What, you want an accurate physical simulation of a nuclear reactor falling onto Manhattan, Marvel comics style?
Well, considering that an actual nuclear bomb going off on Hiroshima killed "only" <150,000 people, I find it pretty unlikely a falling reactor will kill half of Manhattan. So, there.
Glib response aside, I want a credible description of worst case scenario for nuclear disaster. The probability of such an event increases greatly if there are many plants, particularly if there are plants in nations which go into decline or plants built in highly corrupt or inept nations. So how many people die from the initial event? How much residual radiation? How far does it spread? How many people are displaced? Are food supplies effected? It sounds like you haven't really thought it through all the way. My personal belief is that people are not to be trusted with technology with such high stakes. Especially with the onset of climate catastrophe within the century ensuring the decline of our society.
Well, considering that an actual nuclear bomb going off on Hiroshima killed "only" <150,000 people, I find it pretty unlikely a falling reactor will kill half of Manhattan. So, there.