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Fermi 1
Fermi 1 was the United States' only demonstration-scale breeder reactor, built during the 1950s at the Enrico Fermi Nuclear Generating Station on the western shore of Lake Erie south of Detroit, Michigan. It used the sodium-cooled fast reactor cycle, in which liquid sodium metal is used as the primary coolant instead of typical nuclear reactor designs cooled by water. Sodium cooling permits a more compact core, generating surplus neutrons used to produce more fission fuel by converting a surrounding "blanket" of 238U into 239Pu which can be fed back into a reactor. At full power, it would generate 430 MW of heat (MWt), or about 150 MW of electricity (MWe).
The design and construction of Fermi 1 was led by Walker Lee Cisler, president of Detroit Edison. Cisler believed that the breeder cycle would dominate the future commercial market because it would provide an effectively limitless supply of fuel, and he championed efforts to produce Fermi 1 based on the design of the small experimental EBR-I in Idaho. His efforts were supported by Lewis Strauss, chair of the U.S. Atomic Energy Commission (AEC), who was a strong advocate of private companies entering the nuclear field.
On November 29, 1955, EBR-I suffered a partial meltdown for reasons that were not completely understood. Construction licensing for Fermi 1 began in January 1956. The AEC review panel recommended that the design should not proceed until the problems with EBR, and breeder design in general were better understood through testing on newer experimental systems like EBR-II. When their report was cited in congressional hearings, Strauss refused to discuss it and approved construction. This led to a firestorm of debate within Congress and the press, along with a series of lawsuits by the United Auto Workers that briefly led to its construction license being revoked.
Construction was delayed by several years and the budget doubled. Operation had been planned for 1959 or early 1960 but Fermi 1 achieved criticality on August 23, 1963. While slowly increasing its power over the next two years, on October 5, 1966 it suffered a partial meltdown when the flow of sodium was disrupted by blockage of the inlet holes at the bottom of the reactor. The problem was detected early enough to safely scram the reactor and there was no radioactive release outside the containment building. The site was shut down for repairs and restarted in July 1970. It ran only until closing again on November 27, 1972, and was officially decommissioned on December 31, 1975.
Most commercial reactors run on fissile 235U fuel. In nature, 235U is mixed with a much larger amount of non-fissile 238U. There is so much 238U that the 235U atoms are so sparse that a chain reaction is impossible. In most cases, this is overcome by two methods. One method is to "enrich" the fuel, concentrating the 235U so the neutrons have a greater probability of hitting them. The enrichment leaves a by-product of non-fissile 238U referred to as depleted uranium. The other method is to slow the neutrons, or "moderate" them, which increases the chance they will undergo a reaction.
The most common reactor designs use both of these methods, slightly enriching the fuel to about 3 to 5% 235U and using water as a moderator. In these designs, 238U is still the majority of the fuel. Some neutrons from the fission events hit these atoms and are captured, turning them into 239Pu. These can also undergo fission like the 235U. About 35% of the fission in a typical reactor is of the 239Pu created in this fashion.
It is possible to increase the rate of capture and gain additional fuel. However, this process is much more efficient when the neutrons have higher energy, which works counter to the moderation needed for the 235U. This leads to a class of designs that are optimized for the production of 239Pu. These breeders are generally built in two parts, a "core" of fuel that is enriched to the point where it can maintain a chain reaction without a moderator, and a "blanket" of 238U surrounding it that is designed to capture any spare neutrons and create 239Pu.
235U and 238U are chemically identical, and are difficult to separate using mechanical processes based on their slightly different mass. In contrast, uranium and plutonium have different chemistry, and can be separated with chemical processes. This results in relatively pure plutonium that can be used for fuel in the core of the breeder with no further enrichment. Once the system is running, it is possible to produce enough plutonium to replace the original uranium fuel entirely, and still have more left over. This additional plutonium can then be used in the core of other breeders, or mixed with uranium and burned in conventional (non-breeder) reactors.
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Fermi 1
Fermi 1 was the United States' only demonstration-scale breeder reactor, built during the 1950s at the Enrico Fermi Nuclear Generating Station on the western shore of Lake Erie south of Detroit, Michigan. It used the sodium-cooled fast reactor cycle, in which liquid sodium metal is used as the primary coolant instead of typical nuclear reactor designs cooled by water. Sodium cooling permits a more compact core, generating surplus neutrons used to produce more fission fuel by converting a surrounding "blanket" of 238U into 239Pu which can be fed back into a reactor. At full power, it would generate 430 MW of heat (MWt), or about 150 MW of electricity (MWe).
The design and construction of Fermi 1 was led by Walker Lee Cisler, president of Detroit Edison. Cisler believed that the breeder cycle would dominate the future commercial market because it would provide an effectively limitless supply of fuel, and he championed efforts to produce Fermi 1 based on the design of the small experimental EBR-I in Idaho. His efforts were supported by Lewis Strauss, chair of the U.S. Atomic Energy Commission (AEC), who was a strong advocate of private companies entering the nuclear field.
On November 29, 1955, EBR-I suffered a partial meltdown for reasons that were not completely understood. Construction licensing for Fermi 1 began in January 1956. The AEC review panel recommended that the design should not proceed until the problems with EBR, and breeder design in general were better understood through testing on newer experimental systems like EBR-II. When their report was cited in congressional hearings, Strauss refused to discuss it and approved construction. This led to a firestorm of debate within Congress and the press, along with a series of lawsuits by the United Auto Workers that briefly led to its construction license being revoked.
Construction was delayed by several years and the budget doubled. Operation had been planned for 1959 or early 1960 but Fermi 1 achieved criticality on August 23, 1963. While slowly increasing its power over the next two years, on October 5, 1966 it suffered a partial meltdown when the flow of sodium was disrupted by blockage of the inlet holes at the bottom of the reactor. The problem was detected early enough to safely scram the reactor and there was no radioactive release outside the containment building. The site was shut down for repairs and restarted in July 1970. It ran only until closing again on November 27, 1972, and was officially decommissioned on December 31, 1975.
Most commercial reactors run on fissile 235U fuel. In nature, 235U is mixed with a much larger amount of non-fissile 238U. There is so much 238U that the 235U atoms are so sparse that a chain reaction is impossible. In most cases, this is overcome by two methods. One method is to "enrich" the fuel, concentrating the 235U so the neutrons have a greater probability of hitting them. The enrichment leaves a by-product of non-fissile 238U referred to as depleted uranium. The other method is to slow the neutrons, or "moderate" them, which increases the chance they will undergo a reaction.
The most common reactor designs use both of these methods, slightly enriching the fuel to about 3 to 5% 235U and using water as a moderator. In these designs, 238U is still the majority of the fuel. Some neutrons from the fission events hit these atoms and are captured, turning them into 239Pu. These can also undergo fission like the 235U. About 35% of the fission in a typical reactor is of the 239Pu created in this fashion.
It is possible to increase the rate of capture and gain additional fuel. However, this process is much more efficient when the neutrons have higher energy, which works counter to the moderation needed for the 235U. This leads to a class of designs that are optimized for the production of 239Pu. These breeders are generally built in two parts, a "core" of fuel that is enriched to the point where it can maintain a chain reaction without a moderator, and a "blanket" of 238U surrounding it that is designed to capture any spare neutrons and create 239Pu.
235U and 238U are chemically identical, and are difficult to separate using mechanical processes based on their slightly different mass. In contrast, uranium and plutonium have different chemistry, and can be separated with chemical processes. This results in relatively pure plutonium that can be used for fuel in the core of the breeder with no further enrichment. Once the system is running, it is possible to produce enough plutonium to replace the original uranium fuel entirely, and still have more left over. This additional plutonium can then be used in the core of other breeders, or mixed with uranium and burned in conventional (non-breeder) reactors.