INL's 55 Nuclear Reactors
Pioneering reactor designs that have shaped the future of nuclear energy since 1949.
INL's Nuclear Heritage
On Feb. 18, 1949, the recently formed Atomic Energy Commission (predecessor to today’s U.S. Department of Energy) selected a Naval Proving Ground in Idaho as the site of a planned National Reactor Testing Station. Within just a few years, the site was home to numerous milestones that paved the way for the peaceful use of nuclear energy to generate safe, reliable electricity.
Although names at the site changed over the years, nearly every operating reactor in the world has technological roots in Idaho. That proud tradition continues at what is now Idaho National Laboratory, the nation’s center for nuclear energy research and development.
These mission- and function-based categories help organize the dozens of reactors built at the INL site. Many were later repurposed and can fit into multiple categories.
“What happened here merely raised the curtain on a promising drama in our long journey to a better life.”
— President Lyndon B. Johnson during the dedication of Experimental Breeder Reactor-I as a National Historic Landmark in 1966
Demonstration & Prototyping
Demonstration reactors prove the feasibility of new reactor designs as a complete system, including fuels, cooling systems and other plant infrastructure. These facilities advanced innovative ideas from research into practical technologies for many applications, which included testing, power generation, national security and more.
Aalo Atomics' Aalo-X microreactor
Aalo Atomics developed the Aalo-X microreactor to provide reliable, scalable power for industrial, remote and energy-intensive applications. Aalo-X achieved criticality during its demonstration at a site outside of Idaho National Laboratory’s Materials and Fuels Complex as part of the U.S. Department of Energy Reactor Pilot Program, becoming the fourth DOE-authorized advanced reactor to reach this milestone.
Deployable Energy's Unity microreactor
Deployable Energy developed the Unity microreactor to provide resilient, transportable nuclear power for remote, defense and infrastructure-constrained applications. Unity achieved criticality during its demonstration at the Hot Fuel Examination Facility at the Materials and Fuels Complex, marking another modern milestone in advancing innovative microreactor technologies through DOE’s national laboratory capabilities.
Antares Mark-0 microreactor
Antares Nuclear developed the Mark-0 advanced microreactor to provide resilient power for remote environments, military installations and space applications. The Reactor and Criticality Experiment (RACE) at INL’s Materials and Fuels Complex supports Mark-0 criticality testing.
Experimental Breeder Reactor-II
EBR-II was built to scale up the concepts demonstrated with EBR-I, and to demonstrate on-site fuel reprocessing as an add-on to a liquid-metal-cooled, fast breeder reactor power plant using metallic fuels. The reactor operated submerged in a tank of liquid sodium coolant.
Experimental Breeder Reactor-I
Known as the world’s first nuclear power plant, EBR-I produced the world’s first usable electricity from nuclear energy. It used unmoderated enriched uranium for fuel and sodium-potassium alloy (NaK) as coolant. It was designated a National Historic Landmark in 1966 and remains open for visits and tours.
Boiling Water Reactor Experiment No. 5
BORAX-V tested the feasibility of an integral, nuclear superheat system, producing superheated or dry steam entirely by nuclear means for the first time.
Boiling Water Reactor Experiment No. 4
BORAX-IV, at 20 megawatts, tested fuel elements made from uranium and thorium mixed oxides (ceramics). These materials could operate longer in a reactor’s extreme heat before failing.
Boiling Water Reactor Experiment No. 3
BORAX-III was 15 megawatts and connected to a 2,000-kilowatt turbine/generator. On the night of July 17, 1955, it produced sufficient power to momentarily light the city of Arco, Idaho.
Boiling Water Reactor Experiment No. 2
BORAX-II continued testing boiling water reactors, this time at a power level capacity of 6 megawatts. Tests used fuels with varying enrichments of uranium-235.
Boiling Water Reactor Experiment No. 1
BORAX-I as the first in the series, was designed to test boiling water as a reactor moderator and coolant. Operating at 1.4 megawatts, it was deliberately blown up in 1954 to learn more about its operating limits.
Since the site’s earliest days, INL’s longest-running mission has relied on a special class of reactors. These high-performance research reactors operate at much higher power levels than most other research reactors, subjecting experiments to very high levels of neutron irradiation. The resulting data help qualify new reactor fuels and materials and provide scientists with the information needed to predict reactor performance over decades of operation.
Advanced Test Reactor
At 250 megawatts, ATR is the world’s most powerful and highest capacity test reactor. It simulates the environment within a wide variety of Navy propulsion reactors, current fleet reactors and advanced reactors still under development. It accelerates evaluations of how the reactor environment will affect fuel and many structural materials like steel and zirconium.
Engineering Test Reactor
ETR featured big gains in power and capacity over the Materials Testing Reactor and could much more effectively evaluate fuel, coolant and moderator materials in conditions similar to many types of reactors. It was the first test reactor to use experiment loops within the core, including a sodium loop for evaluating fast spectrum breeder reactors.
Materials Testing Reactor
As the world’s first high-flux research reactor, MTR accelerated the testing and evaluation of potential reactor fuels and materials. In 1958, it became the first reactor to operate using plutonium-239 as fuel. Its materials testing was supplanted in the late 1950s by the Engineering Test Reactor.
Safety Testing
This class of powerful research reactors have been an essential part of INL’s mission for more than 70 years to study how nuclear systems respond under abnormal or accident conditions. These facilities provided critical insights into reactor behavior, fuel performance and safety systems, helping improve the safety of commercial and advanced reactors.
Transient Reactor Test Facility
TREAT is a uranium-oxide-fueled, graphite-moderated, air-cooled reactor designed to produce short, controlled bursts of nuclear energy up to 19 gigawatts to simulate accident conditions leading to fuel damage. The data helps refine computer simulations of reactor accidents, leading to better, safer reactors and fuels.
Loss of Fluid Test Facility
LOFT was a scale-model version of a commercial pressurized water plant built to explore the effects of loss-of-coolant accidents. Researchers conducted 38 nuclear power tests in this reactor, with the entire reactor assembly able to be disconnected and moved into a massive nearby hot cell for remote examination and analysis.
Power Burst Facility
PBF’s ability to create millisecond bursts of energy up to 240 gigawatts was valuable for testing light water reactor fuel rods under the most extreme accident conditions. Data from these tests were used to develop and validate fuel behavior computer codes for the Nuclear Regulatory Commission.
Special Power Excursion Reactor Test No. IV
SPERT-IV was an open-tank, twin-pool facility that permitted detailed studies of reactor stability as affected by hydrodynamic effects such as forced coolant flow.
Special Power Excursion Reactor Test No. III
SPERT-III was developed to study nuclear reactors’ inherent safety characteristics, providing the widest practical range of control over three variables: temperature, pressure and coolant flow.
Special Power Excursion Reactor Test No. II
SPERT-II was a closed pressurized water reactor with coolant flow systems designed for light or heavy water.
Special Power Excursion Reactor Test No. I
SPERT-I was the first in a series of four safety-testing reactors designed to study the behavior of reactors when their power levels changed rapidly. The tests demonstrated the damage-resistant capabilities of low-enrichment uranium-oxide fuel pins.
Marine Propulsion
Driven by the pressures of the Cold War, the promise of naval nuclear propulsion led to the construction of prototype reactors that helped establish and advance nuclear propulsion for the nation’s submarines and aircraft carriers. The technology was also demonstrated for commercial shipping, an application now drawing renewed interest. Nuclear propulsion enables vessels to travel farther, remain at sea longer and operate without frequent refueling.
Natural Circulation Reactor
The S5G was the prototype of a pressurized-water reactor for USS Narwhal, capable of operating in either a forced or natural circulation flow mode. To prove that the design concept would work at sea, the prototype was built in a submarine hull section capable of simulating a ship’s rolling motion.
High Temperature Marine Propulsion Reactor
A low-power critical experiment operated at Test Area North to explore the feasibility of an air-cooled, water-moderated system for nuclear-powered merchant ships. This project was discontinued in December 1964.
Large Ship Reactor B
A1W-B was the second pair of prototype reactors for the USS Enterprise (CVN-65). The A1W A and B plants were the first in which two reactors powered one ship propeller shaft. Enterprise had four pairs of reactors driving the ship to record setting speeds.
Large Ship Reactor A
The A1W-A plant was one of a pair of prototype reactors (A1W A and B) for the USS Enterprise, the U.S. Navy’s first nuclear-powered aircraft carrier. Located at the Naval Reactors Facility, these reactors simulated one of four pairs that would power the Enterprise (CVN-65).
Submarine Thermal Reactor
The S1W was installed inside two submarine hull sections matching the size and specifications of the USS Nautilus. After startup, S1W accomplished a simulated 96-hour voyage nonstop from Newfoundland to Ireland, proving the feasibility of atomic ship propulsion long before Nautilus went to sea. It was later used to test advanced equipment and provide training for Navy personnel.
Air & Space
Researchers developed many different types of experimental reactors and reactor systems for power and propulsion in aviation and space flight. These projects explored how nuclear energy could extend the range and capabilities of aircraft, spacecraft, and future exploration missions. Although the air propulsion work ended in 1961, the potential for nuclear rocket propulsion has again picked up steam.
Spherical Cavity Reactor Critical Experiment
SCRCE was the final experiment for NASA to determine the feasibility of a reactor going critical with a gaseous core of uranium. The spherical shape was considered a more likely geometry for application in a rocket.
Cavity Reactor Critical Experiment
CRCE came from a NASA program to investigate nuclear propulsion in space. The concept called for uranium in a gaseous state in the reactor cavity with hydrogen propellant flowing around it.
Systems for Nuclear Auxiliary Power 10A Transient No. 3
SNAPTRAN-3 simulated the accidental fall of a space reactor into water or wet earth. It demonstrated that the reactor would be destroyed immediately instead of building up a high inventory of radioactive fission products.
Systems for Nuclear Auxiliary Power 10A Transient No. 2
This test version of the small space reactor, SNAP 10A/2, was intentionally destroyed to provide information on the dynamic response, fuel behavior and inherent shutdown mechanisms of these reactors in an open-air environment.
Systems for Nuclear Auxiliary Power 10A Transient No. 1
The SNAPTRAN program at Test Area North involved three test series and three reactors investigating the behavior of fuels under large-transient, power-excursion conditions.
Fast Spectrum Refractory Metals Reactor
This low-power critical facility collected data for the 710 Reactor, a fast-spectrum refractory-metal reactor concept that was considered for generating power in space.
Hot Critical Experiment
HOTCE was designed to obtain information on measurement techniques for high-temperature reactors. Part of the Aircraft Nuclear Propulsion program, it operated in the Critical Experiment cell of the Low Power Test Facility at TAN.
Heat Transfer Experiment No. 3
Anticipating use in an airframe, HTRE-I’s reactor, engine, shielding and heat transfer systems were arranged in a horizontal configuration. President John F. Kennedy canceled the Aircraft Nuclear Propulsion program in March 1961, and the reactors are now displayed at EBR-I.
Heat Transfer Experiment No. 2
To irradiate fuel elements too large to fit in the MTR, researchers on the Aircraft Nuclear Propulsion program drilled a hole in the center of HTRE-I and converted it to a materials test reactor, advancing the technology of high-heat ceramic reactor fuels.
Heat Transfer Experiment No. 1
During the 1950s, the U.S. Air Force sought to build a nuclear-powered jet airplane using direct-cycle heat transfer engineering. HTRE-I produced 20 megawatts of heat energy on a test stand using enriched uranium fuel clad in nickel-chromium.
Military & Deployable Power
These reactors were designed to provide reliable power in remote locations, support defense and national security missions as well as for civilian uses. These concepts explored transportable and self-contained nuclear systems that could reduce logistical challenges while delivering dependable energy where it was needed.
Mobile Low-Power Reactor No. 1
ML-1 was a mobile, low-power nuclear plant for the U.S. Army designed to be transported either by standard cargo plane or standard Army low-bed trailers. The Army phased out its reactor development program around 1965.
Gas Cooled Reactor Experiment
GCRE generated heat but no electricity for the U.S. Army, which wanted to develop mobile nuclear power plants. GCRE provided engineering data for improved components and training.
Stationary Low-Power Reactor
The SL-1 reactor was designed for the U.S. Army as a prototype of a low-power, boiling-water reactor plant to be used in geographically remote locations. It malfunctioned in January 1961, killing three men.
Zero Power Reactors
Also known as Critical Facilities, these no- or low-power reactors are used to provide valuable reactor physics data, validate core designs, and support larger testing programs. These facilities supplied the measurements and analysis needed to safely design, operate, and improve reactor systems.
Neutron Radiography Facility
Using two neutron beams from a 250-kilowatt reactor, NRAD produces neutron radiographs showing the internal condition of highly irradiated test specimens without physically cutting into the specimen.
Zero Power Physics Reactor
A low-power critical facility, ZPPR provided reactor physics data for fast neutron spectrum reactors, from tiny space-power reactors to large commercial breeder reactors.
Coupled Fast Reactivity Measurement Facility
Advanced Reactivity Measurement Facility No. 2 was modified in 1968 and renamed CFRMF. A section of the core was modified to provide data on unmoderated neutrons, contributing to the development of fast neutron reactors.
Advanced Test Reactor Critical Facility
ATRC is used for preliminary low-power testing of materials before they go into ATR, and also to verify experiment safety.
Thermal Reactor Idaho Test Station
Located at Test Area North, THRITS’ nuclear core was arranged in two halves, in a vertical honeycomb matrix. The two halves could be brought together to form a critical fuel mass, allowing operators to obtain basic physics and design data.
Advanced Reactivity Measurement Facility No. 2
A refinement on ARMF-I, ARMF-II had a “readout” system that automatically recorded measurements on IBM data cards, speeding up data processing.
Advanced Reactivity Measurement Facility No. 1
ARMF-I was used to characterize reactor fuels and materials for testing in the Materials Test Reactor, to improve core component performance and reliability.
Argonne Fast Source Reactor
Operating at 1 kilowatt, AFSR was used to calibrate instruments and study fast reactor physics. It contributed to better measurement techniques with experimental data instruments.
Shield Test Pool Facility
Situated in a water-filled pool at Test Area North, the reactor, nicknamed ‘SUSIE’, was used for bulk shielding experiments. After the Aircraft Nuclear Propulsion program was discontinued, SUSIE supported other National Reactor Testing Station programs.
Engineering Test Reactor Critical Facility
ETRC was a full-scale, low-power nuclear facsimile of the ETR, used to test the characteristics of experiments planned for ETR, saving time and money.
Zero Power Reactor No. 3
ZPR-III was a low-power, split-table reactor that achieved criticality by bringing two halves of a fuel configuration together. Experimental critical assembly results in this field were almost completely lacking before this reactor started up.
Reactivity Measurement Facility
RMF was a detector reactor that measured reactivity changes in materials irradiated in the MTR or ETR. It was used to assay new and spent fuel elements and to assist in experiment scheduling.
Focused Experiments
Specialized reactors and experimental facilities were developed to investigate specific technologies, fuels, coolants or reactor concepts. These targeted programs answered key scientific questions and helped guide the direction of nuclear research and development.
Nuclear Effects Reactor
The Nuclear Effects Reactor, nicknamed ‘FRAN,’ was a small-pulsed reactor used to test the performance of new detection instruments being developed for reactor control purposes. It was moved back to DOE’s Lawrence Livermore National Laboratory in 1970.
Critical Experiment Tank
The low-power CET reactor was part of the Aircraft Nuclear Propulsion program. It produced neutrons used to calibrate various types of neutron sensors and chambers.
Organic Moderated Reactor Experiment
OMRE demonstrated the feasibility of using a liquid hydrocarbon as both coolant and moderator. The waxy coolant was considered promising because it liquified at high temperatures but didn’t corrode metal.
Experimental Organic Cooled Reactor
EOCR was intended to build on research from OMRE but was placed on standby in 1962 when Atomic Energy Commission leadership decided the concept would not meaningfully improve nuclear power plant performance.
Experimental Beryllium Oxide Reactor
EBOR’s purpose was to develop beryllium oxide as a neutron moderator in high-temperature gas-cooled reactors. The project was canceled in 1966 as graphite gained favor as a moderator.
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