THERE are times when letting go is the best way to move forward. When the US abandoned plans for a nuclear waste repository at Yucca mountain, Nevada, there was no alternative in sight. Now, less than two months after that decision to walk away from a decades-long, multibillion-dollar boondoggle, a promising solution is coming into view.
What is being proposed is not another Yucca mountain-style set of tunnels in an even more remote location, but hundreds of boreholes that could be spread nationwide, where waste would be sealed several kilometres down in impermeable rock. The approach was discussed by the world’s leading experts on deep borehole repositories at a brainstorming meeting in Washington DC on 15 March. The meeting was organised by geochemist of Sandia National Laboratories and was sponsored by Sandia and the Massachusetts Institute of Technology.
The borehole approach would adapt technology already used by the oil and gas industry, to drill holes roughly half a metre in diameter and 5 kilometres deep. Repositories could be at any location where the bore would penetrate 3 kilometres or more into hard, crystalline basement rock (see map) – meaning most of the US and over a third of the UK would be suitable. Canisters of spent fuel lowered into the borehole would end up stacked one on top of the other, filling the bottom 2 kilometres. This stack would then be sealed in place with a cap of clay, asphalt and concrete.
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The approach relies on the deep rock and geological features to contain the waste – doing away with the need for the carefully engineered corrosion-resistant canisters and protective shields envisaged for Yucca mountain. “Any repository will depend on its geologic barriers to last the million years that are required for the waste to decay,” says of the University of Sheffield, UK. “Other engineered barriers don’t really matter.”
Deep boreholes offer distinct advantages over mined repositories such as Yucca mountain, which would have been about 300 metres below ground. In addition to the physical barrier offered by kilometres of rock, deep boreholes ensure that waste is unlikely to seep to the surface in groundwater. Water found below 2 kilometres or so is highly saline, and therefore far heavier than water closer to the surface. As a result, water at depth – and any radioactive material it could transport – stays at depth. Samples so far taken from basement rock show that water has been stagnating there for hundreds of thousands of years or more.
When the US and other countries started looking into nuclear repositories in the 1970s and 1980s, deep boreholes were not an option, as the drilling technology was not available. Once investment in shallower repositories such as Yucca mountain was under way inertia took hold, as governments were understandably reluctant to take the financial and political risks of switching.
If the US moves ahead with deep boreholes, Yucca mountain may not be the only centralised repository to be abandoned. Studies at the Äspö Hard Rock Laboratory in Sweden suggest that the copper-encapsulated canisters Sweden and Finland plan to use in their shallow mine repositories will corrode much faster than previously expected.
“It now seems that all mined repositories with engineered barriers will fail and that deep boreholes may be a better alternative,” says Johan Swahn of , a Swedish NGO charged with reviewing the country’s plans for storing nuclear waste.
