Science Behind The Bombs

When a uranium atom absorbs a neutron and fissions into two new atoms, it releases three new neutrons and some binding energy. Two neutrons do not continue the reaction because they are lost or absorbed by a uranium atom. However, one neutron does collide with an atom of uranium, which then fissions and releases two neutrons and some binding energy. Both of those neutrons collide with uranium atoms, each of which fission and release between one and three neutrons, and so on. This causes a nuclear chain reaction.

In order to detonate an atomic weapon, you need a critical mass of fissionable material. This means you need enough U-235 or Pu-239 to ensure that neutrons released by fission will strike another nucleus which produces a chain reaction. The more fissionable material you have, the greater the odds that such an event will occur. Critical mass is defined as the amount of material at which a neutron produced by a fission process will, on average, create another fission event.