Nuclear Bombs

The amount of energy associated with nuclear reactions is massive compared to the amount of energy associated with chemical reactions. Although this is true in power plants and engines, it is much more obvious in explosions. A chemical explosion can destroy a large building, but even a little nuclear explosion can destroy a small city.

In the bomb that the United States dropped on Hiroshima Japan, the reaction was the fission of Uranium.  In this reaction, a neutron added to a uranium atom causes the uranium to split, spitting out additional neutrons. Each uranium atom, has the potential to "set off" three additional uranium atoms. As a result a few uranium atoms splitting can very quickly turn into a massive reaction which releases a tremendous amount of energy.

Nuclear reactions, unlike chemical reactions are single atom events governed by probability. This means, that an individual atom of uranium could decompose and spit out several neutrons. Those neutrons could hit other uranium atoms and start a chain reaction. However, this is unlikely in a small amount of uranium, of if the uranium is spread out enough, because the neutrons are more likely to be lost than to set off a chain reaction. A sample of uranium is called a critical mass if it is large enough and packed tightly enough that neutrons are likely to start a chain reaction. A smaller or less packed sample is called sub-critical.

 The trick, then, is to build a bomb of sub-critical samples and then to make them suddenly critical at the moment that you want the bomb to explode. This can be done several ways. 

One way to make a sample suddenly critical is to have several sub-critical masses suddenly pushed together. Imagine you had an incomplete sphere of uranium that was just short of critical. The "missing piece" of the sphere is separated along a tube. At the correct moment, that piece is "shot" into the sphere, reaching a critical mass.

Another way to do it is to have a critical mass, but to arrange it in a sphere that is not tightly packed enough to be critical. The entire sphere is wrapped in coventional explosives. Setting off those explosives, forces the uranium into a tighter sphere, reaching criticality.

Of course, the problem is, that uranium can emit a neutron on its own, so that if you had a large enough pile of uranium, it might go off on its own, when you didn't want it to. That "enough" is called a critical mass, and bomb makers are careful to never get a critical mass together while building their bomb.