I concur with your assessment concerning that sequence, it really did not seem all that plausible. However, I have found one
source which argues that is not as implausible as we believe. But, please allow me go ahead and throw some grains of salt in the air before you follow the link or read the excerpts I'll provide. I found this source on a forum that was discussing that sequence of the film. In this forum there is a lengthy quotation from an uncited source who knows "a bit about nukes" who discusses the issue. I have not been able to track down the source of this lengthy quote, so it could be puppy rolling across a keyboard for all I know. However, since everyone else on the internet is decrying the improbability of that scene, I figured I'd throw out this lone voice of defense.
I found the defense of the film's logic here
(it's in the very first post). It's really long, so I'll quote some highlights:
Actually, it's plausible. The first thing to remember is that when a nuclear bomb detonates, all the actual detonating is done at a single point- where the bomb is, known as the hypocenter. The rest of the effects are due entirely to the tremendous force of that detonation creating lots of light, heat, and radiation that affect its surroundings.
There are three killers from a nuclear detonation- thermal pulse, shockwave, and radiation. The lead lining won't actually help much against radiation because the amount of lead that could plausibly be in the fridge isn't thick enough to block gamma rays- and nothing else could have penetrated the air between him and the blast, let alone a metal fridge door of any kind. On the other hand, most of the gamma rays that reach him will pass right through him. Direct radiation isn't a major killer for people caught far enough from a nuclear detonation that they won't be in the zone of total destruction. Inhaling radioactive isotopes or getting them on your skin from the fallout is, but the way Indy got out of the explosion would reduce that threat.
Indy probably got a considerable dose of radiation. Prompt precautions would help with the relatively low dose he got in the movie. The guys in radiation suits scrubbing him down are a part of those precautions. Drinking lots and lots of water would help too, because he needed to get as much of the radioactive fallout out of his system as fast as possible. Flushing the tubes, so to speak, helps. We didn't see him do that, but it's likely he did.
The shockwave is actually more dangerous to objects with a big surface area facing the blast (like houses) than objects with a small surface area (like people). A shockwave that would collapse a house might merely knock a man down. If nuclear explosions produced nothing but shockwave the smartest thing you could do to protect yourself would be to run out in the street and cover your ears. They don't, so it isn't.
The fridge would do a LOT to protect Indy from the shockwave.
Because he was inside a rigid steel container, the air around Indy would not have been compressed. Being wrapped inside a house would have muffled the shockwave inside the house itself, and being in a rigid steel box would have muffled it further, preventing him from being buffeted by the shockwave personally.
The refrigerator would have done an excellent job of protecting Indy from the thermal pulse. Simply being indoors with a wall between him and the hypocenter would have put him 'in the shade'- the wall would absorb all the direct light and infrared from the blast. He would only have to deal with the great heating of the house itself.
Given that the refrigerator would protect him from both the direct thermal blast and from being killed by falling debris in the shockwave, it is quite plausible that Indy could have survived the immediate effects of the blast at that range. The real threat would be radiation. At the estimated range prompt medical treatment would most likely have saved his life, but quite possibly not his health- certainly he'd have been in no shape to go adventuring later that year, and would most likely not have had a full head of hair ever again- certainly not in time for his interrogation by overzealous McCarthyist FBI agents.
Thus, the only real "Hollywood magic" involved in Indy's survival was:
-that the "lead lined" refrigerator could protect him from picking up a nasty radiation dose. At his range he should have taken several hundred REMs. [...] Of course, to put him down in 'safe' (very unlikely to kill or incapacitate him for long at all) levels the fridge would only have had to cut the radiation levels by a factor of five to ten. It would take at least an inch of lead to do that, almost certainly more, and the refrigerator can't have been that heavy or it would have been nearly impossible to move in the first place.
-that he was thrown so far (itself a piece of Hollywood magic) and survived that. He shouldn't have been thrown. If he was thrown, he certainly shouldn't have survived the experience, no matter how tough he was.
In the end, even this guy admits some mighty impossibilities in the scene, and yeah, Indy would have died. But perhaps its not quite as crazy as we think, even if it is still crazy. But maybe this anonymous source I'm quoting is crazy.