Realm of Pluto
Compared with Plutonium-239, the other chemical elements stands in the same relation as the kings on the stage to the king in real life, for it is Plutonium that will reign for thousands of years. It is extremely toxic and highly chemically reactive, half-life of Plutonium is 24,000 years.
Fortunately Chernobyl was Uranium reactor, all Plutonium that present in the area is a product of Uranium, how much depends on the age of the reactor. Some say, it would still be a small amount, but we don't really need much. By extrapolation, 1 lb (less then half kilogram) of plutonium, universally disposed, would be enough to kill every living human being on the earth.
Plutonium is used as a fuel for atomic bombs or as fuel for "fast-breeder reactors", it is appropriately named after Pluto, god of the dead and ruler of the underworld.