HYDROGEOLOGY OF DEEP BOREHOLE DISPOSAL FOR HIGH-LEVEL RADIOACTIVE WASTE

Deep borehole disposal of high-level radioactive waste and spent nuclear fuel is under consideration as a potential alternative to shallower mined repository options. The disposal concept consists of drilling a borehole into crystalline basement rocks to a depth of 5 km, emplacement of canisters containing solid waste in the lower 2 km, and sealing the upper 3 km of the borehole. A number of factors suggest that deep borehole disposal is viable and safe, including large areas in stable continental regions with depths to crystalline basement of less than 2 km, availability of adequate drilling technology, low bulk permeability and high salinity in deep crystalline rocks, and geochemically reducing conditions, which limit the solubility and mobility of many radionuclides. Indications are that groundwater in the tectonically stable, deep crystalline basement is very old, has a long history of chemical interaction with the rock matrix, and is unlikely to interact with shallower groundwater resources at many locations. Such indications of hydrogeological isolation are critical to establishing the safety of deep borehole disposal; however, data on bulk permeability, hydrochemistry, groundwater age and fluid history in crystalline rocks at depths of several kilometers is limited.

A deep borehole disposal research, development, and demonstration project is planned by the U.S. Department of Energy to address scientific questions in the deep subsurface related to disposal safety and to demonstrate engineering operations of a deep borehole disposal system. Groundwater age, history, and fluid flux can be constrained with multiple environmental tracers, such as radiogenic isotopes of noble gases. Data on groundwater salinity and geochemistry are important in evaluating the stability of density-stratified fluids and predicting waste package corrosion, waste form degradation, and chemical interactions with borehole seals. Potentially overpressured conditions and permeability of the disturbed rock zone near the borehole can be evaluated with shut-in pressure measurements and hydraulic testing. Chemical and mineralogical alteration of borehole seals material can be determined using laboratory testing.