How much was known about radiation before the atomic bombings?

How much was understood about radiation before the atomic bombings of Japan?

Some was known, but the effects of a nuclear explosion and critical-mass physics were still difficult to predict.

Some knowledge of radiation existed before the atomic bombings of Japan. Predicting the radiation effects of a nuclear explosion was still a complex problem. Understanding critical mass and the associated physics remained at an early stage, leaving room for serious failures of prediction. Radiation was known, but that knowledge did not amount to a mature model of every consequence.