Back in the days, in the beginning of the cold war, both U.S. and Russian engineers came up with the idea of creating a nuclear-powered aircraft. Such an aircraft would have the benefit of virtually unlimited operating range as it could stay airborne until the crew couldn’t take it any more.
Both parties didn’t make it beyond testing and prototyping, however they came up with interesting problems and concepts. Both eventually built planes with nuclear reactors in them, which however were propelled by conventional engines, in order to test the manageability of running a nuclear reactor on a plane. The Russians built the Tupolev Tu-95LAL based on the Tu-95, and the Americans built the Convair NB-36H based on the B-36, which were their respective strategic bombers at that time. It is not entirely clear to me, however it seems that only the Russians had the guts to actually run the reactor during test flights.
One major problem was shielding the crew from radiation emerging from the reactor; while land- or ship-based reactors are usually shielded with heavy concrete walls, water and lead, this wasn’t feasible on an airplane due to weight restrictions. They came up with combinations of different materials such as sodium and paraffin wax, which apparently are somewhat effective in absorbing neutron radiation.
The other concern was how to convert nuclear energy into thrust. Candidates were ramjet, turboprop or turbojet engines – all of these rely on expanding gas, conventionally generated by combustion of jet fuel, in this case by heating up air by nuclear means. To accomplish this, there were two main approaches – the so called direct cycle, and indirect cycle which involved a heat exchange system.
In the direct cycle approach, air would be compressed and fed into the reactor core, acting as coolant to the reactor and being heated up, the hot air then would go back to the turbine to produce thrust. The downside of this approach is that the air is more likely to get contaminated in the process, the airplane would constantly be spewing radioactive air out of the exhaust. One such engine was the HTRE-3 shown in the picture to the left.
In the indirect cycle approach, the reactor would heat a coolant which would then transfer the heat to compressed air using a heat exchange system. Unfortunately this approach apparently didn’t turn out to be feasible and never came near to be flight-ready.
Eventually, the projects were stopped as strategic bombers were made obsolete by land- and submarine-based ICBMs.
I recommend watching this documentary on YouTube:
Here is a nice overview on the whole Soviet nuclear aircraft program. This web page has some more photos of the Tupolev Tu-95LAL. More information on the Tupolev, in Russian.
*) I did not find copyright information for the Tupolev pictures. But I think this counts as fair use if not public domain.