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.
Background
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.
A reactor that can run unmoderated still needs cooling. Since water would give undesired moderation, some other form of coolant is needed. Sodium metal makes an excellent coolant with a very low chance of reacting with neutrons, thus improving the neutron economy. It also has excellent thermal transfer. The efficiency of heat transfer is a function of the difference between the maximum and minimum working temperatures. In sodium this is the melting point of 371 Kelvin (K) and boiling point of 1156 K. In contrast, water's equivalent limits at atmospheric pressure are 273 K and 373 K, which results in far less specific heat capacity. To improve this, many water-cooled reactors operate under high pressure in order to increase the boiling point. Sodium designs can run at atmospheric pressure at much higher temperature, offering a safety advantage. Offsetting this is the fact that sodium is highly reactive. Hot sodium will burst into flame on contact with water or oxygen.
History Breeder reactors The breeder concept dominated early nuclear thinking as it offered a way to produce almost unlimited amounts of fuel from what was otherwise a waste product. In the 1950s it was commonly believed that the supply of natural uranium was limited and might run out if fission power became popular. The plutonium created could be used to fuel the breeder core, with enough left over to run other reactors. A breeder potentially generates not only electricity, but also income through fuel sales. The first power-producing reactor was a breeder, the Experimental Breeder Reactor I (EBR-I) at what became the Idaho National Laboratory. On December 20, 1951 it produced enough electrical energy to power a series of light bulbs.
Commercial development Walker Lee Cisler was appointed to president of Detroit Edison in December 1951. He had just delivered a report to the United States Atomic Energy Commission (AEC) which suggested that nuclear energy would be practical for commercial power companies and could lead to an entirely new way of meeting future power needs. Cisler's report centered on the fast breeder, stating that "The breeder reactor would continuously produce amounts of fissionable material in excess of that consumed. Thus breeder reactors would augment rather than consume the world's supply of fissionable materials."
Cisler proposed a design cycle for a commercial breeder in a partnership between Detroit Edison and Dow Chemical. The AEC approved the concept on December 19, 1951. This was only one day before EBR-1 began initial operations. Over the next year, Cisler formed the Nuclear Power Development Department within Detroit Edison, and brought in fifteen other utility companies and suppliers, including Consolidated Edison and Philadelphia Electric. The combined group formed the Atomic Power Development Associates (APDA).
The AEC gave the organization the go-ahead for detailed design on October 19, 1952. Development proceeded slowly until President Eisenhower signed the Atomic Energy Act in August 1954, which allowed private companies to own nuclear plants. Further support for the project came when Lewis Strauss was appointed to chair the AEC. Strauss was very interested in seeing commercial development, and strongly supported both the breeder and light water reactor programs.
Negative review At a review meeting with the AEC on November 10, 1954, Nobel Prize winner Hans Bethe and the designer of EBR Walter Zinn disagreed over the outcome of a meltdown in a breeder. Zinn suggested it might cause an explosion while Bethe felt this was extremely unlikely. Everyone agreed that such an explosion, were it to occur, would be small enough to be contained in a suitable building.
Shortly after the meeting, Alfred Amorosi was named the technical director of the project. He began an in-depth study of the operations and became concerned about one particular problem. One advantage of the sodium coolant was that it expanded when it got hotter, slowing the reaction speed and cooling the reactor back down. This negative temperature coefficient is highly desirable as it offers passive safety. However, EBR demonstrated a positive temperature coefficient under certain conditions. More disconcerting was that calculations of a second similar effect, the Doppler coefficient, appeared to be positive. In this case additional heat might lead to a runaway condition.
Amorosi's work was studied at a June 30, 1955 meeting of the breeder group within the Advisory Committee on Reactor Safeguards, a blue ribbon panel formed by the AEC, which is today part of the Nuclear Regulatory Commission. They were concerned that the Doppler coefficient might overwhelm the thermal effect and any automatic systems that were attempting to control it. The team was ambivalent about the concept on a broader sense, raising several potential serious issues and wondering why they were considering moving ahead with a full-scale plant without waiting for results from tests on smaller prototypes. They concluded that "It must be recognized that the assumptions on which these calculations are based have not been established experimentally, and must be so before the operation of such a reactor could possibly be recommended for a site close to a populated area."
EBR meltdown
To further characterize these effects, in the summer of 1955, EBR began to run experiments to measure the Doppler coefficient. As the effect is more pronounced at higher temperatures, a test was arranged that would run the reactor at , not far below the melting point of the fuel. The reactivity was controlled by a series of motorized control rods that would be slowly extracted to allow the power to rise smoothly. In the case of a problem, a second set of emergency rods could be quickly dropped into the reactor.
The test was carried out on November 29, 1955. As the power was ramped up, at 500 W the temperature reading became confusing. Concerned that it was beginning to surge, the scientist in charge of the test ordered the operator to drop the emergency rods, but the operator pushed the button for the slower motor-controlled ones. The scientist realized what had happened and pushed the correct button, but in those few seconds, the power continued rapidly increasing. A second safety system was then triggered which dropped the entire blanket into a pit below the reactor, which had the effect of stopping the reactions.
About half of the core had melted in seconds, leaving the reactor so radioactive that it could not be approached. Teams were finally able to open the core weeks later and it was not immediately clear what had caused the problems. If it had been caused by Doppler effects, it suggested a larger plant would be completely uncontrollable. It was finally determined that the cause of the problems was that the fuel rods would bend, or droop, as they heated up, allowing them to move closer together and the reaction rate to increase. The issue of the Doppler coefficient remained untested.