[Tristan Lall]

Originally Posted by MikeWasHere05

Imagine if terrorist's set a nuclear bomb off in New York City. With one bomb, millions of people would be killed, hundreds of thousands would later die due to cancer, thousands of square miles of land would be inhabitable for centuries, and the whole world's economy would be in threat.

That's quite an overstatement, unless they managed to find themselves the plans for a high-end hydrogen bomb the size of a bus, and figured out how to put it together and set it off.

Take Hiroshima—one 13 kiloton bomb killed a little less than a hundred thousand instantly, and maybe sixty thousand more afterward due to radiation effects (and that's mostly acute effects—like radiation burns, rather than cancer). The city's population just prior to the nuclear bombing was around 250 000; now, it's over 2.8 million. It is not uninhabitable, and hasn't ever been so, except, broadly speaking, the short period while the contaminated debris was being removed. In fact, for the sort of long-term effects you describe, only something like a very large cobalt bomb would suffice, because of the long half-life of the isotopes of that element. (Those have never been tried, because they're potentially so messy—the terrorists would have to develop it on their own, rather than merely steal one!)

Now, setting off a 13 kiloton bomb is no mean feat. But enough destruction to instantly kill millions of people would probably require something on the order of the Tsar Bomba, the largest nuclear weapon ever detonated, which had a test yield of 50 000 kilotons, and a theoretical yield of 100 000 kilotons. It's said that the 50 megaton explosion would have had a lethal radius of over 100 km, which would certainly accomplish that. In any case, to incinerate millions, you'd need several tens of thousands of kilotons, at least. Once again, those sorts of experimental bombs aren't just lying around for the stealing, even in Russia.

As for whether a terrorist group could manage to make such a device (like the 1940s-era Little Boy, to say nothing of the 1960s-era Tsar Bomba), they would need access to some very specialized equipment (it's not COTS), some significant expertise, and hard-to-find materials like enriched uranium (i.e. with a high proportion of ²³⁵U).

Now, the sort of bombs you might actually have to worry about might include dirty bombs, or even the old-fashioned truckload of something explosive. There's not actually that much you can do to avoid these—short of searching every truck that enters NYC—but the consolation (after a fashion) is that it wouldn't kill nearly so many people. The real trouble is, 50 megatons by H-bomb, or one kiloton by NH₄NO₃-diesel-bomb; it doesn't matter, if it goes off in New York. The world's economy will indeed tremble, and in all likelihood, America will go on another foolish killing spree, if there's no way to strike at the actual responsible party. (But that's for the other thread that Mike spoke of....)