Rock salt domes and old salt mines have been widely hailed as the safest place to dispose of radioactive waste. However, field testing suggests their protective value has been overrated, and it may be back to the drawing board in the quest to find a safe place for the waste from nuclear power stations. The news isn't all that good for the oil and gas industry either, since this work raises the possibility that leaks from drilling sites may reach further than has been acknowledged.
Keeping out groundwater is considered one of the most essential ingredients for long term disposal of any dangerous material. Rock salt domes in sedimentary basins have been considered ideal material in this regard, as thick salt deposits are known to be very resistant to groundwater incursions. However, PhD student Soheil Ghanbarzadeh of the University of Texas, Austin is first author of a paper published in Science showing it isn't quite as resistant as has been believed.
Ghanbarzadeh examined oil and salty water around 48 oil and gas wells in the Gulf of Mexico, which run through large salt bodies. He found oil was getting to places it shouldn't. The deeper the well the more permeable salt became, letting oil escape. This was in line with long standing expectations by geologists and previous evidence that oil can infiltrate salt deposits.
More surprising was that, even at quite shallow depths, fluids were sometimes making their way through salt expected to be impermeable.
Coauthor, and Ghanbarzadeh's supervisor, Dr. Maša Prodanović said in a statement: "What this new information tells us is that the potential for permeability is there and should be a consideration when deciding where and how to store nuclear waste."
Ghanbarzadeh and Prodanović used X-ray microtomography on synthetic rock salt samples to find that even tiny pockets of brine between salt crystals can link up to form a network that allows fluids to make their way through the salt.
Previous studies have shown that pressure applied during engineering work, for example the construction of a waste facility or the drilling of oil wells, can deform salt so that cracks open up. Ghanbarzadeh and Prodanović's work indicates this deformation can happen naturally, making it far harder to control. The paper notes that “viscous flow of rock salt due to the density contrast with the surrounding sediments” may explain the changes, particularly where stresses are high and a small amount of salty water is present.
While nuclear waste is expected to be encased in durable materials before burial, this is not thought to be a permanent solution on its own. If ground water can infiltrate the waste site it may corrode metal casings, and there is a danger it will become contaminated with radioactive elements that then get carried into the wider environment.
Dr. Mark Hesse, another author on the paper, added: "Further work is necessary to study the quantity of flow that can occur.” But Prodanović suggested that even existing waste disposal sites should be re-evaluated.
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