The nuclear Regulatory Commission reprimanded its staff last November for covering up its knowledge of a geological fault under a Virginia nuclear power plant, but did so only in a little-noticed footnote to a commission ruling, it was disclosed yesterday.

The reprimand called "unacceptable" a three-month delay between the time in May, 1973, that the NRC was notified of the existence of a geological fault under the Virginia and Electric Power Co. nuclear plant at North Anna, Va., and the date the fault was reported to the Atomic Safety and Licensing Board.

Members on both sidea of Congress said yesterday the cover-up raised serious questions about overall Nuclear Regulatory Commission practices and confirmed onging investigation into NRC behavior in the Vepco and other cases.

The $2 Billion-lus North Anna project is awaiting action on its application for an operating permit and is scheduled to be operational before the end of year. The North Anna Environmental Coalition, which opposes the plant is part on grounds that the geological fault makes it liable to rupture from earthquakes, saie it plans to ask he licensing board to consider reopening hearings on the permit application because of the Justice Department revelations.

World of the November reprimand came as part of an NRC spokesman's response to a Justice Department probe of the cover-up. The Justice Department had called the NRC's actions "ill-considerd and inept . . . demonstrating a pervasive bias against the scrutiny which a project of this importance deserves and is entitled to under federal law."

The NRC, set up in 1975 to replace the Atomic Energy Commission, is charged with guranteeing the safety and efficiency of nuclear power plants. It licenses their construction and operation through independent panels of judges on each project, called Atomic Safety and Licensing Boards.

The NRC spokesman, Frank Ingram, said the facts of the case as set forth by the Justice Department "seem to be correct. We can't agree with the conclusions, though." He said the NRC had reprimanded its staff in a footnote on page 21 of a 2-page opinion handed down Nov. 12, 1976, that found Vepco guilty of seven "material false statements" in the North Anna fault case and fined the company $32,000. That ruling is under review in the U.S. Fourth Circuit Court of Appeals.

The footnote took note of the Vepco defense, which was that it had notified the NRC of the fault on May 17, 1973, and had received a go-ahead from an NRC inspection team in June of that year and heard nothing further until August. "We find the staff delay in informing the (licensing) board and the explanations given for that delay unacceptable," the footnote said. It rejected the idea that staff evaluation was necessary before the fault could be made public.

"The licensing board, the parties (to the dispute) and the public have a right to be promptly informed of a discovery of this magnitude before staff evaluation of that discovery and regardless of whether the record is technically open," the NRC said in the footnote. It noted that the staff had said current practice is prompty reporting of all information, and directed that it "ensure that his practice is fully enforced."

Sen. Gary Hart (D-Colo.) and Rep. John E. Moss (D-Calif.) said separately that the case "raises serious questions about the NRC" and whether it was abusing its mandate.