Need help Nuclear Fission Question

[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.

[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 :wink:
Look at this (http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/u235chn.html) 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 here (http://www.npp.hu/mukodes/lancreakcio-e.htm) 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 (http://en.wikipedia.org/wiki/Nuclear_fission)
some critical masses at the end of this page (http://nuclearweaponarchive.org/Library/Fission.html)
Lawrence Berkeley National Laboratory (http://www.lbl.gov/abc/wallchart/chapters/14/1.html)
Uni. of Prince Edwards (http://www.upei.ca/~physics/p261/projects/nuclear2/release.html)

[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.

[KaneOris]

Aka, Critical Mass

[maccaman]

cheers people, thanks for that