7 1952 Materials Testing Reactor The second reactor built in Idaho, the Materials Testing Reactor, went critical on March 31, 1952, and was brought up to full power on May 22, 1952, becoming mankind’s first nuclear reactor for testing materials. MTR was a keystone of the AEC’s postwar reactor development program. In addition to subjecting potential reactor fuels and structural materials to irradiation, MTR’s “beam holes” made cross-sectioning and other physics research possible. The reactor’s eventual power output was 40 megawatts. In 1958, it became the first water- cooled reactor to operate using plutonium-239, demonstrating that a plutonium-fueled reactor could be controlled satisfactorily.The pioneering work at MTR influenced reactor design throughout the free world. “ Well, we’ve got us a reactor.” — Leonard “Bill” Johnston, manager of the AEC’s Idaho Field Office, when MTR went critical for the first time