Nuclear Fission: The Nuclear Chain Reaction

Worlds Within Worlds: The Story of Nuclear Energy, Volume 3 (of 3), by Isaac Asimov is part of HackerNoon’s Book Blog Post series. You can jump to any chapter in this book here. Volume III, NUCLEAR FISSION: The Nuclear Chain Reaction

The Nuclear Chain Reaction

Earlier in this history, we discussed chain reactions involving chemical energy. A small bit of energy can ignite a chemical reaction that would produce more than enough energy to ignite a neighboring section of the system, which would in turn produce still more—and so on, and so on. In this way the flame of a single match could start a fire in a leaf that would burn down an entire forest, and the energy given off by the burning forest would be enormously higher than the initial energy of the match flame.

Might there not be such a thing as a “nuclear chain reaction”? Could one initiate a nuclear reaction that would produce something that would initiate more of the same that would produce something that would initiate still more of the same and so on?

In that case, a nuclear reaction, once started, would continue of its own accord, and in return for the trifling investment that would serve to start it—a single neutron, perhaps—a vast amount of breakdowns would result with the delivery of a vast amount of energy. Even if it were necessary to expend quite a bit of energy to produce the 1 neutron that would start the chain reaction, one would end with an enormous profit.

What’s more, since the nuclear reaction would spread from nucleus to nucleus with millionths-of-a-second intervals, there would be, in a very brief time, so many nuclei breaking down that there would be a vast explosion. The explosion was sure to be millions of times as powerful as ordinary chemical explosions involving the same quantity of exploding material, since the latter used only the electromagnetic interaction, while the former used the much stronger nuclear interaction.

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The first to think seriously of such a nuclear chain reaction was the Hungarian physicist Leo Szilard (1898-1964). He was working in Germany in 1933 when Adolf Hitler came to power and, since he was Jewish, he felt it would be wise to leave Germany. He went to Great Britain and there, in 1934, he considered certain new types of nuclear reactions that had been discovered.

In these, it sometimes happened that a fast neutron might strike a nucleus with sufficient energy to cause it to emit 2 neutrons. In that way the nucleus, absorbing 1 neutron and emitting 2, would become a lighter isotope of the same element.

But what would happen if each of the 2 neutrons that emerged from the original target nucleus struck new nuclei and forced the emission of a pair of neutrons from each. There would now be a total of 4 neutrons flying about and if each struck new nuclei there would next be 8 neutrons and so on. From the initial investment of a single neutron there might soon be countless billions initiating nuclear reactions.

Szilard, fearing the inevitability of war and fearing further that the brutal leaders of Germany might seek and use such a nuclear chain reaction as a weapon in warfare, secretly applied for a patent on a device intending to make use of such a nuclear chain reaction. He hoped to turn it over to the British Government, which might then use its possession as a way of restraining the Nazis and keeping the peace.

However, it wouldn’t have worked. It took the impact of a very energetic neutron to bring about the emission of 2 neutrons. The neutrons that then emerged from the nucleus simply didn’t have enough energy to keep things going. (It was like trying to make wet wood catch fire.)

But what about uranium fission? Uranium fission was initiated by slow neutrons. What if uranium fission also produced neutrons as well as being initiated by a neutron? Would not the neutrons produced serve to initiate new 129fissions that would produce new neutrons and so on endlessly?

It seemed very likely that fission produced neutrons and indeed, Fermi, at the conference where fission was first discussed, suggested it at once. Massive nuclei possessed more neutrons per proton than less massive ones did. If a massive nucleus was broken up into 2 considerably less massive ones, there would be a surplus of neutrons. Suppose, for instance, uranium-238 broke down into barium-138 and krypton-86. Barium-138 contains 82 neutrons and krypton-86 50 neutrons for a total of 132. The uranium-238 nucleus, however, contains 146 neutrons.

The uranium fission process was studied at once to see if neutrons were actually given off and a number of different physicists, including Szilard, found that they were.

Now Szilard was faced with a nuclear chain reaction he was certain would work. Only slow neutrons were involved and the individual nuclear breakdowns were far more energetic than anything else that had yet been discovered. If a chain reaction could be started in a sizable piece of uranium, unimaginable quantities of energy would be produced. Just 1 gram of uranium, undergoing complete fission, would deliver the energy derived from the total burning of 3 tons of coal and would deliver that energy in a tiny fraction of a second.

Szilard, who had come to the United States in 1937, clearly visualized the tremendous explosive force of something that would have to be called a “nuclear bomb”. Szilard dreaded the possibility that Hitler might obtain the use of such a bomb through the agency of Germany’s nuclear scientists.

Partly through Szilard’s efforts, physicists in the United States and in other Western nations opposed to Hitler began a program of voluntary secrecy in 1940, to avoid passing along any hints to Germany. What’s more, Szilard enlisted the services of two other Hungarian refugees, the physicists Eugene Paul Wigner (1902- ) and Edward Teller (1908- ) and all approached Einstein, who had also fled Germany and come to America.

Leo Szilard

Eugene P. Wigner

Einstein was the most prestigious scientist then living and it was thought a letter from him to the President of the United States would be most persuasive. Einstein signed such a letter, which explained the possibility of a nuclear bomb and urged that the United States not allow a potential enemy to come into possession of it first.

Largely as a result of this letter, a huge research team was put together in the United States, to which other Western nations also contributed, with but one aim—to develop the nuclear bomb.

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Isaac Asimov. 2015. Worlds Within Worlds: The Story of Nuclear Energy, Volume 3 (of 3). Urbana, Illinois: Project Gutenberg. Retrieved May 2022 from https://www.gutenberg.org/files/49821/49821-h/49821-h.htm#c32

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