[LoSboccacc]

Yeah the artifical constraint of understandable by everyone is quite limiting. I can see two set working best: scary faces and statues for primitives, atom structure of thorium or whatever waste is for educated. Optionally leave some stuff out in a ventilated but inaccesdible area so people die/get sick before releasing the whole toxic dump in the wild. Can be in stages like pyramids: first zone warning, then the second zone trapped. Inhuman, but if your priority is to protect the whole word from releasing the waste a couple curious casualty may be a compromise.