A fundamental study into underground permeability can help society harness nuclear energy as a safe, stable and secure energy source.
As society’s demand for sustainable energy sources continues to grow, nuclear energy is still seen as a viable and productive option for powering the nation. However, concerns over the safe and secure storage of high-level radioactive nuclear waste often keep the technology from being advanced. An example of this is the Yucca Mountain nuclear waste repository, which until recently, was being developed as a deep geological storage facility for spent nuclear fuel.
Now, researchers at Virginia Tech have been awarded major funding by the Department of Energy’s Office of Nuclear Energy to gain deeper insight and knowledge into the permeability of the clay barriers used in nuclear waste repositories, such as those used at Yucca Mountain.
Dr. Cheng Chen, mining and minerals engineering assistant professor, is serving as the lead PI on a project titled, The role of temperature on non-Darcian flows in engineered clay barriers. He is joined by Dr. Rui Qiao of Virginia Tech’s mechanical engineering department, who brings additional expertise in areas such as nanoscale fluid and ion transport and will serve as the project’s Co-PI. The project is funded for three years and is being carried out in collaboration with Sandia National Laboratories.
High-level nuclear waste can be permanently disposed of in subsurface geological formations, first by storing it in a container and then placing that container into a geological host rock. Low-permeability clay, such as compacted bentonite, is filled between the waste container and the host rock underground.
“For this kind of storage system,” explains Chen, “long-term security and stability are critical and require an advanced, fundamental understanding of the geomechanical, hydrogeological, and thermal processes relevant to this waste disposal system.”
The project is centered on the concept of non-Darcian flow as it pertains to nuclear waste repositories. In the field of fluid mechanics, Darcian and non-Darcian flows describe the manner in which fluids flow through porous mediums, with non-Darcian flows being more applicable to the conditions found when storing underground nuclear waste in deep geologic structures.
According to Dr. Chen, having an advanced understanding of non-Darcian flow in the clay buffer used to “pack” the nuclear waste containers is critical. “This understanding would allow us to accurately predict water migration and saturation evolution over a time scale of thousands of years throughout fields and locations near the repository,” he adds.
The role of temperature on non-Darcian flows in saturated and unsaturated clays is also critical. “Unfortunately, experimental data with respect to the role of temperature on non-Darcian flows are rare,” says Chen. “To the best of our knowledge, very limited experimental data are available for saturated flow, and no experimental data are available for unsaturated flow.”
At the Laboratory for Flow and Transport in Porous Media
“The knowledge generated from this project will directly benefit the long-term security and stability of subsurface nuclear waste repositories over a time scale of 100,000 years” --Cheng Chen
Dr. Chen plans to carry out much of his research at his Laboratory for Flow and Transport in Porous Media, located in the Corporate Research Center near campus. Yuntian Teng, a PhD candidate working with Dr. Chen, will assist on the project and research. The team’s first objective will be to develop a predictive, theoretical model to facilitate experimental data interpretation and provide mechanistic insights into the role of temperature on non-Darcian flows in low-permeability engineered clay barriers.
Afterwards, they will conduct laboratory experiments over a wide range of temperatures to unravel the role of temperature on the threshold gradient of non-Darcian flow in both saturated and unsaturated bentonite clays.
Finally, the team plans to use molecular dynamics (MD) simulations in order to obtain a fundamental understanding of non-Darcian flow and its dependence on temperature and the interface of mineral and water chemistry.
Dr. Riu Qiao, along with mechanical engineering graduate researcher Chao Fang, will focus their research contributions on the development of MD simulation capabilities to investigate the influence of temperature on the threshold gradient of non-Darcian flow in low-permeability porous media. The MD simulation results will be fitted to a continuum-scale, two-parameter predictive model proposed by Dr. Chen, and which has been accepted for publication in Hydrogeology Journal.
Fitting the MD simulation results to the continuum-scale model will provide insights into the two parameters and advance the understanding as to how these two parameters change under varying temperatures.
Mechanical Engineering Department Collaborative Team
While this project's first steps focus on answers to critical and fundamental fluid dynamics questions, Dr. Chen emphasizes its significant environmental and societal impacts. “The knowledge generated from this project will directly benefit the long-term security and stability of subsurface nuclear waste repositories over a time scale of 100,000 years,” he explains.
In addition, results of the team’s research may have broader implications for the oil & gas industries. “Understanding Non-Darcian flows in shale can advance our understanding of oil and gas flow in unconventional hydrocarbon reservoirs, such as shale reservoirs,” an equally critical source for future energy security.