SPRING CITY, Tenn. — In an immaculate control room at the Watts Bar nuclear plant, green bars flash on a large screen, signaling something that has not happened in the United States in two decades.

As control rods lift from the water in the core, and neutrons go about the business of splitting uranium atoms, life comes to a new nuclear reactor — the first in the country since its sister reactor here was licensed in 1996. MORE