On 04/01/2010 11:06, The Natural Philosopher wrote:
andrew wrote:
Andrew Gabriel wrote:

The only way to use these is in nuclear reactors. The Sun is
one, the earth itself is one, and we have built our own too.


Parts of the earth were some sort of self moderating chain reaction but I
thought that was no longer self sustaining and it's just the
decomposition
of unstable elements formed from then that keeps the middle warm now?


Its a moot point as to when natural decay becomes a reactor.

Natural decay will occur however concentrated or dilute the isotope in
question is. You get a reactor if you concentrate the material. With
uranium you need about 3% or more of U235 (up from the natural 0.7%
level). When a U235 atom fissions, out come some fast neutrons (and heat
energy). If you slow these down ("moderate" them), these neutrons can
themselves act like bullets which fission more U235 atoms.

In a reactor, you need to keep the number of slow neutrons produced per
U235 atom-fission at around 1.0. A bit above, and the reactor produces
more heat. A bit below and you cool it off. This is done by inserting
control rods which absorb the neutrons. If the ratio gets too much above
1.0, then you are in trouble.

Obviously with a bomb you want it as much above 1.0 as poss.

--
Tim

"That excessive bail ought not to be required, nor excessive fines
imposed, nor cruel and unusual punishments inflicted"

Bill of Rights 1689