Experimental Breeder Reactor (EBR-I)

On December 20, 1951, a marvel of modern science quietly came to pass in the eastern Idaho desert. As a select group of engineers gazed upon a string of four large glowing bulbs ignited by the power of nuclear reaction, relief washed over them. At last their project worked, their theory was confirmed. Electricity produced with nuclear power became a reality at their small Idahoan facility, EBR-I, and the world would never be the same.

The story of EBR-I begins on the periodic table with enormous machines called “atom smashers.” Scientists used these devices in the 1930s to fire neutrons into various elements, among them uranium. It was discovered that some uranium atoms fission, or break apart. This fission produced additional neutrons and resulted in a chain reaction that would cause more atoms to fission. The extraordinary pace of these reactions and the energy released led to the creation of the first atomic bomb.

Yet scientists believed that nuclear technology could be utilized for peaceful purposes as well. In early 1939, theoretically, steam produced by nuclear heat could turn a turbine to propel a ship or submarine or spin a generator to produce electricity. However, since at that time it was assumed that uranium was extremely rare, any use outside of weaponry had to be highly efficient. In 1946, after the Second World War, a proposal by physicist Walter Zinn to help solve the uranium shortage was approved by a top U. S. Military official.

Zinn became the first director of Argonne National Laboratory in Chicago, dedicated to the development of nuclear reactors. His proposal was to build a reactor that would produce, or breed, plutonium, another atomic fuel, while generating electricity. His project would have to wait a few years, but once the civilian Atomic Energy Commission approved, it was clear the city of Chicago was not the place for this type of nuclear experiment. Instead, Zinn’s project needed to take place in a more remote location.

This location was the high desert plains west of Idaho Falls, sparsely populated but populated enough to provide the basics of civilization. The decision was made February 18, 1949, and a section of the vast desert became the National Reactor Testing Station. It was nearly built in Montana near Fort Peck, but that site proved too isolated to provide support to such a technology-intensive project. In early 1949, with enthusiastic support from nearby communities, construction began on the first Experimental Breeder Reactor, EBR-I, at a vacated naval repair and test site. While the reactor housing building was being built, the Korean War broke out in 1950, increasing the urgency of the need for military uranium. An exceptionally cold winter at the end of that year delayed construction on EBR-I as war raged on the other side of the world. By April 1951, the building was finished and the reactor, designed and assembled by Argonne in Chicago, was installed.

At the time, nuclear fuel needed to be “borrowed” by the AEC from the military. The first attempt to activate EBR-I was unsuccessful; it was decided that there was insufficient fuel in the core to reach the critical mass needed for the material to fission. Additional fuel was ordered and at a later test date on August 24, 1951, EBR-I went critical, the first reactor to use enriched U-235 fuel. It was also the first to use a liquid-metal coolant made of sodium (Na) and potassium (K) that was known as NaK. Yet this was not the last breakthrough for the experimental reactor. On December 20, 1951, EBR-I produced enough electricity to light a string of four 200-watt light bulbs by converting the heat produced by nuclear reaction into steam, running a turbine. The next day, power output from the reactor was increased to 100 kilowatts, enough to power the building it was housed in.

Indeed, nuclear power was a reality. But the question remained as to whether plutonium had been produced. In 1953, technicians at EBR-I took samples from around the reactor’s core. Analysis at a laboratory in Chicago revealed that fissionable plutonium was produced from non-fissionable uranium, thereby breeding fuel in the process of burning fuel. During its years of operation, the ratio of new fuel being bred by the reactor went from an initial 1.01 or one percent more fuel bred than consumed up to 1.27 or a notable twenty-seven percent more fuel produced than used. EBR-I succeeded in everything it had set out to do, setting a precedent for future research.

Yet EBR-I still had some additional firsts to add to the field of nuclear science. In 1955, the concept of a new, larger breeder reactor was in the works. Consequently, further tests at EBR-I were run to enhance the design of the new facility which would be called EBR-II. In one of these experiments, the NaK coolant was blocked and automatic safety backups were deactivated in an effort to gauge the effects of high temperatures on the reactivity of the core. The necessary manual reactor scram was delayed slightly too long and the reactor reached temperatures exceeding the melting point of the fuel rods. Although there was no visual or auditory evidence of failure, temporarily elevated radioactivity levels in the control room did cause a personnel evacuation of the building. The damaged core itself needed to be replaced and by carefully examining the approximately 12 inch tall fuel assembly many lessons were learned.

Thus, EBR-I also produced the first unintended nuclear meltdown in America’s history, and provided valuable information to design future reactors. In 1963, the reactor used plutonium to produce a fission chain reaction, another first for the small reactor. Later that year EBR-I was shut down, being too small to provide new data and it stopped producing power. In 1964, the reactor was decommissioned, being replaced by the more advanced version of itself, EBR-II, which operated from 1964-1994. Realizing the valuable contribution made to human understanding of nuclear power, on August 26, 1966, EBR-I was designated as a National Historical Landmark in a ceremony marked with a visit and speech by President Lyndon Johnson.


  • “Experimental Breeder Reactor #1 National Register of Historic Places Inventory – Nomination Form.” United States Department of the Interior, National Park Service. Form dated 12 June 1976.
  • Holl, Jack M. “The National Reactor Testing Station: The Atomic Energy Commission in Idaho, 1949-1962.” Pacific Northwest Quarterly 85 1 (1994): 15-24.
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  • Michal, Rick. “Fifty Years Ago in December: Atomic reactor EBR-I produced first electricity.” Nuclear News 44 12 (2001): 28-29.
  • Michal, Rick. Interview of Leonard Koch. “Koch: Remembering the EBR-I.” Nuclear News 44 12 (2001): 30-35.
  • “Nuclear Pioneers.” 27 minute video, Atomic Heritage Foundation, Argonne Information and Publishing Division, 2003. < https://inlportal.inl.gov/portal/server.pt?open=514&objID=4403&parentname=CommunityPage&parentid=3&mode=2&in_hi_userid=200&cached=true >.
  • Stacy, Susan. Proving the Principle, A History of The Idaho National Engineering and Environmental Laboratory 1949-1999. Idaho Operations Office of the Department of Energy, 2000.