INEEL's Advanced Test Reactor (ATR), located in the Test Reactor Area of the INEEL, is designed to evaluate the effects of intense radiation on material samples, especially nuclear fuels. It also produces rare and valuable medical and industrial isotopes and is the world's largest test reactor. Unique capabilities offer high thermal neutron flux and large test volumes for performing irradiation services.

Built nearly 50 years ago, the ATR has contributed to over 50 new reactor designs and its use is planned for far into the 21st century. ATR provides the infrastructure to support research on plant aging, advanced materials, fuels, and isotopes. Recent upgrades make the ATR a modern state-of-the-art facility with an 80 percent availability factor. Reactor internals are replaced every seven to nine years, making the solid stainless steel core in essentially like-new condition.

The ATR is the most powerful test reactor operating in the United States. One advantage of its design is the precision with which the power level can be adjusted at various test positions. Maximum total power is 250 MW in each lobe; power shifting allows a maximum lobe power of 60 MW and minimums are at approximately 17 MW with "in betweens" available. Additionally, the ATR offers:

• Large test volumes
• Numerous test positions
• High neutron flux
• Variety of fast- to thermal-flux ratios
• Constant axial power profile
• Power tilt capacity
• Individual experiment control
• High reactor availability
• Frequent experiment changes
• Support facilities
• Favorable cost sharing

	The ATR has nine flux traps and 68 other irradiation test positions in or near the core. The curved fuel arrangement brings the fuel closer on all sides of the flux trap positions than is possible in a rectangular grid.
	Serpentine fuel assembly and neck shim rod housing in center of reactor core as seen during initial reactor assembly.
	Serpentine fuel assembly illuminated through several feet of water.

The ATR was designed to provide large-volume, high-flux test locations. It has larger test volumes in high-flux areas than any other reactor. A unique serpentine fuel arrangement provides nine high-intensity neutron flux traps and 68 additional irradiation positions inside the reactor core reflector tank, each of which can contain multiple experiments.

Thirty-four more low-flux irradiation positions are in the two capsule irradiation tanks outside the core. The four flux traps positioned within the corner lobes of the reactor core are almost entirely surrounded by fuel, as is the center position. Four other flux trap positions between the lobes of the core have fuel on three sides. The curved fuel arrangement brings the fuel closer on all sides of the flux trap positions than is possible in a rectangular grid. Effects from years of radiation in a normal power reactor can be duplicated in months or even weeks in the ATR.

The ATR's unique control device design permits large power shifts among the nine flux traps. The reactor uses a combination of control cylinders or drums and neck shim rods. The control cylinders rotate hafnium plates toward and away from the core, and the shim rods, which withdraw vertically, are individually inserted or withdrawn to adjust power. Within bounds, the power level in each corner lobe of the reactor can be controlled independently.

Powered with highly enriched uranium, the ATR has a maximum thermal power rating of 250 MW with a maximum unperturbed thermal flux rating of 1.0x10¹⁵ n/cm²-s. Testing can be performed in three major kinds of experiment facilities in the ATR

• Pressurized water test loops installed in some flux traps that replicate a variety of reactor conditions
• Instrumented lead experiments that provide realtime measurements and temperature and atmosphere control in the experiment capsules
• Simple drop-in capsule experiments in reflector or core irradiation positions.

The duration of operating cycles vary from typically three to seven weeks. Experiments are inserted or removed during operational shutdowns or outages. A wide variety of test positions are available for experiments, some of which are in flux traps. Others are in the beryllium reflector or the neck shim housing.

	The water-filled canal is centrally located down the middle of the photograph above. Operations can be conducted in the underwater facilities using remote tools. The canal serves as interim storage of equipment used in experiments.

The ATR is a U.S. DOE facility operated primarily for material research and materials testing. The facility is available as a user facility on a full cost recovery basis to other federal agencies, states, universities and other qualifying non-governmental and industrial users. Contact the ATR business office to use the facility about nine months to two years ahead of the intended start, depending on the complexity of the experiment. The INEEL has a full complement of technical capabilities to support research and development, including

• Radioanalytical Measurements and Development
• Radiation Measurements Laboratory
• Analytical Radiochemistry
• Radiochemical Research and Development
• Field Measurements and Site Characterization
• Safety and Tritium Applied Research Facility (STAR)
• Mass Separator Laboratory
• Engineering, fabrication, laboratory, and computer facilities
• Waste Generator Services
• INEEL Research Center
• Argonne National Laboratory-West facilities include Electrometallurgical Treatment, Fuel Conditioning Facility, Hot Fuel Examination Facility, Transient Reactor Test Facility, Zero Power Physics Reactor and Sodium Processing Facility