In the past sixty years or so the human race has familiarized itself with the word nuclear. We know what destruction a nuclear missile can cause, fear the effects of a "nuclear winter", and people have taken advantage of nuclear power to successfully fuel things such as power plants, submarines and aircraft carriers.
But in the early days of the Nuclear Age (the 1940's and 50's) atomic energy was all the rage, and people talked about atomic powered cars, trains, using nuclear devices for excavation purposes, and for nuclear powered aircraft. This last item intrigued the United States goverment, eventually leading to the creation of the Nuclear Energy for the Propulsion of Aircraft program or NEPA in 1946.
What the goverment wanted was a bomber (with nuclear weapons on board) that could stay aloft for months at a time. While this idea seems crazy now, due to safety concerns, one has to remember that in 1946, the only way that long range nuclear bombs could be delivered was by airplane, and the United States had a new enemy on its hands, the Soviet Union, who also had nuclear bombs at its disposal. A fleet of bombers that was constantly airborne would hopefully act as a deterrent from the Soviets using their nuclear arms in a preemptive strike against America.
So, research began in 1946 and by 1948 the Air Force had sunk 10 million dollars into the project. In 1951, the Atomic Energy Commission (AEC) was tasked to join the USAF in this venture and the program was renamed ANP (Aircraft Nuclear Propulsion).While the NEPA project was mainly concerned with research, the ANP program had the ambitious goal to turn this research into a working prototype.
While the ANP had several proponents,one being General Curtis E. LeMay head of the Strategic Air Command (SAC), the ANP program faced several large obstacles, in the areas of technology and cost.
First of all they had to build a jet engine that was able to be powered by a nuclear reactor as well as the reactors themselves. Two companies were contracted to build the components, General Electric and Pratt & Whitney.
General Electric was tasked with building a direct cycle system where air would flow into the turbojet (or jets), through the reactor core, be heated and then the heated air would hit the turbine, providing thrust. The Pratt & Whitney system was to rely on a indirect cycle system which was similar to the direct cycle system except that air would not pass through the reactor itself, instead a dense fluid (Pratt & Whitney planned on using pressurized water) was heated by the reactor, then heated the air with the help of a heat exchanger. General Electric built a test reactor (HTRE-1) which would test the feasiblity of the program.This reactor reached critical mass on November 4th 1955. HTRE-1 was then modified to test different reactor fuels in July 1957. This reactor was called (HTRE-2} and ran to the end of the ANP program in 1961. Based on these tests GE built HTRE-3 in 1958, a reactor-turbine assembly that proved that building a reactor for a airplane and using its power to run a jet engine was possible. As for Pratt & Whitney, while progress was made, the end of the ANP project did not see a working reactor from them.
Although the powerplant of a nuclear powered bomber was deemed possible, there were several more hurdles to overcome. One big problem was shielding the crew from the high amounts of radiation the reactor put out. The ANP extensively modified a B-36 Convair bomber (the biggest the USAF had at the time) for testing. A small air cooled reactor was placed in the bomb bay (this reactor did not power the aircraft), and a whole new nose section was built, using 12 tons of lead and rubber to protect the crew. Also, some shielding was added around the reactor, and water jackets lined the fuselage and were also placed behind the crew compartment to asorb radiation.
The converted bomber now known as the NTA (Nuclear Test Aircraft) made numerous test flights from 1955-1957 to test the effectiveness of the shielding, and to gauge the effects of a reactor and radiation on the airframe. Tests were also conducted to see how "safe" flying this type of aircraft would be. The tests were deemed to be successful. All of the components needed to construct a nuclear powered aircraft were there. But the nuclear powered aircraft was not to be. Why?
In fact, there were several reasons. One reason was that the ANP program was getting to be extraordinarily expensive, costing about $1.1 billion (in 1957 dollars) by the time it was cancelled in 1961, and there still was no working prototype. Another reason was that the project had dragged on for 14 years, and other advances in technology (such as rockets and missiles) led some in the Kennedy administration to believe the nuclear powered bomber was soon to be obsolete. President Kennedy axed the ANP program on March 28th 1961.
Sources included www.megazone.org/ANP, www.duotone.com/coldwar/anp and www.inel.gov