Seismic activity collapses the well, trapping the nuke underground, pressure find a way to escape via polluting a "faraway" aquifer, or it doesn't escape and you have a man-made volcano in the works
Not a plausible scenario. The entire point is that by putting these a mile deep that they will self seal if they meltdown. They can't create a volcano because they are only 15MW. If the reactor cracks the radioactive fuel can only contaminate the water at the very bottom of the shaft because it is a mile deep.
I thought the whole point was that the reactor would deliver heated water to the surface for utilization...? But it will magically stop trying to do that if the main shaft is obstructed and there is a separate egress?
Or are you also burying the turbine and power lines a mile deep?