39 1967 Advanced Test Reactor When the AEC and the Navy requested a new test reactor with less neutron flux variation, the result was ATR. It first reached criticality on July 2, 1967, and reached its full 250-megawatt power level in August 1969, becoming the largest test reactor in the world. The reactor’s distinctive cloverleaf design offers a wide range of power levels to nine main test spaces or loops, each with its own distinct environment apart from that of the main reactor core. While its main customer was the U.S. Navy, ATR became the basis of a National Scientific User Facility in 2007, making it available to researchers across the U.S. and around the world. It remains in operation today. “ I reached over across the front seat of the car and with my finger drew four circles for test loops, and then a snakelike fuel line partially around each loop… The more we looked at that strange arrangement, the better it looked.” — Deslonde deBoisblanc, on the inspiration for ATR’s core configuration striking him while driving home from the Site