Splitting Atoms - How does it stop?

[SunOfBeach]

I'm going to talk like a 12yr old here, as I only have a 12yr olds understanding of this:

As I understand it, you shoot a neutron at an atom, and it causes the already unstable atom to get more unstable and hence split. It then releases other neutrons, which then can split other atoms in a chain reaction. So, 1 neutron leads to 2, 4, 8.... etc.

When/how does it 'stop', and cause an explosion? What causes this process to cease from going on forever?

[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.....

[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.

[CORed]

As far as I know, the only isotopes that undergo neutron-initiated fission, and are therefore capable of a fission chain reaction, are Uranium 235, Uranium 233 and Plutonium. In a bomb, where the goal is to have as rapid a reaction as possible, you quickly run out of fuel. Another factor limiting the yield of fission bombs is that above a certain size, the explosion will blow the fuel apart before all of it reacts. The biggest bombs use a fission bomb to intitiate a fusion reaction.

As another poster mentioned, in a reactor, there are "control rods" which contain a material that absorbs neutrons, which can be moved in and out of the core to regulate the speed of the reaction. Also, reactors generally use fuel that is not sufficiently concentrated to cause a nuclear explosion, although if things get screwed up badly enough, the reactor can overheat and trash itself (Three Mile Island) or release large amounts of radioactive material (Chernobyl).

[Metric]

The short answer is that knocking neutrons out of most light nuclei takes energy -- it doesn't release energy (there are exceptions for some isotopes that are unbalanced with too many neutrons). I.E. the binding energy per nucleon increases approximately until you reach Iron, at which point it begins to decrease (making really heavy elements unstable -- wanting to spit into lighter elements which are more tightly bound). This is why you get energy from fusing Hydrogen, but get energy from fissioning e.g. Uranium.