[CharlesGriswold]

As I understand it, Tech Levels have more to do with practical use of technology than with theoretical knowledge. So, yes, we knew about nuclear energy in late TL 6, but the actual use of nuclear energy is solidly TL 7.

On the other hand, I don't think that the instant of the detonation of the first atomic bomb marked a total, worldwide transition to TL 7, it just marked the transition of nuclear energy (in the U.S., at least) and certain branches of physics.