Using new high-performance computing software, the researchers were able to simulate the migration of radionuclides in natural and engineered barrier systems consisting of multiple materials distributed in complex 3D geometries. The models were validated using experimental data from the Mont Terri Project in Switzerland, an international research project for the hydrogeological, geochemical, and geotechnical characterization of an Opalinus Clay formation.
The study, which was coauthored by MIT Ph.D. student Dauren Sarsenbayev and assistant professor Haruko Wainwright, along with Christophe Tournassat and Carl Steefel, appears in the journal PNAS.
Crunch time: The software used in the study, called CrunchODiTi, was developed from established software known as CrunchFlow. Most recently updated this year, the software is designed to be run on many high-performance computers at once in parallel. Unlike other models commonly used to simulate radionuclide interactions with the cement-clay interface, CrunchODiTi accounts for the electrostatic effects associated with the negatively charged clay minerals in the barrier materials, simulating the interactions in 3D space.
For the study, the researchers looked at a 13-year-old experiment at Mont Terri that initially focused on cement-clay rock interactions. Within the past several years, a mix of both negatively and positively charged ions were added to the borehole located near the center of the cement emplaced in the formation. The researchers focused on a 1-centimeter-thick zone between the radionuclides and cement-clay referred to as the “skin.” They compared their experimental results to the software simulation, finding the two datasets aligned.
“The results are quite significant because previously, these models wouldn’t fit field data very well,” said Sarsenbayev. “It’s interesting how fine-scale phenomena at the ‘skin’ between cement and clay, the physical and chemical properties of which changes over time, could be used to reconcile the experimental and simulation data.”
Sarsenbayev added, “It’s been hypothesized that there is mineral precipitation and porosity clogging at this interface, and our results strongly suggest that.”
Further work: According to MIT, the new model could replace older models used to conduct safety and performance assessments of underground geological repositories and improve confidence among policymakers and the public in the long-term safety of nuclear waste disposal.
“If the U.S. eventually decides to dispose nuclear waste in a geological repository, then these models could dictate the most appropriate materials to use,” said Sarsenbayev. “For instance, right now clay is considered an appropriate storage material, but salt formations are another potential medium that could be used.”
Sarsenbayev said the model is reasonably accessible to other researchers and that future efforts may focus on the use of machine learning to develop less computationally expensive surrogate models.
MIT said further data from the Mont Terri experiment will be available later this month and the team plans to compare those data to additional simulations.
