The advanced test reactor 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 (similar to the camunian rose) that allows for a variety of testing locations. The unique design allows for different neutron flux (number of neutrons impacting one square centimeter every second) conditions in various locations.