A nuclear-powered aircraft is a concept for an aircraft intended to be powered by nuclear energy. The intention was to produce a jet engine that would heat compressed air with heat from fission, instead of heat from burning fuel. During the Cold War, the United States and Soviet Union researched nuclear-powered bomber aircraft, the greater endurance of which could enhance nuclear deterrence, but neither country created any such operational aircraft. One inadequately solved design problem was the need for heavy shielding to protect the crew and those on the ground from radiation; other potential problems included dealing with crashes. Some missile designs included nuclear-powered hypersonic cruise missiles. However, the advent of ICBMs and nuclear submarines in the 1960s greatly diminished the strategic advantage of such aircraft, and respective projects were canceled. U.S. programs NEPA and ANP In May 1946, the United States Army Air Forces started the Nuclear Energy for the Propulsion of Aircraft (NEPA) project, which conducted studies until the Aircraft Nuclear Propulsion (ANP) program replaced NEPA in 1951. The ANP program included provisions for studying two different types of nuclear-powered jet engines: General Electric's Direct Air Cycle and Pratt & Whitney's Indirect Air Cycle. ANP planned for Convair to modify two B-36s under the MX-1589 project. One of the B-36s, the NB-36H, was to be used for studying shielding requirements for an airborne reactor, while the other was to be the X-6; however, the program was canceled before the X-6 was completed. The first operation of a nuclear aircraft engine occurred on January 31, 1956 using a modified General Electric J47 turbojet engine. The Aircraft Nuclear Propulsion program was terminated by President Kennedy after his annual budget message to Congress in 1961. The Oak Ridge National Laboratory researched and developed nuclear aircraft engines. Two shielded reactors powered two General Electric J87 turbojet engines to nearly full thrust. Two experimental reactors, HTRE-2 with its turbojet engines intact, and HTRE-3 with its engines removed, are at the EBR-1 facility south of the Idaho National Laboratory. The U.S. designed these engines for use in a new, specially-designed nuclear bomber, the WS-125. Although President Eisenhower eventually terminated it by cutting NEPA and telling Congress that the program was not urgent, he backed a small program for developing high-temperature materials and high-performance reactors; that program was terminated early in the Kennedy administration. Project Pluto In 1957, the Air Force and the U.S. Atomic