Gory imagery always gets the job done.
Gory imagery always gets the job done.
The ancient Egyptians learned this the hard way. Many of their tombs were marked with graphic warnings threatening anyone who disturbed them with various horrible ends. But most of them were robbed regardless, and those that weren't were eventually cracked open by 19th and 20th century archaeologists.
That wasn't because the people violating those tombs didn't understand the warnings (the archaeologists certainly did). It was for two reasons:
1. Some people, like the archaeologists, are going to read the warnings and dismiss them as dumb ancient superstitions; and
2. Other people, like ancient grave robbers, are going to read the warnings and think "Huh. They really want me to stay out of that place. There must be something really valuable in there!"
The only Egyptian tombs that survived to the modern era were the ones so well hidden that nobody had managed to stumble across them.
Then again, we can't really know if the future civilization will consider out spent nuclear fuel as a pretty good store of value.
Not the exact message, but definitely should give pause to anybody visiting the place in the far future.
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.