The dangers of acute radiation poisoning were certainly well known by then. Two Manhattan Project scientists died in 1945 and 1946 from radiation poisoning, and of course there were a lot of Japanese civilian deaths.<p>The long-term danger of lower doses is a rather different thing, though, and that took a lot longer to be well understood. There are a lot of things which are dangerous in large doses but harmless or even beneficial in small doses, after all.