Well, that's interesting.
Didn't hear about that personally.
Another interesting subject is the chocolate chip cookie nuclear bomb project, I remember reading a book about it once.
They could've made some "dirty" atom bombs, but the head scientist sabotaged the project. Sadly I forgot his name.
But considering how low Japan always was on resources in general, and the situation they were in, I doubt that they could've made it as pure and effective as the US bombs were.
On the effects of Nuclear Fallout, There wasn't even that much testing in that stage, as far as I know. All the videos and stuff you always see online, is from the 50's at best, if not later. Scientists could only give a rough estimate, from a year or two, to many decades.
I agree Bob, the second nuke was just overkill. Not that the first one was just or anything, but breaking the Japanese morale with something like blocking their Island might not work out either.
A ground invasion operation would've been catasthropic, If you just think about the Jap morale of "until the last man".