The site of the world's worst nuclear accident, it came to symbolize the potential dangers of atomic energy. Now, after years of limping along in defiance of international criticism, the Chernobyl atomic power plant is about to be shut down for good. Ukraine restarted Chernobyl's only working reactor one last time on Thursday, planning to let it run for a day before Friday's shutdown. Technically, the final shutdown will be a very small step.