How do you think spent uranium interacts with the environment?
There's an estimated 4.5 billion tons of uranium dissolved in seawater. Naturally occurring. I honestly think we missed a trick when we outlawed dumping in the ocean, there's basically no way for human generated nuclear waste to even move the needle on ocean sources.
Lets say I take you completely at face value. Every notion of yours comes to pass. We cask it, and leave it in an underground vault. 9999 years later, a cask fails. Whats the issue? Are you using that vault as a busy thoroughfare? Its still in a big hole in the ground. Maybe theres an earthquake? And the vault shears a little. What is the radiation now doing in your mind that makes it dangerous? TBH we shouldnt leave signs warning people to stay away, we should leave a concrete recipe behind on all the signage.
There's life thriving in Pripyat just past the big concrete dome. There's a war going on there.