Just about the only thing heating up in the eastern half of the U.S. right now is the debate over what power source should be trusted to keep homes warm and the lights on. There are 99 nuclear generating units in the U.S. and every one of them is currently operating -- "an incredible but unsurprising testament to nuclear's reliability," said John Keeley, spokesman for the Nuclear Energy Institute. Coal has become the dominant power-plant fuel across the Midwest, thanks to higher natural gas prices tied to record demand for that rival energy source.