DEEP BOREHOLE DISPOSAL OF HIGH-LEVEL RADIOACTIVE WASTE

Preliminary evaluation of the disposal of high-level radioactive waste and spent nuclear fuel in deep boreholes (3 to 5 km depth) indicates the potential viability and long-term safety of this disposal method. Existing drilling technology permits the construction of deep boreholes at acceptable costs for this purpose. Durable and effective multi-component systems for sealing boreholes after waste emplacement also exist. Low permeability and high salinity at many locations in the deep continental crystalline basement suggest extremely limited interaction between deep saline water and much shallower fresh groundwater resources. Geochemically reducing conditions at depth limit the solubility and enhance the sorption of many radionuclides. Crystalline basement rocks are common at depths greater than 2 km, making this disposal method attractive from the perspective of regional equity and reduced transportation risk and costs.

Analyses show that granite bedrock temperatures near the disposal borehole would increase to a peak about 30^o C above ambient conditions for the disposal of a string of spent nuclear fuel assemblies and by about 120^o C for vitrified high-level waste from fuel reprocessing. Coupled thermal-hydrologic modeling indicates transient upward specific discharge of about 1.5 cm/yr from the thermal expansion of groundwater, which dissipates within several hundred years of waste emplacement. Preliminary analyses of nuclear criticality, molecular diffusion, and thermally induced hydrofracturing of the bedrock suggest that none of these processes would have significant impacts on safety of the disposal system. Preliminary modeling of performance assessment that includes solubility, sorption, groundwater transport, and shallow aquifer pumping indicates that dose to a hypothetical human receptor would be limited to a single radionuclide (Iodine-129) and would be negligible.

Several technical issues related to the deep borehole disposal of radioactive waste are being investigated in greater detail, including: 1) modeling of the coupled thermal-hydrologic-chemical-mechanical behavior near the borehole, 2) long-term behavior of seals, 3) deployment of compounds that sorb/sequester radioactive iodine in the borehole or seals and 4) more detailed performance assessment analyses.