Meltdown Occurs in the First Breeder Reactor Summary

  • Last updated on November 10, 2022

An accidental meltdown in the core of the Experimental Breeder Reactor occurred four years after it became the world’s first nuclear reactor to generate electricity. The accident lent strength the nascent grassroots movement resisting the development or use of nuclear technology.

Summary of Event

On November 29, 1955, an accidental meltdown of about one-half of the reactor core of the Experimental Breeder Reactor 1 (EBR-1) occurred at the National Reactor Testing Station National Reactor Testing Station at Arco, Idaho. The EBR-1 was completed by the Argonne National Laboratory in August, 1951, and became the first nuclear reactor to generate electricity when it produced more than one hundred watts of electrical power on December 20, 1951. Nine years earlier, on December 2, 1942, a team led by Enrico Fermi at the University of Chicago had achieved the first controlled nuclear fission chain reaction as part of the Allied effort to develop an atomic bomb during World War II. Nuclear energy;accidents Experimental Breeder Reactor 1 Breeder reactors [kw]Meltdown Occurs in the First Breeder Reactor (Nov. 29, 1955) [kw]Breeder Reactor, Meltdown Occurs in the First (Nov. 29, 1955) [kw]Reactor, Meltdown Occurs in the First Breeder (Nov. 29, 1955) Nuclear energy;accidents Experimental Breeder Reactor 1 Breeder reactors [g]North America;Nov. 29, 1955: Meltdown Occurs in the First Breeder Reactor[05010] [g]United States;Nov. 29, 1955: Meltdown Occurs in the First Breeder Reactor[05010] [c]Disasters;Nov. 29, 1955: Meltdown Occurs in the First Breeder Reactor[05010] [c]Energy;Nov. 29, 1955: Meltdown Occurs in the First Breeder Reactor[05010] [c]Environmental issues;Nov. 29, 1955: Meltdown Occurs in the First Breeder Reactor[05010] Fermi, Enrico Hahn, Otto Meitner, Lise Seaborg, Glenn Theodore Strauss, Lewis L. Zinn, Walter Henry

Nuclear fission was discovered by Otto Hahn and Fritz Strassmann Strassmann, Fritz in 1938, when they bombarded uranium with neutrons in their laboratory in Berlin, leaving traces of radioactive barium impurities. These results were sent to their former colleague Lise Meitner, who recognized the possibility that neutrons had split uranium atoms (which have 92 protons) into two nearly equal parts, yielding barium (56 protons), krypton (36 protons), and additional neutrons.

Early in 1939, Meitner and her nephew Otto Robert Frisch Frisch, Otto Robert published their calculations, which showed that the enormous energy that would be released in fission reactions would produce radioactive fragments that were isotopes of lighter atoms such as barium, krypton, strontium, and cesium. Fission would also produce excess neutrons that might result in new fissions with even more neutrons, causing a self-sustaining chain reaction. These results were quickly verified in several laboratories in 1939, including observations of simultaneous neutron emission by Fermi, Leo Szilard, and Walter Henry Zinn, among others.

In 1940, Niels Bohr Bohr, Niels and John Archibald Wheeler Wheeler, John Archibald used Bohr’s liquid-drop model of the nucleus to show that the rare uranium Uranium 235 isotope (U-235, containing 235 nucleons: 92 protons and 143 neutrons) is far more likely to fission than the common uranium 238 isotope (U-238), which makes up 99.3 percent of natural uranium. Frisch and Rudolph Peierls then showed that a “critical mass” of pure U-235 could sustain a chain reaction with less than one kilogram of uranium, releasing energy equivalent to burning about three billion tons of coal. They also pointed out the huge effort needed to separate U-235 from U-238, since both isotopes have the same chemical properties and can be separated only by physical means.

Later in 1940, the Americans Edwin Mattison McMillan McMillan, Edwin Mattison and Philip Abelson Abelson, Philip found that U-238 bombarded with neutrons produced a radioactive substance with a 2.3-day half-life, which they identified as the new element neptunium (atomic number 93). A group led by Glenn Theodore Seaborg showed that this new element decayed into another new element they called plutonium Plutonium (atomic number 94). The isotope plutonium 239 (Pu-239) has a half-life of twenty-four thousand years and is fissionable when bombarded with neutrons in the same way as U-235. Since plutonium is chemically different from uranium, Pu-239 can be produced from the plentiful U-238, and then chemically separated much more easily than separating U-235. Thus it was recognized that a nuclear reactor could “breed” plutonium from U-238 as a by-product of a fission chain reaction.

When the first nuclear chain reaction Nuclear fission was demonstrated by Fermi in 1942, he used a pile of graphite blocks as a moderator to slow the neutrons by collisions with carbon atoms. Slow neutrons were needed to increase the probability of fission in natural uranium with only 0.7 percent of U-235. Thus he was able to achieve a critical mass with a lattice consisting of 385 tons of graphite and 40 tons of uranium. Cadmium control rods were inserted to absorb neutrons during the construction, and the chain reaction was observed when the rods were partially withdrawn by Zinn. The success of this reactor led to the construction of several large production reactors to produce plutonium at Hanford, Washington, on the Columbia River where water from the river could be used to cool the reactors. After World War II, several countries began to develop power reactors to generate electric power from the heat produced by fission, including some dual-purpose reactors that also produced plutonium for use as nuclear fuel or weapons.

The possibility of a fast-breeder reactor that could breed more plutonium than the fuel it used for generating power was developed by the Argonne National Laboratory Argonne National Laboratory under the direction of Zinn at the National Reactor Testing Station in Arco, Idaho, resulting in the EBR-1. This type of reactor requires fast neutrons, no moderator, and a fuel with at least 20 percent of fissile material, either U-235 (enriched uranium) or Pu-239. The first core of the EBR-1 consisted of nearly pure U-235 fuel rods clad with stainless steel in a structure about the size of a football. This core was surrounded by a layer of natural uranium metal to act as a reflector of neutrons and as a breeding blanket in which the U-238 captured fast neutrons to form Pu-239.

A liquid sodium-potassium alloy was used as a coolant because of its thermal properties and its ability to reflect neutrons without slowing them down. Heat transferred from the coolant by a heat exchanger generated steam to drive a turbogenerator capable of producing about two hundred kilowatts of electric power. The reactor was completed in August, 1951, and became the first reactor to generate electricity when it produced more than one hundred watts of electrical power on December 20, 1951. The EBR-1 was found to produce an amount of plutonium just over 100 percent of the fuel it used, showing the possibility of breeding more fuel than was required for its operation.

In the fall of 1955, the EBR-1 was given a new core. One test of this Mark II core involved interrupting the flow of the sodium-potassium coolant to check the temperature rise. The test was carried out on November 29, 1955, but when the operator began to shut down the reactor, he mistakenly used the slow-acting control rods instead of the rapid scram rods. The core temperature soared to more than 1,100 degrees Celsius and the uranium fuel began to melt, slumping to the bottom of the containment shield where it solidified again, forming a cup which caught more of the melting fuel. The slow rods shut down the reactor, but the core was destroyed with nearly 50 percent of the fuel melted. There was no report of radiation exposure, nor was the accident exposed in time to affect the subsequent development of Detroit Edison’s Enrico Fermi fast-breeder reactor plant. In 1962, the fourth and last core was installed in the EBR-1 with fuel elements of Pu-239 clad with zirconium. With this plutonium core, the reactor was a true breeder and was successfully operated for another twelve years.


The EBR-1 offered the prospect of a nearly unlimited source of energy, but the Mark II core meltdown set the stage for growing opposition to nuclear power Nuclear energy;opposition in the United States. At first, it appeared that not even Lewis Strauss, chairman of the Atomic Energy Commission Atomic Energy Commission, U.S.;nuclear accidents (AEC), knew about the meltdown. When asked about it by The Wall Street Journal on April 5, 1956, he replied that it was “news to him.” Not until that evening did Strauss direct the AEC to issue a press release admitting the meltdown. The Advisory Committee on Reactor Safeguards Advisory Committee on Reactor Safeguards submitted a report on June 6, 1961, declaring that not enough was known about breeder reactors to guarantee public safety if a fast-reactor station operated near an urban center. This degree of uncertainty did not prevent implementation of Detroit Edison’s Detroit Edison plan based on the EBR-1 for a three-hundred-megawatt fast-reactor power station on Lake Erie approximately twenty-five miles from both Detroit and Toledo.

Application for a construction license for Detroit Edison’s Enrico Fermi fast-breeder reactor station Enrico Fermi fast-breeder reactor station[Enrico Fermi fast breeder reactor station] was filed with the AEC on August 4, 1956, while opposition was led by the United Auto Workers to the Supreme Court. In March, 1957, the AEC published its analysis of the possibility of a nuclear accident in document number WASH-740, entitled Theoretical Possibilities and Consequences of Major Accidents in Large Nuclear Power Plants. Theoretical Possibilities and Consequences of Major Accidents in Large Nuclear Power Plants (government report) The AEC estimated the likelihood of a major accident at one chance in one billion per year per reactor, but the public was more interested in the results of a “maximum credible accident,” which was set at thirty-four hundred deaths, forty-three thousand injuries, and property damage of seven billion dollars.

Four months later, a University of Michigan study of the Detroit Edison plan calculated a worst case outcome of 133,000 fatalities. On June 12, 1961, the Supreme Court Supreme Court, U.S.;nuclear energy in a split decision approved a license to construct the Enrico Fermi power station in spite of the first major U.S. nuclear accident on January 3, 1961, when a control rod was accidentally pulled out too far in a three-megawatt military prototype, the Stationary Low-Power Reactor 1 Stationary Low-Power Reactor 1[Stationary Low Power Reactor 1] (SL-1), at Arco, Idaho. The core went supercritical, and the resulting steam explosion and radiation killed three workers.

The Enrico Fermi plant went critical in August, 1963. Its core was a cylinder about the size of a base drum (seventy-five centimeters in diameter and height) with 14,700 fine fuel pins made of 28 percent enriched uranium (about five hundred kilograms of U-235). This carefully aligned assembly was cooled by liquid sodium flowing through tiny channels between the pins, and it was surrounded by a U-238 breeding blanket with outer dimensions about two meters in diameter and height. A succession of problems kept the plant far below its final design rating of two hundred megawatts for three years.

On October 5, 1966, as the control rods were slowly being removed, unusually high temperatures appeared at two points in the core, radiation alarms began to sound, and scram rods were inserted to shut down the reactor. Part of the fuel in the core had melted, but the shutdown was initiated in time to prevent the highly enriched fuel from producing a surge of reactivity large enough to cause a chemical explosion.

Some reports filed shortly after the accident indicate that an alert had gone out to civil defense authorities to prepare for emergency evacuation of Detroit. Great care was taken not to disturb the equilibrium of the ruptured core, and nearly one year passed before the meltdown’s cause was determined. Shortly before completion of the plant, the Advisory Committee on Reactor Safeguards had insisted that a pyramid-shaped structure be placed below the core to disperse any fuel that might melt in an accident and to prevent the fuel from forming a critical mass that could melt its way through the containment and contaminate the groundwater (the so-called China syndrome). One loosely attached zirconium plate on the pyramid was lifted by the uprushing liquid sodium, partially blocking the flow of fuel and allowing the temperature to rise enough to melt some fuel.

The difficulties and near disasters with fast-breeder reactors in the United States led to a reliance on light-water reactors in the subsequent development of nuclear power for commercial use. In these reactors, ordinary water serves as both moderator and coolant, and any accident in which the water is lost stops the reaction even if the control rods fail, since the reaction cannot continue without a moderator to slow the neutrons. Nuclear energy;accidents Experimental Breeder Reactor 1 Breeder reactors

Further Reading
  • citation-type="booksimple"

    xlink:type="simple">Glasstone, Samuel. Sourcebook on Atomic Energy. Princeton, N.J.: D. Van Nostrand, 1967. An authoritative source about all aspects of nuclear energy. Includes historical background and a sixty-five-page chapter on nuclear reactors, with nearly forty references to further literature on reactors. Many photographs and diagrams.
  • citation-type="booksimple"

    xlink:type="simple">Inglis, David R. Nuclear Energy: Its Physics and Its Social Challenge. Reading, Mass.: Addison-Wesley, 1973. A good introduction for general readers, with a chapter on nuclear power reactors. A number of interesting appendixes provide useful information and documents on nuclear energy.
  • citation-type="booksimple"

    xlink:type="simple">Libmann, Jacques. Elements of Nuclear Safety. Translated by Jean Mary Dalens. Les Ulis, France: Editions de physique, 1996. Text produced by the Institut de Protection et de Sûreté Nucléaire (IPSN; Institute for Nuclear Safety and Protection) detailing hazards of, and proper safety measures for dealing with, both reactors and radioactive materials.
  • citation-type="booksimple"

    xlink:type="simple">Novick, Sheldon. The Careless Atom. New York: Dell, 1969. A good source for information about nuclear accidents, radiation damage, and pollution problems. Well documented.
  • citation-type="booksimple"

    xlink:type="simple">Patterson, Walter C. Nuclear Power. Baltimore: Penguin Books, 1976. A readable introduction to the early development of nuclear power, including descriptions of reactors, the nuclear fuel cycle, and nuclear accidents.
  • citation-type="booksimple"

    xlink:type="simple">Webb, Richard E. The Accident Hazards of Nuclear Power Plants. Amherst: University of Massachusetts Press, 1976. A description of nuclear power plants and types of reactor accidents, including a review of probability estimates. An appendix gives brief descriptions of fourteen accidents and near accidents in reactors.

World’s First Nuclear Reactor Is Activated

Atomic Energy Commission Is Established

Hanford Nuclear Reservation Becomes a Health Concern

World’s First Breeder Reactor Produces Electricity

Chalk River Nuclear Reactor Explosion and Meltdown

Atomic Energy Act

Price-Anderson Act Limits Nuclear Liability

Nuclear Waste Explodes in the Ural Mountains

England’s Windscale Reactor Releases Radiation

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