Isolation from _what_? If you reactor melts down, it will contaminate the water column above it. And by having it inside the shaft, you won't be able to do any maintenance on it.
It's a stupid idea designed to filter out investors who are stupid enough to fall for it.
It boils off the coolant, creating a vapor lock, then water-zirconium reaction happens. Then it explodes and ruins the shaft. So you'll have to drill a rescue well, and then defuel the reactor to avoid any possible contamination.
Again, this is a monumentally stupid idea for no reason whatsoever.
It boils off a 1 mile deep water column? And you don't need a core catcher when you are already 1 mile deep in bedrock. It just goes to the bottom of the hole.
Or partially fill it and drop another reactor on top.
The only projects that overrun more than nuclear is nuclear waste disposal. This kills two birds with one stone.