The Advanced Test Reactor (ATR) is a research reactor at the Idaho National Laboratory, located east of Arco, Idaho. This reactor was designed and is used to test nuclear fuels and materials to be used in power plants, naval propulsion, research and advanced reactors. It can operate at a maximum thermal power of 250 MW and has a "Four Leaf Clover" core design that allows for a variety of testing locations. The unique design allows for different neutron flux conditions in various locations. Six of the test locations allow an experiment to be isolated from the primary cooling system, providing its own environment for temperature, pressure, flow and chemistry, replicating the physical environment while accelerating the nuclear conditions.

Research reactors are nuclear fission-based nuclear reactors that serve primarily as a neutron source. They are also called non-power reactors, in contrast to power reactors that are used for electricity production, heat generation, or maritime propulsion.