The visual mode might be the most universal form of communication. Psychology and biology studies have proven that humans are hard-wired to react in certain ways depending on visuals. For example, the color red instinctively draws a human's attention since it is a sign of danger. That is why stop signs and traffic lights are red, they cause drivers to instantly be alert in order to avoid accidents.
I think that because the visual mode is so universal, it could be used to solve the problem presented by James Conca in his article "Talking to the Future -- Hey, There's Nuclear Waste Buried Here!". Initially I considered a red sign with the classic skull and crossbones to signify a deadly threat. However the truth is that within human history the symbolism for death has been very different for each culture. For example, the Ancient Greeks never portrayed Death as a menacing or evil character, since they knew death was inevitable and sometimes even considered it the path to an honorable sacrifice or a peaceful end of suffering. Since we do not know if future generations will use a dark hooded figure, or a dirty old woman, or a skeleton wielding a scythe as their symbol for Death, we should probably stick to simpler design. I think red should be a main color (since it will evoke the human instinct for danger) along with black (since it has proved to be a basic color to represent evil in most cultures throughout time). Perhaps the simplest solution would be a depiction that would use the human empathy to communicate the hazard of nuclear waste, such as an exaggerated illustration of a suffering human.
Sources:
Conca, James. "Talking to the Future -- Hey, There's Nuclear Waste Buried Here!" Forbes, 17 Apr. 2015, https://www.forbes.com/sites/jamesconca/2015/04/17/talking-to-the-future-hey-theres-nuclear-waste-buried-here. Accessed 2 October 2017.