Consider the dark ages: much knowledge from greek and roman antiquity was lost, only to be rediscovered at the renaissance. It's entirely conceivable that knowledge of ionising radiation could be lost for a similar period of time if, say, a solar flare were to destroy all telecom infrastructure and subsequent wars were to destroy printed works.

Unlikely? Maybe, but certainly possible.

Nobody will be continuing studies in the field of nuclear. Knowledge will be lost.

Heck, 5,000 after the pyramids were built we still don't know how.