There was a site that showed the blast radius of various bombs. Assuming that most terror nukes probably wouldn't go higher than 10kt ground blasts, most any large city would not be totally "destroyed".
Experts in this stuff know that terrorists and rouge nations would go for financial centers and symbolic targets, like the WH and Wall Street. 5 or 6 well placed nukes going off at once would probably bring a nation to its knees economically, followed shortly by the entire developed world. This is the nightmare scenario that terrorists want to carry out. Urban civilization as we know it would be at risk. Just as other civilizations have been attacked and crumbled in the face of new military technology and internal strife, so ours would be under threat in the wake of such an attack, as remaining cities were evacuated or abandoned and political and economic structures reorganized themselves or completely collapsed.
However, by demonstrating fortitude, determination and creativity in attacking organized terror worldwide, this is an entirely avoidable outcome. Attacking finances, organizations and worldwide political support structures as we are now is the only effective way.
Any military expert would say that a good offence is the best defense against this type of aggression, since there are simply too many ways to breech the walls of any given security structure with compact weapons and abundant targets.
GW gets briefed on the likelyhood of this type of attack regularly and this scenario drives much of US policy at the moment, like it or not.
From
www.radshelters4u.com The following four drawings show what level of blast damage (at different psi overpressure) and fire ignition from the thermal pulse might be expected for different strength nuclear explosions (both ground and air bursts) at different distances from ground zero. Take note of the damage range distances from GZ - ground zero. (Courtesy of Nuclear Attack Environment Handbook, FEMA - August, 1990)
Obviously, the bigger the weapon yield the larger the area of overpressure damage from the blast wave. But, notice that the damage range does not increase in a linear fashion with the more powerful explosions. For instance, comparing the 200 KT air burst with the five times more powerful 1 MT air burst, the range of moderate damage and initial fires increased from only 4.3 miles to 7.3 miles. This is because the reach of blast and fire effects varies as the cube root of the weapon yield ratio and the cube root of 5 is 1.71. So, instead of a five-fold increase or 500% we have only about a 70% increase in this comparison.
A readily portable terrorist nuclear bomb would likely be only a fraction as powerful as the examples above, but for reference, the Hiroshima nuclear bomb was only a 15KT air burst. (The RA-115 backpack nukes reported missing from Russian stockpiles are one kiloton yield each, and they would most likely be surface exploded.)