Advanced Test Reactor begins major overhaul to replace core components INL Operations personnel working above the Advanced Test Reactor. The small cylindrical section in the center of the platform has access ports that allow access to the reactor core for refueling and experiment loading and unloading during routine outages. INL This iconic INL photo shows the Advanced Test Reactor’s distinctive cloverleaf core. The blue Cherenkov glow indicates the reactor is in operation. The experiment in-pile tubes are the long tubes extending down into the core from the top of the frame. IDAHO FALLS, Idaho (KIFI) - The Advanced Test Reactor (ATR) at Idaho National Laboratory is getting an overhaul that will shut it down for nine months.