Characterization of Deep Hydrogeology for Borehole Disposal of High-Level Radioactive Waste

Deep borehole disposal of high-level radioactive waste and spent nuclear fuel is being considered as a potential alternative to shallower mined geologic repositories.  The basic concept of deep borehole disposal consists of drilling a borehole to a depth of about 5,000 m into crystalline basement rocks, emplacement of waste canisters in the lower 2,000 m, and sealing the upper 3,000 m of the borehole.  Preliminary evaluations indicate that deep borehole disposal of radioactive waste would be viable, safe, and cost effective.  Several factors, including high-salinity fluids at depth, low average permeability, geochemically reducing conditions in deep crystalline rocks, and long transport lengths to shallow fresh groundwater resources, suggest long-term isolation of radionuclides from the biosphere.  Deep hydrogeological conditions favorable to waste isolation likely exist over broad regions in North America, particularly in the geologically stable continental interior. 

Hydrogeological characterization for deep borehole disposal should be focused on those aspects of the groundwater system most important to disposal safety, including the following.  Saline groundwater residence time is an important indicator of long-term isolation of the deep hydrogeological system and can be interrogated using a number of natural isotopic tracers.  Salinity stratification of deep fluids would oppose thermally driven circulation of fluids from waste heat.  Overpressured conditions at depth would be unfavorable to waste isolation.  Permeability of the host rock must be evaluated and is expected to have a low average value; although, individual fracture zones may have significantly higher permeability, even at great depth.  Deep geochemical conditions have important implications for radionuclide mobility, with reducing conditions causing lower solubility and greater sorption for many radionuclides.  Differential horizontal stress may impact borehole stability and the disturbed rock zone permeability in deep boreholes.  Chemical and mineralogical interactions with borehole seals may affect long-term seals integrity.