The symbol you need is bones. Bones are always associated with death and decay because after death, bones are the thing that’s left.

I think the only way to get it taken seriously is to include depictions of the site and depictions of what radiation sickness looks like.

Make like a 100 page comic that starts with someone opening a cask and removing a fuel element, then dying days later, the element leaving contamination, which kills someone else months later, causes birth defects, etc until it's returned.

You're not going to scare a society into never investigating something, but you might be able to convince them to put it back and bury it again after they realize the predictions are accurate.