HORIZONTAL BOREHOLES IN SHALE FOR DEEP ISOLATION OF SPENT NUCLEAR FUEL
Potential sites are deep (1 to 2 miles), have low permeability (microdarcys), low mobile water levels (“none” by oil and gas industry standards), lie far below aquifers in tight yet ductile formations (brittleness < 10 MPa) with no or few natural fissures; such shale is unusable for hydraulic fracturing and gas/oil recovery. Suitable formations are found in most states that have SNF inventories.
No new drilling technology is needed; current methods are highly developed and relatively inexpensive. A vertical delivery borehole is drilled to a “kickoff” point ≈700 ft above the shale; the borehole then curves to horizontal; the low curvature can easily accommodate 4-meter canisters containing unmodified fuel assemblies. As an initial approach, the unmodified SNF assembly canisters are placed end-to-end in the horizontal section at depth. There is no criticality danger since SNF assemblies are currently safely stored even in water. For a pressurized water reactor, assemblies contain ≈ 265 rods in a diameter ≈ 12 inches and linear mass density ≈ 1 ton/ 5 meters. A typical 20 tons/yr of SNF requires 100 m; a 3-km horizontal hole can hold 30 yrs of waste. For boiling water reactors, the assemblies are smaller (≈ 7 inch diagonal) and could fit into a 9-inch borehole; however the length required is over 3x greater. If the fuel is consolidated into denser structures, the length required is substantially reduced; in principle, a 3 km 9-inch borehole could isolate over 1000 tons; the capacity of 70 boreholes would equal that planned for the Yucca Mountain facility.
The most likely path for leakage is in the vertical delivery borehole or the surrounding damage zone; these might be sealed using the technology being studied for the Department of Energy vertical borehole program. Ability to retrieve stored waste is based on extensive experience in recovery of pumps and instruments from horizontal boreholes. The 50 surface acres above a set of parallel boreholes are left essentially pristine.