He was a "nuclear physicist" like Obama was a "constitutional law professor."...
As I remember it, the first statement was a mistake that the press made early on. It was established that Carter did qualify as a nuclear engineer. Of course, Obama obviously was not a qualified law professor.
Well, sorta. Kinda the same way an train driver is an "engineer." He was trained how to operate and maintain the reactor on a sub. Not great at it, apparently.
BTW, Admiral Hyman Rickover, the guy who got us into nukes for the Navy, thought Carter was a complete idiot. Took him in the program for political reasons ... Sam Nunn.