In the 50's development work was done on nuclear powered aircraft. One engine system, the HTRE-3, that was tested used a reactor to heat air which was then used to run two GE J-47 turbojet engines (first photo). The reactor is above the two engines with ducting in between. The engine was only tested on the ground, but a B-36 was actually flown with a reactor inside for shielding tests.
The engine test facility (second photo) still exists in Idaho with the engine from the first photo installed.
"In 1955, the X-39 was run on this ground test stand in what was called the Heat Transfer Reactor Experiment No l (HTRE-l). Engineers tested a complete aircraft power plant consisting of this reactor, a radiation shield, two X-39 engines, ducting, control parts and instrumentation. In January 1956, the engines were operated successfully but, because there had been no attempt to restrict the weight of the shielding, they would not have been flyable. Later in 1957, other cores that were tested, HTRE-2 and 3, did reduce the weight somewhat. The HTRE-3 assembly produced enough thrust to theoretically sustain a flight at 460 mph for about 30,000 miles [using ~100 pounds of fuel]. However radiation levels were still a problem; at one point in the tests, controls failed and released enough radioactivity to contaminate 1,500 acres."