The NB-36H was built for the Nuclear Powered Aircraft (NPA) program. It was made from parts from a B-36 damaged by a tornado. The original crew and avionics cabin was replaced by a massive lead-lined 11 ton crew section for a pilot, copilot, flight engineer and two nuclear engineers. As the rear section was unmanned, the engines and reactor were monitored via a television camera system. Power was supplied by six Pratt & Whitney propeller and four GE J47 jet engines. The reactor did not power any of the plane's systems, nor did it provide propulsion, but was placed on the NB-36 to verify that the plane could actually carry and run it in the air.
The NB-36 completed 47 test flights and 215 hours of flight time (during 89 of which the reactor was operated) between September 17, 1955, and March 1957 over New Mexico and Texas. The plane was also followed by several support planes. Though it was never needed, there was a direct hotline to the President's office set up in case of a nuclear accident on board the aircraft.
http://en.wikipedia.org/wiki/NB-36
In May 1946, the Air Force began the Nuclear Energy for the Propulsion of Aircraft (NEPA) project which was followed in May 1951 by the Aircraft Nuclear Propulsion (ANP) program. While the NEPA project was mainly concerned with research, the ANP program had the ambitious goal to turn this research into a working prototype. Two different systems were pursued, the Direct Air Cycle, and the Indirect Air Cycle.
The Direct Air Cycle concept was assigned to General Electric based at Evendale, Ohio. This system had advantages in cost, simplicity and reliability. Conventional jet engine compressor and turbine sections were used, with the compressed air run through the reactor itself to heat it before being exhausted through the turbine. This configuration had the additional benefit that the aircraft could take off using on chemical power, then switch to nuclear once the core reached operational temperatures, reducing deployment time.
A series of tests were run known as the Heat Transfer Reactor Experiment (HTRE), involving three different reactors, with the purpose of determining the most efficient method. This program produced the successful X-39 engine, which consisted of two modified General Electric J47s turbojets, with heat supplied by the Heat Transfer Reactor Experiment 3 (HTRE-3).
Development of the Indirect Air Cycle was assigned to Pratt & Whitney. In this configuration, the air did not go through the reactor core, but instead was passed through a heat exchanger. The heat would then be carried by liquid metal or highly pressurized water to the turbines where the hot air would drive the aircraft. Being a closed system, this concept would have produced far less external radioactive pollution. While progress was made, by the end of the ANP project, Pratt & Whitney failed to produce a workable system.
After establishing the parameters for the power plant and the transfer mechanism, engineers commenced work on the difficult issue of shielding for the crew and avionics systems. Initial plans proposed shielding the reactor with massive layers of cadmium, paraffin wax, beryllium oxide and steel. The idea being that the more protection the reactor had, the less shielding the crew cabin would require.
In theory this approach was workable. However in the context of aircraft design, weight or rather that lack of it is all important and this design was rejected. It was decided that the best approach was to use a concept known as "shadow shielding". The layers of protection would be equally divided between the reactor and the crew cabin (also referred to as the divided shield concept). The crew being in the 'shadow' created by the shields. This system satisfied weight requirements, but meant that both the aircraft and the ground staff and equipment were exposed the significantly higher levels of radiation.
One unverified proposal for the problem of shielding was to use only elderly air crew. The reasoning being that the effects of radiation injury and potential sterilisation were of less significance to crew and pilots over a certain age.
http://www.sonicbomb.com/modules.php?name=News&file=article&sid=117
