Deep borehole disposal

Deep borehole disposal is the concept of disposing of high-level radioactive waste from nuclear reactors in extremely deep boreholes. Deep borehole disposal seeks to place the waste as much as five kilometers beneath the surface of the Earth and relies primarily on the thickness of the natural geological barrier to safely isolate the waste from the biosphere for a very long period of time so that it should not pose a threat to man and the environment. In the diagram the solution domain is used for the purpose of computer modelling of heat flow around the borehole.[1]



Deep borehole nuclear waste disposal works by drilling deep into the Earth's crust


The concept involves drilling a borehole about 5 km down into the Earth's crust. High level waste, like spent nuclear fuel, would be sealed in strong steel containers and lowered down the borehole, filling the bottom one or two kilometres of the hole. The rest of the borehole is then sealed with appropriate materials, including perhaps clay, cement, crushed rock backfill, and asphalt, to ensure a low-permeability barrier between the waste and the land surface. In some concepts, waste may be surrounded by cementitious grout or a highly-compacted bentonite buffer matrix to provide improved containment and to attenuate the impact of rock movements onto the canisters integrity. A high-temperature scenario involves very young hot waste in the containers which releases enough heat to create a melt zone around the borehole. As the waste decays and cools, the melt zone resolidifies, forming a solid granite sarcophagus around the containers, entombing the waste forever.[2] Under both scenarios, chemically reducing conditions adjacent to the borehole will reduce the transport of most radionuclides.

The deep borehole concept can be applied to any amount of waste. For countries that do not rely on nuclear power plants, their entire inventory of high-level nuclear waste could perhaps be disposed of in a single borehole. Current estimates suggest that spent fuel generated from a single large nuclear power plant operating for multiple decades could be disposed of in fewer than ten boreholes. Borehole disposal programs could be terminated at any time with little loss of investment because each borehole is independent. The modular nature of borehole disposal would lend itself to regional, or on-site, disposal of nuclear waste. Another attraction of the deep borehole option is that holes might be drilled and waste emplaced using modifications of existing oil and gas drilling technologies.

Finally, the environmental impact is small. The waste handling facility at the wellhead, plus a temporary security buffer zone, would require about one square kilometer. When the borehole is filled and sealed, the land can be returned to a pristine condition.


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