By Alexandra Branscombe

WASHINGTON, DC – There are so many ways that repositories for canisters of nuclear waste can leak that at least one country, Sweden, is engineering the canisters themselves to last a million years. In general,  however, the integrity of nuclear waste repositories depends on a host of geologic factors, Jean Bahr, a professor of geoscience at the University of Wisconsin-Madison said at a briefing earlier this month on Capitol Hill.

Sweden plans to build a nuclear waste repository at a granite site. But despite its solid crystalline structure, the formation still contains some fractures. So, the country intends to rely on super-long-lived copper waste canisters to survive corrosion and disturbance for a thousand millennia. Bahr discussed the Swedish plans with GeoSpace in an interview following the March 11 briefing.

Mining crews at the Department of Energy’s Waste Isolation Pilot Plant (WIPP) in Carlsbad, New Mexico. WIPP is built in salt rock, an impermeable and heat tolerant geological structure that is ideal for storing nuclear waste products. Finding a repository site that fits all or most of the geological safety criteria can be very difficult, said Jean Bahr, a professor of geoscience at the University of Wisconsin-Madison.
Credit: U.S. Department of Energy

“The question is how much engineering do you do to get around geographic factors that aren’t ideal?” Bahr said.

Sweden isn’t the only country to face this challenge—other nations are scoping out the best natural geological formations to safely contain nuclear waste. Select rock types, formations and locations can safely store spent nuclear fuel for thousands of years, Bahr said during the briefing.

“It’s the properties of the rocks and the geologic [characteristics] that determine to a great extent if we actually have the feasibility to isolate and contain these hazardous materials for very long periods of time,” she said. That is because chemical, hydraulic, and thermal dynamics can all influence if radioactive waste and spent fuel can be safely contained.

Citing one important rock property, Bahr noted that a potential repository site must have rocks that have low porosity—or empty space—so it is impermeable to flowing water. Low porosity is critical for ensuring that a repository cannot be corroded by water and won’t contaminate groundwater.

Along with porosity, geologists must also consider other factors. Granite makes a good waste storage site because it lacks microscopic pores, Bahr said. But, to the chagrin of the Swedes and others, it can still fracture.

A safe repository must also be able to withstand any possible apocalyptic scenario that could expose or compromise it, Bahr said. These include earthquakes, volcanos, tsunamis, or even a future ice age.

Finding a repository site that fits all the necessary criteria is daunting, admitted Bahr in a post-briefing interview. “If you were to throw a dart at a map and ask, ‘what is the probability that point is a suitable site for a repository,’ the answer is it is probably … very small.” she said.

Two other speakers, Susan Hall, a uranium expert with the U.S. Geological Survey, and Annie Kammerer, a seismologist with Bechtel Corporation, also spoke at the briefing about geoscience-related aspects of nuclear power generation. Nevada State Geologist Emeritus Jonathan G. Price, of the University of Nevada, Reno, moderated the panel.

The briefing was part of the Energy Geosciences Consortium’s Energy from the Earth briefing series highlighting geoscience information on various energy issues for policymakers. The consortium includes the American Geophysical Union, American Association for the Advancement of Science, American Association of Petroleum Geologists, American Geosciences Institute, Association of American State Geologists, Geological Society of America, National Science Foundation Directorate for Geosciences, and the U.S. Geological Survey.

 – Alexandra Branscombe is a science writing intern in AGU’s Public Information department