2 december 1942 - a half watt

4:00 PM 2-Dec-1942

Compton: The Italian navigator has landed in the New World.

Conant: How were the natives?

Compton: Very friendly

 

Almost exactly 80 years ago the first human-made self sustaining nuclear reaction took place under the stands of a vacant football field at the University of Chicago.  It's a remarkable story and something of a race with Nazi Germany.  Fortunately so many competent physicists had fled Germany that the Nazi bomb project was a failure.  It turns out the wikipedia article on the subject is an accurate and readable summary of the dawn of the pre-manhattan project effort to get a handle on nuclear fission.  The piece  gets a bit technical here and there, but you can skip over those parts if you like.  I'll try and give a high level view of what a sustained reaction is without going into the quantum mechanics of the process.

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Nuclear fission is the process where the nucleus of an atom breaks apart into smaller pieces releasing energy (often quite a bit) in the process.  The nucleus of an atom is a tight cluster made up of positively charged protons and electrically neutral neutrons.

All of those positive changes packed together - you might ask why don't they fly apart? After all, electromagnetism causes like charges repel.  It turns out a very strong force, cleverly called the strong force, acts on neutrons and protons binding them together.  A difference between the two forces becomes important.  Electromagnetism is a long range force while the strong force only acts over short subatomic distances. To first order you can think of the strong force acting only between immediate neighbors while the electromagnetic force acts on all of the protons.

If the nucleus gets big enough the total repulsive electromagnetic force of all of the protons overcomes the closest-neighbor strong force and breaks the nucleus up in to smaller pieces releasing energy in the process (nuclear fission)

Most of the elements we're used to have nuclei that are too small to break up on their own.  A few are on the borderline. In the case of uranium the addition of a single neutron is enough to undergo fission and in the process it releases energy and, on average somewhat more than two extra neutrons along with a couple of nuclei that are smaller than uranium.¹

In nature uranium usually isn't concentrated and the neutrons from fission are absorbed by other materials.² But if you concentrate uranium you get to a point where each fission has a high probability of striking another uranium nucleus and so on - creating a sustained reaction. Unchecked, with enough purity, and you get an atomic bomb. At lower concentrations and with the ability control the amount of neutron absorption you get a controlled nuclear reactor that liberates a useful amount of energy.

 

A number of interesting characters were involved in these early experiments.  Enrico Fermi was the key player in the first sustained reaction.  Physics was getting specialized and he was probably the last physicist who was both a great theorist and experimentalist. 

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¹ I've left out a lot.  I haven't mentioned which isotope of uranium for example.  Also - you may have noticed that adding an extra neutron shouldn't increase the electromagnetic force as the neutron is neutral.  It turns out the uranium nuclei is so close to breaking up that the kinetic energy of the addition neutron adds just enough to break things up.  A nice example is adding a neutron to U-235.  One possible fission chain  n + U-235 → Rb-92 + Cs-140 + 3n + 200 MeV of energy. 

Also details of the strong force weren't known at the time..  A clear hypothesis didn't arrive until about 30 years later and deeper details of the mechanism took almost thirty more..

² A natural formations of uranium t pure enough to initiate sustained nuclear reactions was discovered in Gabon.  It operated for a few hundred thousand years producing less than a megawatt of power.