The operators of the vast majority of commercial nuclear power plants in the U.S. have reported to the Nuclear Regulatory Commission that their systems might not function as required during an emergency, according to an analysis of government data by Critical Mass, a nuclear safety group. Of 111 reactors licensed between 1996 and 1999, 102 reported problems ranging from a fire door that failed to close during a test to wiring problems that would have prevented a safety system from working.