Nearly 30% of U.S. nuclear-power plants fail to report equipment defects that could pose substantial safety risks, a flaw in federal oversight that could make it harder for regulators to spot troublesome trends across the industry, the Nuclear Regulatory Commission's inspector general said Thursday.

The Office of Inspector General said nuclear-plant operators were confused about what they were required to report to regulators about manufacturing defects. One section of federal law, known as Part 21, requires them to report...