It's interesting how, when communicating with hypothetical future people in written form, we have to choose language we have the best hope of them understanding. A center of emanating energy that is dangerous to the body and kills.
Really makes you empathize with religious writers trying to communicate often second hand divine truths that people thousands of years later will have to figure out without the understanding the original prophet had.
Honestly, the language they have the best hope of understanding is... English! English is, for better or worse, the working language of much of the world (particularly in business, diplomacy, and technology), even if it is not a native language, and there's no language that seems poised to replace it in this regard. Even if English is somehow lost, there is a plethora of bilingual inscriptions that could serve as potential Rosetta stones.
Of course, the real problem with this endeavor is that human nature is going to be that explorers ignorant of The Old World will happily ignore any signs saying "Danger! Keep Out! We Really Mean It! This Will Kill You (Slowly)!" looking for treasure. That's exactly what happened with the pyramids--they were looted within a few centuries of being constructed. If humanity regresses to a point that they don't understand radiation poisoning, they are not going to be stopped by even the clearest explanation of what will befall them because they won't believe it.
This exercise is more talking to ourselves than the future. It is a sort of design fiction[1], attempting to cope with the concept of consequences that last much longer than the initiating actors' whole civilization.
That it isn't fiction is what gives it salience, and the requisite paternalistic 'talking down' to a pre-technical future aspect provides emotional and/or moralistic flavor, if you like that sort of thing.
Eventually, it turns into bike-shedding scary apocalyptic artworks.
Also on point, if they have forgotten about the concept of nuclear radiation then discovering a nuclear waste dump will be a massive boon and cause for celebration. When we zoom out enough to see entire civilisations at once, a few people dying just isn't an issue; it is routine. We try our best to keep everyone healthy, but at the end of the day the benefits of progress outway handfuls of dead astronauts, scientists, explorers and early colonists.
We don't sit around moping that Marie Curie & co died doing research, everyone involved gets recognised for helping to usher in a new age of scientific progress.
This is HN and nuclear energy byproduct. There is no danger to worry about. Future cave men are more likely to die cutting themselves on discarded solar panels in landfills!
Really makes you empathize with religious writers trying to communicate often second hand divine truths that people thousands of years later will have to figure out without the understanding the original prophet had.