Overexposure to plutonium
Plutonium, a radioactive waste product of nuclear production, is carcinogenic. Even in minute quantities it is highly toxic. It is expensive to secure, chemically unstable and easy to fashion into a crude nuclear device. The plants which manufacture it are aging, and there is possibility that quantities of the element could escape should there be any accident. There is no known use for plutonium other than in weapons and bombs. Uncontrolled pollution coupled with mass destruction arising from the use of plutonium bombs.
There is no real distinction between weapons-grade and reactor-grade plutonium, in that both make very satisfactory bombs. Less than nine kg of plutonium (a piece the size of a grapefruit) is sufficient for a bomb. One ten-thousanth of a gram is the human cancer dose.
Tiny quantities of plutonium are instantly fatal. A few hundred grams placed in a municipal water supply would kill hundreds of thousands of people.
Plutonium is highly toxic, but hardly the most dangerous material known. Sudden death from radiation poisoning would occur if you ate or swallowed 0.5 grams of plutonium. Other chemicals much easier to obtain can kill at lower levels. Ingesting less than 0.1 gram of cyanide, for instance, is lethal. If three hundred grams of plutonium were dropped into a water supply, most of it would sink, remain in the sediment and not pose an health hazard. Only 3 milligrams would be dissolved and that amount would be unlikely to cause even one additional cancer death.