Heavy nuclei split into two fragments of roughly equal mass. Energy is released in the process. Fission powers nuclear reactors and "small" nuclear weapons.
|23592U||+||10n||→||fission fragments||+||2.4 neutrons||+||192.9 MeV|
|23994Pu||+||10n||→||fission fragments||+||2.9 neutrons||+||198.5 MeV|
chain reaction: subcritical, critical, supercritical
Cartoon. Alternating series of parents and daughters and parents and daughters. At the end its nothing but fission fragments and free neutrons. Watch out.
chain reaction timeline
Leo Szilard recalls the day
As the light changed to green and I crossed the street, it … suddenly occurred to me that if we could find an element which is split by neutrons and which would emit two neutrons when it absorbs one neutron, such an element, if assembled in sufficiently large mass, could sustain a nuclear chain reaction …. I didn't see at the moment just how one would go about finding such an element, or what experiments would be needed, but the idea never left me. In certain circumstances it might be possible to set up a nuclear chain reaction, liberate energy on an industrial scale, and construct atomic bombs.
I found myself in London about the time of the British Association meeting in  September 1933. I read in the newspapers a speech by Lord Rutherford, who was quoted as saying that he who talks about the liberation of atomic energy on an industrial basis is talking moonshine. This set me pondering as I was walking the streets of London, and I remember that I stopped for a red light at the intersection of Southampton Row [at Russell Square]. As the light changed to green and I crossed the street, it suddenly occurred to me that if we could find an element which is split by neutrons and emit two neutrons when it absorbed one neutron, such an element, if assembled in sufficiently large mass, could sustain a nuclear chain reaction. I didn't see at the moment just how one would go about finding such an element, or what experiments would be needed, but the idea never left me. Soon thereafter, when the discovery of artificial radioactivity by Joliot and Mme. Joliot was announced, I suddenly saw that tools were at hand to explore the possibility of such a chain reaction. I talked to a number of people about this …. [I]in the spring of 1934 I had applied for a patent which described the laws governing such a chain reaction. It was the first time, I think, that the concept of critical mass was developed and that a chain reaction was seriously discussed. Knowing what this would mean - and I knew it because I had read H.G. Wells - I did not want this patent to become public. The only way to keep it from becoming public was to assign it to the government. So I assigned this patent to the British Admiralty.
On Tuesday, September 12, 1933, while waiting at the lights to cross the road to the British Museum in Bloomsbury, Leo Szilard, a Hungarian theoretical physicist, had the flash of insight which was to result in the Little Boy and Fat Man bombs being dropped on Hiroshima and Nagasaki less than 12 years later. "As the light changed to green and I crossed the street, it suddenly occurred to me that if we could find an element which is split by neutrons, and which would emit two neutrons when it absorbs one, such an element could sustain a nuclear chain reaction."
Just take excerpts from this letter by Leo Szilard
I feel that I ought to let you know of a very sensational new development in nuclear physics. In a paper in the Naturwissenschaften Hahn reports that he finds when bombarding uranium with neutrons the uranium breaking up into two halves giving elements of about half the atomic weight of uranium. This is entirely unexpected and exciting news for the average physicist. The Department of Physics at Princeton, where I spent the last few days, was like a stirred-up ant heap.
Apart from the purely scientific interest there may be another aspect of this discovery, which so far does not seem to have caught the attention of those to whom I spoke. First of all it is obvious that the energy released in this new reaction must be very much higher than in all previously known cases. It may be 200 million (electron-) volts instead of the usual 3-10 mil-lion volts. This in itself might make it possible to produce power by means of nuclear energy, but I do not think that this possibility is very exciting, for if the energy output is only two or three times the energy input, the cost of investment would probably be too high to make the process worthwhile.
Unfortunately, most of the energy is released in the form of heat and not in the form of radioactivity.
I see, however, in connection with this new discovery potential possibilities in another direction. These might lead to a large-scale production of energy and radioactive elements, unfortunately also perhaps to atomic bombs. This new discovery revives all the hopes and fears in this respect which I had in 1934 and 1935, and which I have as good as abandoned in the course of the last two years. At present I am running a high temperature and am therefore confined to my four walls, but perhaps I can tell you more about these new developments some other time. Meanwhile you may look out for a paper in "Nature" by Frisch and Meitner which will soon appear and which might give you some information about this new discovery.
23390Th + 10n → 23392U + 20−1e
and U233 fissions into junk and neutrons. Neutrons hit thoriums making more U233s and life goes on.