But what about if we had a Reactor Dump storing 100's of these reactors, and then something happened to that dump, either it was hit by a meter or it was intentionally attacked in some way. Could that result in an apocalyptical type event?
It would basically be a few Chernobyls in the worst case (a 1 in 1000 year asteroid impact winning the 1 in 100 million lottery and hitting the dump) and that mostly means extra birth defects and cancer (you wouldn't want to live somewhere that gave you a 10% chance if dying due to cancer age 40, but you could still have a civilization in such a location). It doesn't end life in the impacted area.
For intentional attacks there are much much easier ways of killing a lot of people.
You cannot do that. There is no place on Earth that we can be sure is geologically stable over that time period.
What a selfish thing to do: Make future generations pay for our current consumption for millennia.
Yes, there's a 1 in 10000000 chance that 2000 years from now industrial age humans recovering from the apocalypse might dig a well in exactly that location and thousands will get sick with hundreds of deaths. You could multiply that by a hundred thousand cores and it would still be negligible compared to the hundreds of millions likely to die due to climate change in the next century