So there are two extremes to radioactive waste / leakage. One extreme is the "ideal dispersal" leak, like the Fukushima leaks into the ocean. In this case, th

So there are two extremes to radioactive waste / leakage. One extreme is the "ideal dispersal" leak, like the Fukushima leaks into the ocean. In this case, the radioactive material does not "clump up" enough to be dangerous -- it's not that some cubic meters of ocean are fine and some contain deadly, deadly poison, it's that each cubic meter of ocean contains the same minuscule amount of radioactive material, too low to be a threat. The other extreme is the "immobile clump" of radioactive material, like the Elephant's Foot at the heart of Chernobyl. Here you have enough radioactivity to cause serious harm or death if you get too close for too long. It's scary, but it's also pretty stationary. It's scary like a hole in the ground. You stay away, you warn people not to fall in, but it's not going to creep up on you without warning, like a sinkhole in Florida. The really scary kind of radioactive waste isn't either of these two extremes, but a dangerous middle-ground: radioactive waste which is clumpy enough to give off deadly amounts of radiation, but mobile enough to get everywhere. You occasionally get this from reactor leaks (that's the primary reason for the exclusion zone around Chernobyl; on the other hand, they could never find any evidence of this sort of leak after Three Mile Isle). You also see it in non-power-plant nuclear accidents, like the Goiânia Accident, which involved a radiation source used in medical imagine. You also worry about it in coal ash, because the ash can be blown and scattered but is clumpy enough that the radioactive particulate doesn't disperse.