Nuclear Weapons
The development of a nuclear weapon was the driving force behind early research into nuclear fission during the 1940s. The term “atomic bomb” conventionally refers to weapons that rely exclusively on nuclear fission to generate an explosive output of energy. Nuclear weapons are designed to release as much energy as possible as quickly as possible. This is very different from a nuclear reactor, which is designed to maintain a constant power output. The total energy produced by fission bombs can range from 1 to 500,000 tons of explosives.
The development of a nuclear weapon was the driving force behind early research into nuclear fission during the 1940s. The term “atomic bomb” conventionally refers to weapons that rely exclusively on nuclear fission to generate an explosive output of energy. Nuclear weapons are designed to release as much energy as possible as quickly as possible. This is very different from a nuclear reactor, which is designed to maintain a constant power output. The total energy produced by fission bombs can range from 1 to 500,000 tons of explosives.
Neutrons emitted during fission reactions will travel through the material until they collide with another uranium nucleus and trigger another fission event. If the mass of uranium used in a bomb is too small, the neutrons will “leak” through the surface of the material before encountering another uranium nucleus. For larger masses of fuel, neutrons will penetrate through more material and are more likely to generate another fission event before escaping. The critical mass is the minimum amount of nuclear material necessary to sustain a nuclear chain reaction. At the critical mass, each fission event causes an average of one additional fission event. The critical mass of weapons-grade uranium-235 is a sphere about 6.8 inches in diameter, about the size of a large cantaloupe. The most commonly used fuel materials for nuclear weapons are uranium-235 and plutonium-239.
Critical mass has important implications for the design of nuclear weapons. The nuclear fuel must be kept below critical until the time of detonation. In practice, this is achieved by splitting the fuel into multiple pieces, each below the critical mass, and then rapidly driving them together at the instant of detonation. The two primary means for achieving critical mass are the “gun-type” method, in which two subcritical pieces are fired together using conventional explosives; and the “implosion” method, in which a subcritical mass is compressed inward by explosives, thereby increasing its density above criticality (Figure 7). Modern nuclear weapons have largely abandoned the gun-type method in favor of the implosion method.
The Manhattan Project
In the early months of World War II, several notable physicists became concerned about the possibility of Nazi Germany creating a nuclear weapon. In 1939, Leo Szilárd drafted a letter to President Franklin Roosevelt, signed by Albert Einstein, urging the United States to initiate its own nuclear program. In response, the United States government funded pilot research to determine how soon a bomb could be developed, and at what cost. Various methods for enriching nuclear material were explored during this phase.
In the early months of World War II, several notable physicists became concerned about the possibility of Nazi Germany creating a nuclear weapon. In 1939, Leo Szilárd drafted a letter to President Franklin Roosevelt, signed by Albert Einstein, urging the United States to initiate its own nuclear program. In response, the United States government funded pilot research to determine how soon a bomb could be developed, and at what cost. Various methods for enriching nuclear material were explored during this phase.
Under the direction of General Leslie Groves of the Army Corps of Engineers, the United States Army assumed control of the nuclear weapon program, codenamed the Manhattan Project. The primary uranium enrichment site for the Manhattan Project was established in Oak Ridge, Tennessee. Plutonium generation was carried out at the B Reactor at the Hanford plant in Washington State.
Research and development of the atomic bomb was conducted at Los Alamos Scientific Laboratory, a top-secret site in New Mexico. The director of Los Alamos, physicist J. Robert Oppenheimer, recruited a team of top scientists, including several former and future Nobel Prize winners, to design the atomic bomb. Fuel production issues prompted several design revisions by the Los Alamos group. Although the team started with a uranium gun-type design, a plutonium bomb could only be detonated with a more complicated implosion design.
The first detonation of a nuclear weapon took place during the Trinity test in the New Mexico desert on July 16, 1945 (Figure 8). The test was successful and demonstrated that the plutonium implosion design was ready to be deployed. Following the success of the Trinity test, President Truman issued terms of surrender to Japan and threatened swift destructive force if they were not accepted.
On August 6, 1945, the United States detonated a uranium bomb over Hiroshima, Japan, completely destroying five square miles of the city and resulting in a total of approximately 140,000 deaths (Figure 9). After receiving no indication of surrender, the U.S. detonated a plutonium bomb over the city of Nagasaki three days later, resulting in another 74,000 deaths. On August 15, Japan announced its unconditional surrender, marking the end of World War II.
After the war, the United States Atomic Energy Commission (AEC) was established to assume control of the American atomic energy program. The postwar “Atomic Age” was characterized by a widespread combination of fascination and fear over the potential of atomic energy. Many scientists sought to educate the public about the dangers of nuclear weapons through public speeches or newsletters.
The ensuing Cold War between the United States and the Soviet Union was marked by a race for more powerful nuclear weaponry. The Soviet Union detonated a fission weapon of their own in 1949, and both nations developed thermonuclear weapons a few years later.
The late 20th century marked a turn toward increased international oversight of nuclear activity. In 1968, the U.S. joined 61 other nations in signing the Nuclear Non-Proliferation Treaty to limit the further spread of nuclear weapons. Today, nuclear proliferation remains a pressing issue of global security. Through various treaties and international sanctions, the United States works to ensure that nations throughout the world can benefit from peaceful applications of nuclear energy while limiting the spread or misuse of nuclear materials.
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