If the nuke is placed at the NY Stock Exchange most New Yorkers would not be affected by the blast due to the unusually dense architecture surrounding the exchange -- much denser than now allowed by zoning and building codes. And many of the older buildings are substantially built of heavy stone and steel, far more so than recent flimsy buildings like WTC and those in midtown.
Pretty good fire protection required by code would also inhibit spread of fire -- NYC instituted those after several disastrous fires in the 18th and 19th centuries -- the last in early skyscrapers. Moreover, the tip of downtown Manhattan is within a few blocks of water on three sides, and winds usually blow toward Brooklyn, the Atlantic or New Jersey, on the east, south and west, respectively not the upper part of Manhattan to the north where the super rich and super poor live on Park/5th Avenues and Harlem. The smoke and toxic plumes from WTC saturated Brooklyn at first then shifted to the south and west but not north. Coincidentally contaminating the home neighborhoods of many who worked in WTC (not many of which were Manhattanites). For maximum effect on people -- for blast and for toxic plumes -- the weapon should be located in Central Park, or best, several of them placed at its east, south and west periphery. The park is in the heartland of Manhattan -- cultural, political and economic -- and thousands of the nation's vainglorious personages live adjacent to it. (Why the Atta bunch wasted their efforts on laborers in WTC rather than creme-de-la-creme Park and 5th is a mysterious conspiracy.) Delivery would be a cinch by way of the multiple subway lines that lie just feet down around the park and serviced by plentiful access stairs, transformer rooms and ventilation areaways. Still, for pinpoint dual-use targeting, devices placed atop the five or six Trump hi-rises (four or five adjacent to Central Park, one adjacent to the UN), most of the Western Hemisphere's villains would be left homeless, though bunkered safely within the several dozen caverns under and near the city like the several IBM has in nearby Westchester County, some dating from the Cold War. But why waste a nuke on NYC when there are such bountiful opportunities for less cumbersome and likely much more devastating weapons? The answer to that seems to be that attackers want dramatic strutting and not only to kill, as the US does in Afganistan. A nuke would make a great thermobaric video compared to the pale gasses or invisible germs. If scaring the shit out of people to get obedience is the strategic intention.
