Splitting Atoms - How does it stop?

[TomCowley]

Not all elements/isotopes are unstable when they pick up another neutron. Even in a sample of fissionable material, not every neutron released hits another fissionable atom (or initiates fission if it does).

http://en.wikipedia.org/wiki/Nuclear_reactor

[furyshade]

in a reactor, you have rods of material which absorb neutrons, so you get a reaction going, then the rods slow/stop the reaction, repeat to get power. in a nuclear explosion this just happens but it isnt controlled so it takes a lot longer to stop

[SunOfBeach]

Ahhhhh.....