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Unit 12: Kinetics and Nuclear Chemistry—Rates of Reaction

Section 10: The Development of Nuclear Weapons

When a neutron causes a uranium-235 nucleus to undergo fission, three more neutrons are released along with the energy and the smaller nuclei (Figure 12-18). The equation is shown below:

Neutron-Induced Fission of Uranium-235

Figure 12-18. Neutron-Induced Fission of Uranium-235

© Science Media Group.

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Neutron-Induced Fission of Uranium-235

Figure 12-18. Neutron-Induced Fission of Uranium-235

When a neutron strikes a uranium-235 nucleus, it causes it to undergo nuclear fission. Along with energy, three neutrons and two smaller nuclei are produced.

$↖1↙0$n + $↖235↙92$U → 3$↖1↙0$n + $↖90↙37$Rb + $↖143↙55$Cs

If those neutrons go on to strike other uranium nuclei, those nuclei split and produce more neutrons, which split more nuclei, which makes more neutrons, which split more nuclei, and so on—a fission chain reaction occurs. Shortly after Meitner and Hahn's discovery of neutron-induced fission, physicists Leó Szilárd (1898–1964) and Enrico Fermi (1901–1954) found that fission chain reactions can and do occur in uranium. Recognizing the potential for such a chain reaction to be used as a bomb, Szilárd and Albert Einstein wrote a letter to President Franklin D. Roosevelt warning that the Nazis might be working along these lines, and they urged the president to begin a competing research effort to construct an atomic bomb first. Headed by J. Robert Oppenheimer (1904–1967), the Manhattan Project called on the best physicists and engineers from top universities across the United States.

Working for the Manhattan Project, Fermi and Szilárd created the first manmade fission chain reaction in 1942 near the old football stadium at the University of Chicago. Uranium and graphite bricks were stacked together into a "nuclear pile," and the chain reaction was allowed to run for 28 minutes with very little in the way of safety precautions. Having proved that fission chain reactions were possible, research in the Manhattan Project proceeded to weaponize the technology.

After five years of strenuous effort, the team detonated the first nuclear bomb in the New Mexico desert on July 16, 1945. Less than a month later, on August 6 and 9, the United States destroyed the Japanese cities, Hiroshima and Nagasaki, with atomic bombs. Japan surrendered to the Allies on August 15, ending World War II.

Two veterans of the Manhattan Project, Edward Teller (1908–2003) and Stanislaw Ulam (1909–1984) went on to develop the next generation of nuclear weapons: weapons powered by nuclear fusion instead of nuclear fission. Nuclear fusion happens when two small nuclei come together to form a larger one, as in the following reaction:

$↖1↙1$H + $↖3↙1$H → $↖4↙2$He

This process also releases an enormous quantity of energy. In fact, fusion releases much more energy than fission, and fusion bombs (called "hydrogen bombs" or "H-bombs") are thousands of times more powerful than fission bombs. (Figure 12-19)

Atomic Bomb (Nuclear Fission) vs. Hydrogen Bomb (Nuclear Fusion)

Figure 12-19. Atomic Bomb (Nuclear Fission) vs. Hydrogen Bomb (Nuclear Fusion)

© Left: Wikimedia Commons, Public Domain. Right: Wikimedia Commons, Public Domain.

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Atomic Bomb (Nuclear Fission) vs. Hydrogen Bomb (Nuclear Fusion)

Figure 12-19. Atomic Bomb (Nuclear Fission) vs. Hydrogen Bomb (Nuclear Fusion)

Fusion releases much more energy than fission. The Fat Man A-bomb blast over Nagasaki (15 kilotons) is shown on the left. Castle Bravo H-bomb test (15 megatons) is shown on the right.

For two nuclei to get close enough to fuse, they must be moving extraordinarily fast; fusion reactions require very high temperatures to occur. A fission reaction generates the necessary heat, so a fusion bomb is actually two bombs in one. A small fission bomb explodes first, which heats up enough hydrogen to make it fuse.

Fusion reactions also power the stars; the sun is an enormous fusion reactor. Small atoms such as hydrogen and helium fuse inside the sun, making larger atoms and releasing huge quantities of heat and light.


Chain reaction

A reaction in which the products cause further reactions to take place.


The combining of smaller atomic nuclei to make a larger nucleus.


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