[Hebrews 11:6]

scientists knew next to nothing about radiation or radiation sickness at the time

That scientists actually knew so little says nothing about these questions that I've long wondered over:
Could radiation sickness have been anticipated?
Should radiation sickness have been anticipated?

[colorado tanker]

I'm sure the scientists were aware of the high levels of radiation the bomb would produce. It was also pretty well known that overexposure could produce sickness. There was the sensational story of the so-called "Radium Girls." They were hired by U.S. Radium Corporation to paint watch faces with luminous radium paint with no protection at all. Many of the girls got sick and some sued the company in a case that got a lot of publicity.

But, if you are deciding whether to drop an atomic bomb on a populous city, resulting radiation sickness is probably down the list of considerations.

What I'm reasonably confident they did not anticipate was radioactive fallout or how persistent the radioactive contamination might be. There was no precedent by which they would anticipate it.