You just need to find a geologically safe place to put it and you need to make sure everyone involved follows safety protocols to the letter. And you can't have anyone cutting corners to save money. You need to spare no expense when it comes to safety.
The only issue is that people don't stay strict with keeping everything safe sometimes. People are terrified of it because when something goes wrong, everyone can see the very gruesome results very quickly
But I don't think microsoft or any company should be making an AI at the rate they are if it's going to take as much resources as it seems.
Capital costs is incomplete, you need to look at lifetime costs versus lifetime production to get a more useful average - Levelized cost of energy (LCOE)