The Manhattan Project

The Manhattan Project

The Manhattan Project was the code name of the United State's attempt to construct an atomic bomb during World War II. It earned its name after the Manhattan Engineer District of the U.S. Army Corps of Engineers, because most of its earlier research was done in New York City. An atomic bomb is a weapon that uses the energy from a nuclear reaction called Fission for its destruction. The idea that mass could be changed into energy was predicted by Albert Einstein in the earlier part of the 1900's. John D. Cockcroft and Ernest Walton confirmed this by experiments in 1932. Then in 1938, nuclear fission was discovered by German scientists, and it was feared by many of the U.S. scientists, that Hitler would try to build a fission bomb. Three Hungarian-born physicists, Leo Szilard, Eugene Wigner, and Edward Teller asked Albert Einstein to send a letter to Franklin Roosevelt. Compelled by the letter in late 1939, Roosevelt ordered an effort to obtain an atomic weapon before Germany. At first, this program was led by Vannevar Bush, head of the National Defense Research committee and the Office of Scientific Research and Development. Then it came under control of Leslie Groves of the Army Corps of Engineers. Groves quickly bought a site in Oak Ridge, Tennessee, as a place for processing the Uranium-235 from the more common Uranium-238. Uranium-235 is used because it is fissionable, it releases many neutrons, and does not capture many. However, 99.3% of uranium in nature is the U-238 isotope, and only .7% is the lighter, more "fissionable" isotope U-235. Next, he gathered and combined research from many East Coast universities under direction of Arthur Compton, at the University of Chicago. He appointed theoretical physicist, J. Robert Oppenheimer as the director of the weapons laboratory, which was built on an isolated mesa located at Los Alamos, New Mexico. After much work, a porous barrier that could separate the isotopes of uranium was made, and it was installed in the Oak ridge gaseous diffusion plant. In 1945, uranium-235, pure enough for use in a bomb was produce and sent to Los Alamos, where it was made into a gun-type weapon. One small piece of Uranium-235, which was not big enough to hold a chain reaction itself, was fired at another small piece. This was done by means of explosive charge, inside a cylinder shaped tube, which formed a supercritical mass that exploded instantly. They were so sure that this would work, that they did not even test it. Its first use was made in military action over Hiroshima, Japan, on August 6, 1945. The bomb uses a device called an altimeter to measure how far it is from the ground. It sends out radio frequencies which are bounced back to it. Microchips in the bomb determine how far it still has to fall, and when to detonate. The bombs also have fuses in the front which actually arm the bomb. They are not inserted until the bomb is ready to be launched. Before this bomb was developed, another kind was proposed. Uranium-238 could capture a neutron and become Uranium-239. All uranium has 92 protons. U-238 has 146 neutrons, and the added neutron raised the mass to 239. But U-239 is very unstable and it decays to neptunium-239 (93 protons, 146 neutrons), and plutonium-239 (94 protons and 145 neutrons). Plutonium-239 was fissionable, and could be separated from uranium by chemical techniques ( much easier than physical process of separating the different isotopes of 235 and 238 of the same element). The first successful reactor was made at the University of Chicago under the Italian physicist Enrico Fermi. On December 2, 1942 it made a controlled chain reaction. Five large reactors were built at Hanford, Washington, where U-238 was blasted with neutrons to make plutonium. It was then sent to Los Alamos. Since another isotope of plutonium was also fissionable, there was a fear that a chain reaction could start to soon when the pieces of plutonium where brought together, making it blow apart before it was consumed. To overthrow this problem, the plutonium would have to be brought together much faster than the methods use for the uranium bomb. A technique called implosion was used to make the plutonium bomb work. A noncritical shell of plutonium was surrounded by chemical high-explosives. When detonated, it squeezed the plutonium into a very dense supercritical mass, that in chain reaction lasted long enough for a large and destructive explosion. This type of bomb was tested 60 miles northwest of Alamogordo on what is now the White Sands Missile Range on July 16, 1945. This bomb was used on Nagasaki, Japan, on August 9, 1945. When the "uranium based" atomic bomb, (nick-named Little Boy), was dropped (by the Enola Gay , flown by Colonel Paul Tibbets), on Hiroshima, Japan, on August 6, 1945, at 8:15 a.m., five square miles of the city were completely destroyed in seconds, and most buildings in the city were destroyed or damaged. The bomb weighed 9,700 pounds. It detonated 1900 feet above the city, and exploded with a force of 20,000 tons of TNT. About 75,000 people (including 20 American airmen held as POWs)were killed. Another 70,000 were injured. By the end of the year the death number had risen to 140,000 from radiation sickness. Five years later it had reached 200,000. The Peace Memorial park was made in memory of the bombing. It has a monument and a marble tomb, in memory of the bomb's victims, and the remains of the Industrial Exhibition Hall. The Peace Memorial Museum in the park has relics of the attack. Nearby is the Atomic Bomb Casualty Commission, which studies the effects of radiation. Clinics have been set up to fight radiation illness and other effects of the bomb. The "plutonium-based"