Chris Schneidmiller
WC Monitor
12/11/2015
Rock salt could be less effective than believed as a subsurface container for nuclear waste, according to new research from the University of Texas at Austin.
Salt beds are already used for safe holding of low-level nuclear waste at the Waste Isolation Pilot Plant in New Mexico, and have been discussed as a storage option should the stalled Yucca Mountain repository in Nevada go permanently moribund. They are also employed for waste storage in Germany, according to a UT press release.
While salt in the ground has been considered as a “sealant” around nuclear waste in storage, as reported by The New York Times last year, UT researchers via field tests and laboratory-based 3-D micro-CT imaging determined that rock salt might be more porous than believed and less capable than figured of keeping nuclear waste from contaminating groundwater following potential failure of the storage container.
Using synthetic rock salt, one researcher affirmed long-held suspicions that salt grows more porous at increased depths. Another member of the research team found proof of that in analyzing fluid distribution in 48 Gulf of Mexico hydrocarbon wells owned by oil giant Statoil. Unexpectedly, the researchers also found that fluids at times could move through salt even closer to the surface, which commonly is not the case.
The problem might be “deformation” of rock salt, the university said: “Deformation can stretch the tiny isolated pockets of brine that form between salt crystals and link them into a connected pore network that allows fluid to move.”
The research paper, though, notes that the dihedral angle – the angle between two planes – at the depths generally planned for geologic repositories should block “brine percolation in rock salt,” and that “Lower differential stresses recorded in shallow bedded rock salt suggest that it is more likely to provide an impermeable barrier.” WIPP site disposal rooms are more than 2,000 feet underground, relatively shallow in geologic terms.
“We’re not saying, ‘Oh my God, evacuate the WIPP site’ and whatnot,” researcher Marc Hesse, a computational geoscientist at the university, said in a telephone interview Wednesday.
“I think the best takeaway – The first thing is we found fluids, hydrocarbons in places where we didn’t expect them, based on what we consider sort of the standard theory, and so that just points to maybe we don’t understand this as well as we should. Which doesn’t mean that we can’t go ahead, it just would be nicer if we had a complete control on the fluid distribution,” Hesse said. “The other takeaway message … is that shallow is certainly better than deeper, and undeformed salt is better than deformed salt.”
Further research is needed into the levels of fluid flow in rock salt, according to Hesse.
The study, conducted by researchers from the university’s Cockrell School of Engineering and Jackson School of Geosciences, was published in the Nov. 27 issue of Science magazine.
This study is an example of ongoing research on the performance of salt as a repository medium. Department of Energy policy calls for evaluating the ramifications of potential new scientific discoveries in this area. A DOE spokesman said there was “exhaustive scientific study” in the decades before WIPP opened into how salt formations functioned as a geological repository for transuranic waste in storage.
“The results of these studies were peer reviewed and supported by world-renowned experts,” the spokesman said by email. “In addition, every five years since WIPP opened in 1999, the U.S. Environmental Protection Agency (EPA) evaluates changes in geological conditions and practices at WIPP as part of the recertification process. EPA has continued to determine that the facility meets its performance objectives and all the requirements of EPA’s disposal regulations."
