Physical effects could be used in combination with symbols. The current trefoil is enough to warn the people of today, so that should continue to be used. If

Physical effects could be used in combination with symbols. The current trefoil is enough to warn the people of today, so that should continue to be used. If knowledge of the trefoil is lost, however, the actual radiation would serve as a warning. Much smaller, but still dangerous quantities of waste could be stored around the world, with just enough of a barrier around it to remain confined while curious individuals exploring around it would contract radiation sickness, but survive for long enough to warn others about the danger and the associated trefoil symbol. The main storage area should have much bigger versions of the trefoil, but not so big that a person would lose perspective and not know what they're looking at. The size should convey to anybody who knows the meaning of the symbol that this is serious business. If the smaller quantities of waste are abundant and accessible enough, and the main storage area inaccessible enough, then it would be unlikely for anyone to come across the main storage area before learning the meaning of the symbol.