Splitting Atoms - How does it stop?

[MaxWeiss]

First, you have to fire the neutron with enough energy to split the atom, and second, the split atom's neutrons have to hit something. Eventually the energy dies out and also the neutrons miss, but not before on big [censored] explosion.

[eurythmech]

For the split atom's neutrons to continue splitting further atoms (causing an exponential growth in fission), the material has to be of "critical mass", should be a wikipedia entry for that as well.