I wonder why this idea wouldn't work for disposing of nuclear waste. It's similar to something that I had thought of. The basic idea presented here is that radioactive waste would be buried on a tectonic plate in a subduction zone. This would over the course of time pull the waste into the core of the earth. This would seem to avoid all of the problems with conventional nuclear waste disposal sites, such as groundwater and aquifer contamination. One concern that I can see would be corrosion of the containment vessel.
My original idea was to dig a mine that would go beneath all the underground water. This could be prohibitively expensive, but it has been done. An added bonus to this type of mine is that it ends up being deep enough that the heat can be used to generate geothermal energy.