The inquiry by the inspector general of the Nuclear Regulatory Commission also raised questions about the agency’s oversight, saying reporting guidelines for the nuclear industry are “contradictory and unclear.”

The study comes as questions are raised about the safety of U.S. nuclear facilities in the wake of the nuclear crisis in Japan. The NRC voted Wednesday to conduct two safety reviews of the 104 nuclear reactors operating in the U.S.

Unless the NRC takes steps to improve its reporting guidelines, “the margin of safety for operating reactors could be reduced,” the report said.

NRC inspectors found at least 24 instances where possible equipment defects were identified but not reported to the agency from December 2009 through September 2010, according to the study.

NRC staff reviewed a draft of the report but opted not to provide formal comments. An NRC spokesman declined to comment.