What kind of steel will be used and how thick?

The casing that lines the drillhole is typically made of carbon steel, but other materials could be used. The space between the casing and the drillhole could be filled with cement. The canister can be made of a corrosion-resistant alloy that is expected to endure underground for thousands of years. The very long-term isolation (over tens of thousands of years to even a million or more) is provided by the deep, highly impermeable and stable geologic formations.