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19.14.1: Nuclear Fission and WWII

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    World War Two brought about many advances in weaponry. The Nazis used the immense science ability of their newly conquered countries to attempt to create super weapons to keep the new territory. The weapons included new tanks like the feared Tiger Tank, or the highly advanced Luftwaffe air force. However, the one weapon the Nazi's really wanted to hold was the atomic bomb. The atomic bomb would end the war. No country would be able to keep fighting after such a huge attack with a single weapon. The problem that the Nazi's began to have though was that many of the great minds they once had left the country and came to the United States. Also, in the end Allied troops got to Hitler and his scientists just before they could complete any atomic bombs. The U.S. also wanted to create an atom bomb, and for the same reasons. The Manhattan Project was started to form this new weapon.

    Figure \(\PageIndex{1}\): Atomic Cloud over Hiroshima

    The Manhattan Project was successful in the United States because of the vast resources that could be pulled together for the project. The great minds worked under the University of Chicago football field to try and create fission. A plant in Tennessee had the sole purpose of producing enough uranium for atomic bomb testing. Finally, New Mexico was the epicenter of the whole operation. This is where the bombs were constructed and tested. The resources needed were immense. There had to be enough fuel for the bomb, and everything required to make it. There had to be housing and food for all the scientists. Maybe most importantly, there had to be enough money to support the project. The Unites States was able to effectively use all of these resources, while the Germans did not have the resources, and didn't use them as effectively. Finally, after years of hard work an atomic bomb test took place of July 16, 1945. The bomb was held together in some parts by just tape, yet it was a huge success. The problem was that the war in Europe was already over, so President Truman had to decide whether or not to use the bomb on Japan.

    Figure \(\PageIndex{2}\): Fat Man Atomic Bomb

    The war in the Pacific was a lot different than the war in Europe. In Europe, you landed once and fought your way through to Berlin to defeat the Nazi's. However, in the Pacific Japan controlled hundreds of Islands before you got to the mainland. The United States had to battle through every little island before they were even close enough where bombers could reach Japan. Battling through every little island was extremely costly for U.S. Troops. Also, the U.S. had very little allied support, and the Japanese were ready to fight until every last soldier was killed or captured. This mentality brought about the idea of using the atomic bomb on Japan. If an invasion was used, it was estimated that there could be as many as 1 million casualties and the invasion could have lasted as long as 6 months. The invasion would have cost huge amounts of American lives and resources. As a result, when the bomb had a successful test President Truman said he hardly hesitated to approve of the usage of the atomic bomb on Japan.

    The first atomic bomb was dropped on Hiroshima on August 6, 1945 by the Enola Gay. The bomb destroyed the city, killing almost 100,000 people. The city of Hiroshima was essentially leveled. Following this attack, on August 9, 1945 another atomic bomb was dropped on Nagasaki. The same effects were felt in this city. The United States was ready to drop another atomic bomb in the third week of August, 3 more in September, and 3 more in October if necessary. However, on August 15 Japan surrendered to the allies and World War Two was officially over.

    Figure \(\PageIndex{3}\): (left) Hiroshima before the atomic bomb (right) Hiroshima after the atomic bomb

    The Workings of the Atomic Bomb

    A crucial feature of the fission of uranium without which an atom bomb is impossible is that fission produces more neutrons than it consumes. As can be seen from Eqs. (1), for every neutron captured by a \(\ce{^{235}_{92}U}\) nucleus, between two and four neutrons are produced. Suppose now that we have a very large sample of the pure \(\ce{^{235}_{92}U}\) isotope and a stray neutron enters this sample. As soon as it hits a \(\ce{^{235}_{92}U}\) nucleus, fission will take place and about three neutrons will be produced. These in turn will fission three more 235U nuclei, producing a total of nine neutrons. A third repetition will produce 27 neutrons. a fourth 81. and so on. This process (which is called a chain reaction) escalates very rapidly. Within a few microseconds a very large number of nuclei fission, with the release of a tremendous amount of energy, and an atomic explosion results.


    There are two reasons why a normal sample of uranium metal does not spontaneously explode in this way. In the first place natural uranium consists mainly of the isotope \(\ce{^{238}_{92}U}\), while the fissionable isotope \(\ce{^{235}_{92}U}\) comprises only 0.7 percent of the total. Most of the neutrons produced in a given fission process are captured by \(\ce{^{238}_{92}U}\) nuclei without any further production of neutrons. The escalation of the fission process thus becomes impossible. However, even a sample of pure 235U will not always explode spontaneously. If it is sufficiently small, many of the neutrons will escape into the surroundings without causing further fission. The sample must exceed a critical mass before an explosion results. In an atomic bomb several pieces of fissionable material, all of which are below the critical mass, are held sufficiently far apart for no chain reaction to occur. When these are suddenly brought together, an atomic explosion results immediately.

    A great deal of the five years of the Manhattan Project was spent in separating the 0.7 percent of 235U from the more abundant 238U. This was done by preparing the gaseous compound UF6 and allowing it to effuse through a porous screen. (the "Kinetic Theory of Gases" sections discuss that the rate of effusion is inversely proportional to the square root of molar mass.) Each effusion resulted in a gas which was slightly richer in the lighter isotope. Repeating this process eventually produced a compound rich enough in 235U for the purposes of bomb manufacture.

    Only the first bomb dropped on Japan used uranium. The second bomb used the artificial element plutonium, produced by the neutron bombardment of 238U:

    \[ \ce{^{235}_{92} U + ^{1}_0n \rightarrow ^{239}_{94}Pu + 2 ^{0}_{-1}\beta } \nonumber \]

    Fission of Pu-239 occurs in much the same way as for U-235, giving a variety of products; for example,

    \[ \ce{^{239}_{94} Pu + ^{1}_0n \rightarrow ^{90}_{38}Pu + ^{147}_{56}Ba + 3 ^{1}_{0}n } \nonumber \]

    Again this is a highly exothermic reaction yielding about the same energy per mole (20 000 GJ mol–1) as 235U.

    This immense amount of energy released is the reason why plutonium and uranium were used. By comparison, 1 ton of TNT yields about 4 GJ mol–1 of energy. This means that the atomic bombs had about 5,000 times more energy than 1 ton of TNT, that is a huge amount of energy, and explains why the bomb was so effective and destructive.

    We take a look at the size and power of explosions created from a hand grenade to the Tsar Bomba and beyond. Its not just nukes but supervolcano eruptions & meteor impacts as well. It is indeed a terrifying true scale of nuclear weapons.

    Although the Fat Man atomic bomb was extremely powerful and destructive, it is not even close to the power of subsequent nuclear bombs. The atomic age was set forth by the United States on those two fateful days in August. In the subsequent years many other countries harnessed the power of nuclear weapons, including Russia, China, Britain, and France. The "highpoint" of the atomic age came with the explosion of the Tsar Bomba by Russia. This bomb was equal to 50 megatons of TNT, or 2,500 times more powerful than the Fat Man bomb. It was said that when the bomb was detonated the shock wave in the ground could be felt over 600 miles away (the bomb wasn't even detonated on the ground either, but rather in mid air!). The advance and use of nuclear weapons seemed to be growing at an extraordinarily fast pace. However, countries stepped up and signed various treaties, including the Nuclear Non-Proliferation Treaty, to stop the increase in nuclear weapons and hopefully the use of any more in the future.

    This page titled 19.14.1: Nuclear Fission and WWII is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Ed Vitz, John W. Moore, Justin Shorb, Xavier Prat-Resina, Tim Wendorff, & Adam Hahn.

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