On: How to build something that lasts 10k years

[smacktoward]

"Somebody made that place super dangerous to get into. There must be something really valuable in there!"

[scj → smacktoward]

I'd go the other way... Give the impression there was something really valuable in there.

Have a main chamber with a solid pedestal in the center. Then, on walls throughout the complex, depict some golden treasure that never existed resting on said pedestal using wall art.

Imagine a scavenger, with a clear vision in their head of what the chamber looks like, arriving in an empty room: "Someone must have already raided this place."

Meanwhile, the nuclear material is under that chamber.

[Retra → smacktoward]

Then don't make the place inaccessible. If you're warning people about a threat, you need to show them where the threat is, not hide it.