The Manhattan Project
General History: The Manhattan Project to the Cold War
The first successful experiments in splitting a uranium atom occurred at the Kaiser Wilhelm Institute in Berlin in 1938. The first scientists in the United States to become aware of the dangerous potential associated with these experiments were a small group of refugee physicists who has fled Europe and the growing Nazi menace. If given sufficient time and resources, these scientists had little doubt that Nazi Germany would eventually use this information in the development of a potential super weapon – an atomic bomb. In the summer of 1939, three of these scientists, Leo Szliard, Eugene Wigner, and Edward Teller – convinced Albert Einstein of the potential danger of such a weapon, and also convinced him to write a letter to U.S. President Franklin Roosevelt, in an attempt to spur the government to action. This letter, eventually known as the Szliard-Einstein letter, resulted in the creation of a secret uranium committee on October 19,1939, which ultimately led to the creation of the Manhattan Project in August 13, 1942. In September 17. 1942 Colonel Leslie R. Groves is appointed head of the project and is then promoted to Brigadier general six days later. This set into motion an incredible series of events that ultimately resulted in the creation of three atomic bombs: Trinity (a prototype plutonium based atomic bomb this tested on July 16, 1945), Little Boy (a gun model uranium bomb that is dropped on Hiroshima on August 6, 1945), and Fat Man (an implosion model plutonium bomb) that is dropped on Nagasaki on August 9, 1945).
The Two Paths of the Manhattan Project
The Manhattan Project pursued two parallel paths in the development of an atomic weapon: 1) the development of a uranium based bomb using a highly enriched form of uranium isotope U-235 (the only fissionable form of uranium), and 2) a plutonium based bomb using the element plutonium, an extremely rare element that could only be produced in sufficient quantities as a by-product of a nuclear reaction in a nuclear reactor (which did not exist at the time) by bombarding uranium isotope U-238 with neutrons resulting from the fission of U-235. However, in order to create a sustainable nuclear reaction for use in the production of plutonium, scientists would need uranium containing at least 5% U-235. Because natural occurring uranium only contains about 0.7% U-235, scientist were faced with the challenge of having to come up with a way to enrich uranium from 0.7% to at least 5%. Further, in order to produce a uranium based bomb, scientist calculated they would need approximately 64 kilograms of highly enriched uranium (i.e. uranium made up of at least 80% U-235). Thus, in both cases, a method for enriching uranium was needed.
While the Manhattan project occurred at several locations across the United States, the three main facilities were at Oak Ridge, Tennessee (where the initial uranium enrichment work was pursued); Hanford Washington (where the initial plutonium development was pursued), and Los Alamos, New Mexico (where the bombs were initially developed and tested).
Since no proven method for uranium enrichment existed at the beginning of the Manhattan Project, four possible strategies were initially tried:
- Electromagnetic separation – which used huge magnets to separate the U235 from the U238.
- Thermal diffusion – which used the temperature difference between cold and hot plates to separate the U235 from the U238.
- Centrifuge technology – in which the heavier U238 atoms would be flung to the outside of a rotating cylinder while more of the lighter U235 atoms could be “scooped” and separated from the U238.
- Gaseous Diffusion – which mixed uranium with a fluorine gas which was then pumped though huge “converters” which had a special type of nickel barrier or filter which would allow more of the smaller U235 atoms to pass through, thereby separating the U235 from the U238. A schematic of one of these converters is shown in the figure below. Unfortunately, this was a very inefficient process, so that several hundred converters all strung together (called a cascade) were needed just to enrich the U235 just a few percentage points.
Four Enrichment Technologies Reduced to Three:
Ultimately, the centrifuge approach was abandoned at the beginning of the project. An initial pilot facility to test the thermal diffusion process was built (the S50 building), but the thermal diffusion process proved to be very inefficient and the facility was shut down shortly after the end of the war. As a result, the electromagnetic approach and the gaseous diffusion approach were continued. This resulted in the construction of two huge facilities at Oak Ridge: the Y-12 facility (based on the electromagnetic approach), and the K-25 building based on the gaseous diffusion approach. A third major facility, the X-10, was also built at Oak Ridge and housed the first plutonium production reactor to be built in the United States and served as the pilot for several additional plutonium production reactors to be built at Hanford Washington and then later at Savanah River South Carolina.
Map of the Oak Ridge Nuclear Facilities (link)
Picture of the Oak Ridge Power Plant and S50 Facility (link)
Picture of the X-10 Complex
Schematic of the X-10 Graphite Nuclear Reactor at Oak Ridge Tennessee (link)
Loading Face of the X-10 Reactor (link)
Picture of the K-25 building at Oak Ridge (link)
At the time of its construction, the K-25 building was the largest building in the world.
Following the end of World War II, the gaseous diffusion process was finally selected as the sole strategy for use in enriching uranium. The thermal diffusion process was the least efficient of the remaining three and the electromagnetic separation process used ten times the amount of electricity as the gaseous diffusion process (at one time, it was estimated that 10% of the nation’s electricity was being consumed by Oak Ridge Tennessee). As a result, both the S50 facility and the Y-12 facilities were shut down in September of 1945.
In order to increase the U.S. nuclear weapons arsenal, additional enrichment facilities were built in Oak Ridge between 1945 and 1951. These included: 1) the K27 facility, which operated from 1945 to 1985 and was capable of enriching U235 up to 20%, and 2) the K29 facility, which operated from 1951 to 1985 and was capable of enriching U235 up to 10%. Two additional facilities, K31 and K33 were built in 1951 and 1954 respectively to provide enriched uranium for use in fuel rods for commercial nuclear power plants. Together, along with the K25 building, all five of these facilities became known as the K25 complex. In 1964, the K-25 and K-27 facilities were shut down, while enrichment operations for commercial reactor applications continued at the K29, K31, and K33 facilities until 1985. In 1969, the K25 facility again saw operation as part of a “toll” enrichment program in which private customers supplied uranium compounds to K-25, for subsequent enrichment and purchase. This program also ended in 1985.
In addition to the expansion of the K25 complex, the cold war with the Soviet Union brought with it the construction of two more uranium enrichment facilities: 1) The Paducah Gaseous Diffusion Plant (which was completed in 1952), and 2) the Portsmouth Gaseous Diffusion Plant (which was completed in 1956). In the 1960s, the Portsmouth plant's mission changed from enriching uranium for nuclear weapons to one focused on producing fuel for commercial nuclear power plants. Until 2001, Portsmouth and its sister plant in Paducah, Kentucky, worked in tandem to enrich uranium for use in nuclear power plants. The Paducah plant enriched uranium up to 2.75% and then shipped it to Portsmouth for further enrichment to approximately 4% - 5%. In 2001, enrichment operations at the Portsmouth Plant end, and all operations were consolidated at the Paducah Plant. That same year, an assay upgrade program was completed, enabling the Paducah plant to enrich uranium at levels up to 5% (USEC, 2010).
Map of the K25 Complex (link)
Aerial Photograph of the K25 Complex
Picture of the K-27 Building with the adjacent remaining footprint of the K-29 building at Oak Ridge (link)
Paducah Gaeous Diffusion Plant (link)
Paducah Gaseous Diffusion Plant
Portsmouth Facility