# Need help Nuclear Fission Question

1. Oct 8, 2004

### maccaman

This is probably an easy question for most of you, but me nor my teacher knew. Today we learned that nuclear fission can occur when a neutron "hits" the nucleus of Uranium-235. What i wanted to know is, where does this neutron come from, and how can they make it hit this uranium-235 atom.

Any help would be greatly appreciated.

2. Oct 8, 2004

### humanino

The incoming neutron generally comes from... a previous fission ! It may seem like the problem of the egg and the cow and which came first
Look at this schematic reaction. An incoming neutron causes fission of $$^{235}U$$ in $$^{144}Ba$$ plus $$^{89}Kr$$ thus liberating three neutrons : each of them could trigger one reaction, overall producing 9 neutrons... So basically, if you stack more than a critical mass of $$^{235}U$$, byitself it will radiate energy. Random processes will eventually cause a few neutrons to be liberated, spontaneous fission for instance, and as you can imagine the process is divergent : if you actually stack really too much, it would blow.

This is controlled in nuclear plants, which do not aim at blowing stuff. For instance http://www.npp.hu/mukodes/lancreakcio-e.htm [Broken] is a drawing illustrating the need for a moderator to cool down the neutrons. An absorber is usually also used to control the rate of reactions.

wikipedia
Lawrence Berkeley National Laboratory
Uni. of Prince Edwards

Last edited by a moderator: May 1, 2017
3. Oct 8, 2004

### arivero

I'd add, that uranium spontaneously disintegrates producing alpha and gamma radiation, and this is enough energy source to start natural reactors if the concentration of uranium is big.

4. Oct 8, 2004

### KaneOris

Aka, Critical Mass

5. Oct 8, 2004

### maccaman

cheers people, thanks for that