I lived in Chattanooga for about 12 years. Somewhere around 1998 or 1999 I visited the exact place you are talking about with a friend of mine (who was then and still is a TVA IT employee). I was just becoming interested in computers at the time and he told me that he would show me where the TVA kept "the brains" of their operation as he put it. Even at that time I questioned whether or not he could get me into this area and his reply was "nobody will know and even if they did nobody will care".
I went back to Chattannoga to visit my old friend a couple of years ago. He had since moved up the govermental ladder and was more or less "in charge" of certain systems. During my visit he took me back to the place, on a Saturday. There was a single security guard on duty at the entrance. As we entered the guard didn't give us much notice as my friend swiped his ID card and entered a key code. I was holding my friend's laptop bag and the guard never asked to look at it or see it. For all he knew I could have been carrying in a brick of explosives. Once inside, I had access to pretty much any place in the "bunker" that I wanted to go, including various control panels, servers, etc.
Short answer, no their security is not better than it was back then.He has not acquired a fortune; the fortune has acquired him. -- Bion