Nuclear Fission

As we just learned, the mass defect can potentially lead to an enormous release of energy. Read this text to learn about this type of nuclear reaction. Fission means to break up. This text shows the chain reaction that can occur when a nuclear fission reaction is started. In the example in the text, each reaction between a neutron and a uranium isotope produces three more neutrons, which can each react in turn with another uranium. A certain amount of fission material must be available to sustain this reaction. We call this the critical mass.

While the liberation of large amounts of energy results from the splitting of the atom as a consequence of the mass defect, another observation is key to the successful harnessing of nuclear fission.

It should be noted from Equation 3 (^{235}_{92}\text{U}+ ^{1}_{0}n\rightarrow\ ^{140}_{56}\text{Ba}+^{93}_{36}\text{Kr}+3\ ^{1}_{0}n) and Equation 4 (^{235}_{92}\text{U}+^{1}_{0}n\rightarrow\ \ ^{137}_{52}\text{Te}+ ^{97}_{40}\text{Zr}+2\ ^{1}_{0}n) from the previous section that additional neutrons are created in addition to the splitting of uranium-235 by a single neutron, and the formation of two new atoms.

Clearly, if these have sufficient energy they can continue and react with another uranium-235 atom. The reaction of a single uranium-235 atom with a single neutron can generate three additional neutrons. Each of these three neutrons can react with three uranium-235 atoms, from each of which will come a total of nine neutrons. In this process more neutrons are generated than used in the original reaction that is the basis of a chain reaction.

Representation of a chain reaction and the increase in total number of neutrons with each reaction step.

Despite the view of a chain reaction in the figure above, there needs to be a minimum amount of mass to ensure that each neutron is absorbed and allow for the chain reaction to continue. This is known as the critical mass, which is defined as the smallest amount of fissile material needed for a sustained nuclear chain reaction.

Once the idea of uranium fission was reported several people started to think about the atomic bomb H. G. Wells proposed. Hungarian physicist Leó Szilárd had filed for a patent on the concept of nuclear fission, but his attempt to create a chain reaction using beryllium and indium, was unsuccessful. In 1936, he assigned the chain-reaction patent to the British Admiralty to ensure its secrecy (GB Patent 630726).

Szilárd was also the co-holder, with Enrico Fermi, of the patent on the nuclear reactor, and, possibly most importantly, he drafted a confidential letter to the U.S. President Franklin D. Roosevelt explaining the possibility of nuclear weapons. In order to receive the weight the letter deserved, Szilárd persuaded his friend Albert Einstein to sign the letter. It was this letter that initiated the Manhattan project that led to the creation of the first atomic bombs.

Photo of Leo Szilard

Leó Szilárd (1898 – 1964).

Roosevelt signed the executive order to start the Manhattan project on a Saturday before listening to a baseball game on the radio. If he had waited until Monday, the order may have been forgotten since he signed the order on Dec. 6, 1941 – the Japanese attacked Pearl Harbor on the next day which brought the United States into World War II.

The Manhattan Project resulted in the manufacture of two atomic bombs after a test of a plutonium-239 device at Trinity Site, New Mexico, on July 16, 1945. Little Boy was made from uranium-235, and was dropped on Hiroshima on August 6, 1945. Fat Man, which was dropped on Nagasaki on August 9, 1945, was made primarily of plutonium-239. The physical effect of these bombs was dramatic and catastrophic. Temperatures of over 3000 °C were observed more than two miles from ground zero. To put this into perspective, the boiling point of iron is 2750 °C, while the boiling point of the sand in concrete is 2350 °C.

Photo of the atomic bomb called Little boyPhoto of the atomic bomb called Fat man

Photos of the two atomic bombs called the Little Boy and the Fat Man atomic bomb. Images from Wikipedia.

Source: Andrew R. Barron,
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Last modified: Monday, May 17, 2021, 3:46 PM