ORNL-TM-13553

107 4.11.1 General Description The deep-borehole concept for geologic disposal of excess 233U-bearing material includes the emplacement of the excess material into the lower part of one or more underground boreholes, each of which is drilled into a stable rock formation. The stability of the rock formation must be based on appropriate criteria that cover tectonic, hydrology, thermal, and geochemical characteristics. Application of deep boreholes as a disposition option for excess WGP was studied extensively by LLNL and is documented in Alternative Technical Summary Report for Immobilized Disposition in Deep Boreholes, UCRL-LR-121736 (Wijesinghe et al. August 23, 1996).

If the borehole disposition concept is adopted, 233U-bearing materials would be emplaced and sealed in a region of stable rock that lies 2 to 4 km below the surface. This region is called the emplacement zone.

A suitable rock identified for borehole emplacement is granite. Following emplacement, the region above the disposed material, extending from the top of the emplacement zone to the ground surface (and called the isolation zone), is filled and sealed with appropriate backfill materials. The isolation zone provides a large transport barrier to any disposed material.

As noted in the LLNL disposition study for WGP (Wijesinghe et al. August 23, 1996), the deep borehole-emplacement depths can be several thousands of meters greater than those of mined geologic repositories. At such depths, the groundwater is expected to be relatively stagnant and have temperatures of 75 to 150bC, pressures of 7,500 to 15,000 psi, and dissolved salt content of up to 40 wt %.

Table 4.19 summarizes the major features of the deep borehole application for the disposition of 233Ubearing materials. Figure 4.18 shows the general features of the option.

4.11.2 Waste Form and Site Characteristics Desired characteristics of waste forms suitable for immobilized disposition in deep boreholes include a stable solid form, high resistance to dissolution by subsurface brines, thermal stability, and compositional stability over the long periods of time. The host rock formation into which the excess material is buried should be a plutonic-metamorphic, crystalline rock (e.g., granite) in a tectonically, hydrologically, thermally, and geochemically stable region (Wijesinghe et al. August 23, 1996).