A Drop-In Concept for Deep Borehole Canister Emplacement

Abstract

Disposal of high-level nuclear waste in deep boreholes drilled into crystalline bedrock (i.e., “granite”) is an interesting repository alternative of long standing. Work at MIT over the past two decades, and more recently in collaboration with the Sandia National Laboratory, has examined a broad spectrum of design aspects associated with this approach. For emplacement, past reports suggest using steel cables to lower each canister into the borehole. This process would require many years to complete and precise control to safely lower the canisters thousands of meters. The current study evaluated a simple, rapid, “passive” procedure for emplacement of canisters in a deep borehole: free-fall release into a water-flooded borehole. The project involves both analytic modeling and 1/5th scale experiments on a laboratory mockup. Experiments showed good agreement and validated the model. Depending on the inputs used for the mass and dimensions of the full scale canister and the viscosity of water, the model predicted terminal velocities of 2.4-2.6 m/s (4.5-5.8 mph). Further experiments showed that this could be reduced by 50% by making the surface hydraulically rough. Based on these predictions and a structural analysis, there seems to be little risk of damage when a canister reaches the bottom of the borehole or impacts the stack of previously loaded canisters. For reference, dropping the canister in air from a height of only 0.3 m (1 ft) would result in an impact velocity of 2.44 m/s. Cost estimates for the conventional drill string based method were developed, and the drop-in method was concluded to reduce emplacement costs and time by a minimum of 70%, down to $700,000 per borehole. It is concluded that a simple drop-in procedure deserves serious consideration for adoption as a standard procedure for borehole loading.