On: How to build something that lasts 10k years

[Tade0]

How about actual corpses arranged so that they look as if they died violently?

Not the exact message, but definitely should give pause to anybody visiting the place in the far future.

[smacktoward → Tade0]

"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.