Re: Where or how did Enrico Fermi get the neutrons to split uranium 235?

The short answer to your question is that the neutrons came from the Uranium-235.  
U-235 is a "naturally occuring radioisotope"....in other words Uranium is a 
radioactive element which naturally decomposes, breaking up into smaller elements 
and neutrons.   This may seem a bit confusing to you, so I will explain the whole 
idea thoroughly below, starting off with what radioactivity is and ending with how 
the nuclear reactor works.

There are two fundamental forces which we must consider when talking about 
radioactivity:  the electrostatic force and strong nuclear force.  The electrostatic 
force is what gives all atomic particles their charges (protons have a positive charge, 
electrons a negative charge, and neutrons have a neutral, or no charge).  This forces 
is repulsive between like charges (like positive with positive, and negative with 
negative) and attractive between dislike charges (like positive with negative).  
The strong nuclear force is sort of like a gravitational force, but only between 
atomic particles.  This force is attractive between all three atomic particles 
and has nothing to do with their electric charge.

The main difference between these two forces is their relative strengths.  The 
electrostatic force is considerably weaker than the strong nuclear force, but 
it has the ability to act over long distances.  On the other hand, the strong 
nuclear force is very powerful, but only has the ability to act over very small 
distances of about 2fm - that is two femtometers or 0.000000000000002 meters.

Now, lets just look at the nucleus and forget about the electrons for a while.  
With any nucleus being made up of just protons and neutrons, all of the positive 
charges on of the protons will be repelling each other away.  You would think that 
the nucleus would surely not be able to hold itself together.  This is where the 
strong nuclear force comes in.  There is an attracive strong nuclear force between 
all of the protons and neutrons in the nucleus.  This force is enough to overpower 
the electrostatic force of the protons pushing each other away and hold the nucleus 
together.  Radioactivity occcurs when these two forces can no longer counteract each 
other.....I will explain below.

Now to help visualize this next part, try looking at a the Periodic Table of Elements 
(check in a chemistry text or ask your teacher).  For elements with small numbers of 
particles in the nucleus, such as Carbon (C) and Iron (Fe), the nuclear force can easily 
hold the nucleus together.  As the elements get larger, and the number of protons in the 
nucleus increases, the electrostatic force between the protons also increases.  To 
counteract this repulsion with a strong nuclear force, more neutrons must be added to 
the nucleus.  Now, since the strong nuclear force can only act over small distances, as 
you add more and more neutrons, in larger elements, the size of the nucleus will eventually 
get larger than the range which the strong nuclear force can act in.  This means that 
eventually the electrostatic force (which is still effective over long distances) will be 
able to over power the strong nuclear force.  This this imbalance of forces makes the 
nucleus unstable and causes it to decompose, or break up, into smaller atoms and 
particles............like neutrons!!!

As you may have already guessed, this is exactly what happens to U-235 and is also called 
'radioactive decay'.  This process can happen naturally, like I described above, or can be 
induced to happen through a process called 'fission'.  Fission is the central process around 
which nuclear reactors operate.  In fission a neutron collides with a U-235 atom.  This neutron 
is captured by the atom and causes the U-235 to become unstable.  The atom begins to oscillate, 
eventually breaking into two pieces, along with releasing a few neutrons and heat.  These 
neutrons then go on to be captured by other U-235 atoms, which then repeat the process.  Here 
is an equation which describes what I have just said:

	n + U-235 ==> U-236* ==> Xe-140 + Sr-94 + 2n + Q

where:	n = a neutron
	U-236* = the U-235 with the extra neutron 
		    (the '*' means the atom is in an excited state, where it is oscillating)
	Q = heat

In a nuclear reactor, the stray neutrons (given off by natural radioactive
decay) can be enough to start a nuclear reaction.  Once started, it will be 
able to create more neutrons through the fission process and start a chain 
reaction, like the one in the above equation.  This is most likely how Enrico 
Fermi started his reactor.  The main problem with this method is that it may 
take a long time to get started - U-235 atoms are only slightly radioactive 
and only decay naturally once in a long while.

A much more efficient method, and one used in modern reactors, is to put a 
small amount of an element which decays quite often, in with the U-235, to kind 
of jump-start the reactor and get the fission going.  I found this information 
out by talking to someone I know at the McMaster University Nuclear Reactor (Simon Day).  
He told me that they use a compound made of Se/Be (that is Selenium and Beryllium) to speed 
things up in their reactor.

I hope this answer has helped to explain where the neutrons come from in a neuclear reactor.  
If you have any more questions about how nuclear reactors work, just send me an email.

	Kurt Frost
	kfrost@sympatico.ca

Also check out these websites:
	Enrico Fermi: 
           a page with a short history on Enrico Fermi

	The Canadian Nuclear Society Homepage: 
            they have lots of nuclear info....look at the pages for education and links


REFERENCES:

Benson, H.  1991.  University Physics.  New York: John Wiley & Sons, Inc.

Day, Simon.  Phd. Student McMaster University.  Personal Conference. Hamilton, Ontario, Canada.

Harms, A.A.  1987.  Principles of Nuclear Science and Engineering.  Toronto: Research
	Studies Press Ltd.

Gillespie, R.J., D.A. Humphreys, N.C. Baird, and E.A. Robinson.  1989.  Chemistry 2nd.
	ed.  Needham Heights, Massachusetts: Allyn and Bacon, Inc.