Could the plume from an exploded nuclear reactor corrode helicopter blades? I was recently watching the new TV series Chernobyl and was curious if something that occurred was even physically possible. In the show, the physicist instructs the pilots of a helicopter not to fly any closer than 10 meters to the plumes that are being emitted from reactor 4. The pilots disregard the physicist's instructions and fly directly through the plume. Upon emerging on the other side, the blades seem to have corroded to the point that they are slung from the helicopter itself. Is there any way this could physically happen or is it purely cinematic drama?
 A: This moment is one of the most glaring historical inaccuracies in the series. The helicopter crash happened on October 2, 1986, 5 month after the disaster started, and the reactor fire was already put out by this point. The helicopter crashed because its rotors touched a cable, radiation had nothing to do with it. Here’s an article (in Russian) with detailed discussion and real video of that crash.
Among the helicopters which dropped sandbags into the burning core, none crashed. The series authors moved the event  for dramatic effect.
As for radiation, it does not cause significant degradation in metals by itself, at least not this quick. There is a thing called induced radioactivity, which may lead to elements turning into other elements, but usually the fraction of atoms involved is not enough to cause significant mechanical degradation.
A: The helicopter that flew through the plume and then lost its main rotor blades flew under the bucket cable for a crane that was being used to dump sand on the burning core. the blades struck the cable and sheared off, and the rest of the helicopter fell to the ground and crashed. 
A: 
Upon emerging on the other side, the blades seem to have corroded to the point that they are slung from the helicopter itself

It seems you've misinterpreted the events as depicted in the clip. There is no corrosion of the blades shown $-$ the blades snap off because the helicopter flies into a crane. (As others have mentioned, a helicopter crash of this nature did occur, but not during the firefighting action.)
The implication isn't that the helicopter itself was damaged, but rather that the pilot himself was incapacitated by the radiation (i.e. the prompt gamma radiation from the core shining upwards $-$ the same which Legasov warns his helicopter pilot will be deadly when he and Shcherbina arrive to the site), and that this led the pilot to fly into the crane.
