As is the case with explosions caused by conventional weapons, most of the damage to buildings and other structures from a nuclear explosion results, directly or indirectly, from the effects of blast.
The very rapid expansion of the bomb materials produces a high-pressure pulse, or shock wave, that moves rapidly outward from the exploding bomb (SEE FIGURE 1). In air, this shock wave is called a blast wave because it is equivalent to and is accompanied by powerful winds of much greater than hurricane force. Damage is caused both by the high excess (or overpressure) of air at the front of the blast wave and by the extremely strong winds that persist after the wave front has passed.
Figure 1- Fission of a Uranium 235 Nucleaus
In general, large buildings are destroyed by the change in air pressure, while people and objects such as trees and utility poles are destroyed by the wind. The degree of blast damage suffered on the ground depends on several things. Some of these things are a direct result of:
1) The TNT equivalent of the explosion;
2) The altitude at which the bomb is exploded (the height of burst);
3) The distance of the structure from ground zero (the point directly under the bomb).
The isotopes important for the large scale release of energy through fission are uranium-235 (U-235), plutonium-239 (Pu-239), and uranium- 233 (U-233). The binding energy of these three isotopes is so low that when a neutron is captured, the energy released by rearrangement exceeds it. The nucleus is then no longer stable and must either shed the excess energy, or split into two pieces. Since fission occurs regardless of the neutron's kinetic energy
Typical figures for critical masses for bare (unreflected) spheres of fissionable materials are:
U-233 16 kg
U-235 52 kg
Pu-239 (alpha phase) 10 kg
INFO: A 10-kiloton bomb will cause severe damage to wood-frame houses, such as are common in the United States, to a distance of more than 1 mile from ground zero and moderate damage as far as1.5 miles. With this, the severely damaged houses are probably beyond repair. The damage radius increases with the power of the bomb, approximately in proportion to its cube root. If exploded at the optimum height, therefore, a 10-megaton weapon, which is 1,000 times as powerful as a 10-kiloton weapon, will increase the distance ten times that amount, that is, out to 11 miles for severe damage and 15 miles for moderate damage of a frame house. (SEE FIGURE 2)
Figure 2 - Destruction of Nagasaki after the explosion of the "H-Bomb"
When a nuclear weapon is detonated on or near Earth's surface, the blast digs out a large crater. Some of the material that used in be in the crater is deposited on the rim of the crater; the rest is carried up into the air and returns to Earth as radioactive fallout. An explosion that is farther above the Earth's surface than the radius of the fireball does not dig a crater and produces negligible immediate fallout. For the most part, a nuclear blast kills people by indirect means.
Image: Above: The March 1, 1954 Bravo hydrogen bomb crater. Photo © Hiro Toyosaki