What keeps the radioactivity from reaching the surface?
The disposal section of the drillhole is buried thousands of feet underground, under a billion tons of rock, including layers that have held volatiles (methane) for millions of years. The potential release path is for the radioactive material (e.g. radioactive iodine found in the pellets) to dissolve in deep water which could then be transported to the surface.
We plan to prevent this by a combination of engineered barriers (low-corrosion metal for the canisters) and the geologic barrier which is the rock. Engineered barriers include the ceramic pellets themselves, the metal rods that contain them, the sealed corrosion-resistant canisters that hold the rod assemblies, the steel casing that lines the drillhole, and the sealed drillhole. These engineered barriers are expected to provide protection for tens of thousands of years.
For the geologic barrier, a formation will be considered suitable if we can demonstrate beforehand that the water at depth is extremely stagnant and has been out of contact with the surface for hundreds of thousands to millions of years.