Core meltdown and containment failure in (or near and upwind of) a megacity would be the obvious one. Another exciting option: unnoticed containment failure of

Core meltdown and containment failure in (or near and upwind of) a megacity would be the obvious one. Another exciting option: unnoticed containment failure of waste storage, which contaminates huge amounts of groundwater over weeks and months before the contaminated water starts reaching a densely populated area. And don't tell me these things can't happen outside the Soviet Union. Greed and complacenty can always find a way.