In the mid-1980s, operating problems took [nuclear] plants off-line so often that, on an annual basis, they operated at only about 55 percent of their rated total generating capacity. Today, as a result of several decades of experience and an intense focus on performance ... nuclear plants in the United States operate at over 90 percent of capacity. That improvement in operating efficiently is so significant in its impact that it can almost be seen as a new source in electric power itself.