The Nuclear Regulatory Commission is investigating why its staff gave contradictory dates on when it discovered that the North Anna nuclear plant in Louisa, Va., was being built on a geologic fault.

Virginia Electric and Power Co. maintains that it "promptly notified" the commission on May 17, 1973, that there was an inactive fault under the plant, which began supplying electricity this month.

But in Senate testimony last fall an NRC official testified that the agency did not learn of the fault until its staff made an inspection of the Vepco site June 18, 1973, a month later.

After that testimony Sen. Gary W. Hart (D. Colo.), chairman of the nuclear regulation subcommittee, accused Edson G. Case, who gave the June 18 date of "a serious misrepresentation of the facts."

More recently NRC Chairman Joseph M. Hendrie wrote Hart that "I cannot conclusively establish a date when the commission was informed" of the fault.

The question of the dates is crucial, according to critics of the Vepco plant, because a federal licensing board was holding hearings on proposed plant construction between the May 17 and June 18, 1973. If the staff knew about the fault at the earlier date, then the Atomic Safety and Licensing Board should have been told, the critics say.

The existence of the fault has embroiled Vepco, its North Anna plant and the NRC in controversy for more than five years.

Vepco was fined $60,000 by the commission in 1976 for making allegedly false statements about the fault. The North Anna Environmental Coalition, which led opposition to construction of the plant, has accused the NRC of withholding information that could have stopped construction of two additional units.

"The date when the NRC found out about the fault is not in itself significant," June Allen, who heads the coalition, said. "What is significant is why they (NRC staff) are working so hard to change the date."

The NRC spokesman declined to give any details on what the agency's investigation included, saying: "Its purpose is to get to the bottom of what happened and when."

A spokesman for Hart's subcommittee said: "We're not satisfied that these peopld (NRC staff) are sufficiently clear so we can drop the matter."