John Beswick opines that the possibility of disposing of radioactive waste in very deep boreholes has been considered for more than 50 years. A deep borehole disposal (DBD) concept was researched in some detail by the U.S. Department of Energy’s Office of Nuclear Waste Isolation (ONWI) in the early 1980s and reported on in 1983. At that time, the ONWI report rejected DBD of radioactive waste on the basis that the necessary hole sizes and depths were not achievable. Since the 1980s, the technology for drilling and supporting deep boreholes has advanced dramatically, such that this is no longer true.
In recent years, the idea of DBD has been considered by a few waste management organizations, notably Svensk Känbränslehantering AB (SKB), which first reported in the late 1980s, and reviewed again in 2000-2001, a study on deep drilling considerations in which the author was involved. Other studies were reported by Sheffield University in the United Kingdom and the Massachusetts Institute of Technology in 2003. Further studies were carried out in the U.K. for the Radioactive Waste Management division of the Nuclear Decommissioning Authority (NDA) in 2008.
To date, most considerations of DBD for radioactive waste have focused on a disposal zone (DZ) for the waste in the crystalline rock basement. The limited number of deep and ultra-deep boreholes drilled primarily for geoscientific exploration and engineered geothermal systems (EGS) in the basement over the last 45 years have provided much detail about the geology at depth at the locations concerned and, of particular importance for DBD, experience and information on the drilling methods and tools that will withstand the severe environment in these hot, hard, stressed, and abrasive formations [to drilling bits].
Historically, however, there has been little demand for deep and very deep holes in crystalline rock to these depths in these formations, and no experience of drilling deep holes to 5,000 meters in large diameters. Hence the need, at this stage, to extrapolate data from experiences in deep and very deep small-diameter holes and large-diameter drilling technology used for mining applications. This contrasts with the vast amount of data that is available from the many thousands of deep wells drilled in the oil industry, but drilled mainly in sedimentary formations.
A deep borehole for radioactive waste disposal comprises two fundamentally different stages. The first is the borehole construction itself, which, as in all deep drilling, poses risks. While these risks may give rise to troublesome situations during drilling, they can usually be remediated. Once the borehole has been successfully drilled, the status changes and the borehole becomes a radioactive waste facility. Hence, the second phase, comprising the waste emplacement and final sealing, must have a very high level of guaranteed success or an extremely low probability of failure. This unusual requirement for deep boreholes must pervade the design and execution of all the activities related to such a project.
Granitic basement rock at suitable depths for DBD underlies much of the continental crust. Experience over the last 45 years in geoscientific and geothermal energy boreholes provide considerable data on drilling in granitic rocks. While very different from the geological conditions generally encountered in the oil and gas industry, this data allows a detailed design to be undertaken with confidence.
[FYI: Work done in the 1970s on groundwater in igneous and metaphoric (aka crystalline or hard) rocks indicates that flow in the fractures and joints may allow deep radioactive waste fluids to move into shallow (<1,500 feet) zones often used as drinking water. See: https://i2mconsultants.com/downloads/ela-iet/GWinHardRxUNESCO.pdf
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