Narrow vertical holes drilled deep into the earth’s crust have been considered an option for the geological isolation of radioactive waste since the 1950s. Deep borehole concepts were developed but never implemented.
In a first test of its kind, Deep Isolation, a California-based company, has successfully demonstrated a new technique of placing a canister of nuclear waste hundreds of metres underground through a drill hole and then retrieving it.
The prototype canister, which held a steel rod to simulate the weight of radioactive waste, was lowered over 600 metres into an existing drill hole using a wire cable, then pushed using an underground “tractor” about 120 m into a horizontal storage section. The canister was released and the cable withdrawn. Several hours later, the tractor was placed back into the hole, where it latched onto and retrieved the canister, bringing it back to the surface.
In 2016, a team led by the Battelle Memorial Institute was selected by the United States Department of Energy (DOE) to drill a 4,875 m test borehole into a crystalline basement rock formation in North Dakota as part of studies into the feasibility of using boreholes for nuclear waste disposal. In the same year, the International Atomic Energy Agency (IAEA) announced the completion of “proof of concept” tests for borehole disposal of small volumes of radioactive waste. However, the concept has been considered to be an expensive option for large volumes of waste compared with a mined underground repository, and the borehole disposal was also regarded as irretrievable.
Deep Isolation’s technology employs standard drilling technology using off-the-shelf tools and equipment common in the oil and gas drilling industry.
It envisages emplacing nuclear waste in corrosion-resistant canisters, typically 22-33 cm in diameter and 4.3 m long, into drill holes in rock that has been stable for tens to hundreds of millions of years. The drill hole is lined with a steel casing. It has a vertical access section in the beginning which then gradually curves until it is nearly horizontal, with a slight upward tilt. This horizontal “disposal section” will be up to 3.2 km long and lie a few hundred metres to 1-1.5 km beneath the surface, depending on geology. Once the waste is in place, the vertical access section of the drill hole and the beginning of its horizontal disposal section would be sealed using rock, bentonite and other materials.
According to the company, the key advantages of the method are the depth of burial and the fact that the waste is stored in a suitable geologic formation far below the water table, in rock that is saturated with brine that has been virtually stagnant for millions of years.
In addition, small diameter drill holes require less disturbance of the rock than a mined repository. “A drilled repository allows you to go deeper while disturbing less rock. It is both safer and less expensive than a mined repository,” the company stated.