to be used by countries across the world for engineered barrier systems and geological repositories for nuclear waste.”

A significant hurdle in modeling nuclear waste interactions with cement-clay barriers has been accounting for electrostatic effects associated with negatively charged clay minerals. To overcome this, Tournassat and Steefel developed CrunchODiTi, an advanced software built upon the established CrunchFlow platform. This unique software is the only one capable of simulating these complex interactions in three-dimensional space, and it is designed for parallel execution on high-performance computing systems.

The researchers applied CrunchODiTi to analyze a 13-year-old experiment, focusing on a critical 1-centimeter “skin” zone between radionuclides and cement-clay barriers. The simulations successfully replicated the experimental data, particularly in accounting for electrostatic effects and the dynamic changes within this interface over time. Sarsenbayev noted the significance of these findings, stating, “The results are quite significant because previously, these models wouldn’t fit field data very well. It’s interesting how fine-scale phenomena at the ‘skin’ between cement and clay… could be used to reconcile the experimental and simulation data.”

This validated model holds immense promise for improving future safety and performance assessments of underground geological repositories. It could guide the selection of appropriate materials, such as clay or salt formations, for long-term storage, providing predictions for the fate of radionuclides over millennia.

Looking ahead, the team anticipates further data from the Mont Terri experiment and plans to compare these with additional simulations. There is also potential for integrating machine learning to develop more computationally efficient surrogate models. The researchers hope their work fosters a broadly supported, long-term solution for nuclear waste storage, embodying the motto of MIT’s Department of Nuclear Science and Engineering: ‘Science. Systems. Society.’