DEEP BOREHOLE DISPOSAL OF SPENT NUCLEAR FUEL AND HIGH-LEVEL WASTE

Deep borehole disposal of spent nuclear fuel (SNF) from nuclear power plants or solidified high-level radioactive waste (HLW) from the reprocessing of nuclear fuel is a concept dating from the mid-1970s. The current concept of deep borehole disposal proposed by U.S. Department of Energy (DOE)-funded researchers involves drilling a large diameter (~ 0.5 m) borehole to a depth of 5 km in crystalline basement rock, emplacing waste packages of consolidated SNF assemblies or solidified HLW in the lower 2 km of the borehole, and sealing the upper 3 km of the borehole. The U.S. Nuclear Waste Technical Review Board (NWTRB), established by the Nuclear Waste Policy Amendments Act (NWPAA), evaluates the technical and scientific validity of DOE activities related to implementing the NWPAA and provides objective expert technical advice on nuclear waste management to Congress and the Secretary of Energy. The NWTRB has identified a number of technical challenges associated with the proposed deep borehole disposal concept. These challenges include drilling technology (e.g., drilling to 5 km with a diameter of 0.5 m has never been demonstrated), well casing and sealing technology, consolidating and repackaging waste (e.g., all solidified HLW is in 0.61m diameter canisters, wider than proposed borehole diameter), emplacement of the waste, sealing the boreholes, retrieval of the waste, and characterization of the site at depth. Assumptions that less site characterization would be needed at great depth because conditions likely would be more homogeneous, and that potentially advantageous conditions (i.e., a reducing environment, low isotropic permeability, and highly saline, density-stratified conditions) are found everywhere, are not supported by data (e.g., deeply buried basement rock can have considerable variability in chemical and physical properties). The deep borehole concept does not eliminate the need for a mined geologic repository for disposal of wastes that are deemed unsuitable for deep borehole disposal.