The Nuclear Regulatory Commission has concluded that its own probe last year of safety misconduct charges at a nuclear power plant under construction in Ohio was "unsatisfactory" and is repeating the investigation.

The finding on the William H. Zimmer site, near Moscow, Ohio, is apparently the first time the NRC has publicly repudiated some of its own work.

The NRC study is a three-inch-thick report made public this week to the Government Accountability Project, a private, self-described "whistleblower support organization" of the Institute for Policy Studies. It details the case of Thomas Applegate, 29, a private investigator who was hired by the Cincinnati Gas & Electric Co. in late 1979 to check possible time-card padding at Zimmer.

Applegate reported to CG&E that he had found not only time-card padding but also that faulty pipe welds were being accepted, firearms were being sold on the site illegally and construction officials were making private use of workers and materials.

The utility, Applegate told NRC regional officials in early 1980, had taken action on the time cards but was ignoring the other problems.

The resulting NRC investigation, conducted by the regional office of inspection and enforcement, found the pipe weld charges to be "unsubstantiated" and that the utility had acted responsibly in taking no action against its construction officials.

Applegate, charging that the NRC was covering up for the utility, went to the Government Accountability Project, which pushed NRC headquarters to reopen the case.

In its report dated Aug. 7 but just released, NRC's office of inspection and auditing said the regional probe was more the result of incompetence than a cover-up.

It "was neither vigorous nor sufficiently broad in scope," the study said. Its contention that the pipe-weld problem had not been substantiated is "not consistent with the facts," the report said, noting that the original investigators never inspected the welds, reviewed major documents or fully checked the charges.

"In fact, Applegate was correct in saying that defective welds in safety-related systems had been accepted," the report said. "The overall finding is that the investigation . . . was unsatisfactory."

Applegate, who is still a private investigator, said at a Cincinnati news conference that he was "very satisfied" with the findings. "After they called me a nut for over a year, they've finally confirmed I was right all along," he said.

An NRC official said the agency was considering what punitive action to take against CG&E and would probably announce "a substantial fine" sometime in the next two weeks.

NRC Chairman Nunzio J. Palladino said in a letter Monday to Rep. Morris K. Udall (D-Ariz.), chairman of the House Interior Committee that has jurisdiction over the NRC, that the problems have not affected the overall safety of the plant.

"Shortcomings in the earlier investigation of Zimmer reveal a generic problem" in the way such whistleblower probes are handled, he wrote. "Steps are being taken to remedy that problem."