WAYNESBORO, Ga. — Towering over the Savannah River in Georgia, the first nuclear reactors built from scratch in the United States in more than 30 years illustrate the enormous promise of nuclear power — and its most glaring weakness. The two new reactors at the Vogtle nuclear power plant will join two older units to create enough electricity to power 2 million homes, 24 hours a day, without emitting any of the carbon dioxide that is dangerously heating the planet. Sign up for The Morning newslet