The Advanced Test Reactor (ATR) at Idaho National Laboratory is one of the largest research reactors in the world.

It’s been smacking large volume experiments with high-flux neutrons since 1967—the same year the Big Mac debuted.

But unlike the Big Mac, ATR doesn’t require a special sauce—just a unique core design that helps researchers quickly study the long-term impact radiation has on nuclear technologies.

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Watch and learn about the largest test reactor in the world, located at Idaho National Laboratory.

Video courtesy of the Department of Energy

Hungry for more?

Sink your teeth into these seven fast facts about ATR.

1. ATR is a pressurized water test reactor.

ATR is a unique pressurized water test reactor that operates at low pressure and temperature compared with most nuclear power plant reactors. It contains a beryllium reflector to help concentrate neutrons on the fuels and materials it is testing.

2. ATR has lots of test positions.

ATR has 75 testing locations. They vary in size and can handle large test volumes up to 48 inches long and 5 inches in diameter. Six locations contain pressurized water loops to replicate the physical environment (i.e. temperature, pressure, flow and chemistry) of an operating power plant.

3. ATR has four operating cycles.

On average, ATR performs four operating cycles of up to 60 days per year. The remainder of the time is committed to planned outages for swapping experiments in and out, refueling, routine maintenance and upgrades.

4. ATR can do multiple experiments at the same time.

ATR's serpentine core

ATR's serpentine core allows the reactor to operate at different power levels.

Idaho National Laboratory

ATR’s innovative design allows for unmatched versatility in nuclear research. Its serpentine core allows the reactor’s corner lobes to operate at different power levels. This makes it possible to conduct multiple experiments simultaneously—all under different testing conditions.

5. ATR testing helps fuel the Navy.

Since it came online in 1967, ATR has supported the United States Navy in developing its nuclear propulsion program by providing irradiations of its fuels and materials. The Navy continues to be a prime user of the facility and owns a safety and performance record that’s unmatched by any fleet in the world.

6. ATR helps save lives.

ATR is the source of medical-grade cobalt 60–an isotope that is used for cancer treatments. The U.S. Department of Energy (DOE) is expanding its use to produce other isotopes for medical and industrial applications.

7. ATR is a historic landmark.

Fifty years and counting. The ATR complex was named a historic landmark by the American Nuclear Society in 2016 for its countless contributions to nuclear research. DOE plans to keep the test reactor up and running for decades to come.

Learn more about ATR in this fact sheet by Idaho National Laboratory.

Explore more about the Office of Nuclear Energy.