Salt formations, used by the United States and Germany for storage of nuclear waste, might not be as impermeable to groundwater as once believed.
That's the worrisome conclusion of a new study published in the journal Science by University of Texas at Austin researchers, who warn that nuclear waste might leak if storage vessels fail.
The researchers used field testing and 3-D micro-CT imaging of laboratory experiments to study salt deposits' ability to stop fluid flow at shallow depths, a quality that allows oil reservoirs to form. Scientists have long suspected that salt becomes permeable at greater depth.
When the researchers studied salt formations in oil and gas wells, however, they discovered that not only was it permeable deep down, but that fluids sometimes flowed through the salt even at shallow depth.
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The problem may be the plasticity of rock salt. When it deforms, tiny isolated pockets of brine, or salty water, form between salt crystals and link them to a pore network that allows fluids to move. This can happen naturally, even when the salt deposit isn't disturbed further by humans.
"The critical takeaway is that salt can develop permeability, even in absence of mining activity," assistant professor Marc A. Hesse of the Jackson School's Department of Geological Sciences, said in a press release. "Further work is necessary to study the quantity of flow that can occur."
The research has important implications for the U.S. Department of Energy's Waste Isolation Pilot Plant, a 26-year-old facility near Carlsbad, N.M., where the government has placed 55-gallon drums containing 91,000 cubic meters of radioactive waste from its nuclear weapons program.
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The waste is stored in immense rooms carved from the salt formations, 2,150 feet below the ground. The facility has room for more than 175,000 cubic meters of material.
In February 2014, a fire involving a salt-hauling truck occurred inside the underground facility. Nine days later, in a separate incident, a chemical reaction inside a drum cracked a lid and caused the release of plutonium and americium from one container, according to the Albuquerque Journal. It was later discovered that the drum and hundreds of others had been improperly packed, according to the newspaper.
The U.S. once had a plan to store nuclear waste from power plants in Yucca Mountain, Nev., but that plan was shelved in 2009. Today, waste is stored on-site at the plants where it is generated.
Germany has three such repositories for nuclear waste, though two of them have been closed.