In 1945 how much was really known about the effects of the bomb?

Answer 1

Quite a lot was known by the scientists, especially about the potential effects of fallout as radioactivity had been studied since the 1890s and with the isolation of Radium by the Curies its damaging health effects were well known and many of the isotopes that the bomb would produce and would be present in the fallout are much more radioactive than Radium.

However the blast effects could only be guessed at as the explosion released orders of magnitudes more energy than the largest conventional explosions had.

It took the Operation Crossroads weapons effects test to prove that the blast could sink and severely damage naval warships. But even that did not sink enough ships to convince the Navy of the power of the atomic bomb to make equipment unusable through fallout contamination (but see the first paragraph above, if the Navy had listened to the scientists they would have already known this!).

The main problem was regardless of what they were told by the scientists a significant number of military people could not see the atomic bomb as being anything other than a big conventional bomb: you can't see, taste, smell, feel, or hear radioactivity without special instruments, so many military people refused to believe it could affect troops.