The question of "what did they know and when did they know it" during the Three Mile Island nuclear power plant accident resurfaced yesterday in the Nuclear Regulatory Commission's debate over a very pertinent issue: what to do about TMI's undamaged companion reactor, which has not been allowed to start up since the incident in 1979.

The nuclear industry considers the future of the intact Unit One reactor crucial to the financial health of both the owner, Metropolitan Edison Co., and the entire industry because a precedent could be set.

Finding that Met Ed deliberately misled regulators could mean disciplinary or even legal action against some management officials, and it could irreparably damage the image of managerial competence that the utility must have to win NRC permission to start up the undamaged reactor.

Under NRC Commissioner Victor Gilinsky's questioning, the NRC staff deputy executive director, Victor Stello, said yesterday he "substantially agreed" with a congressional report earlier this year that the information was "knowingly" withheld.

But Stello, who probed TMI as NRC's enforcement chief, said withholding information "knowingly" did not mean Met Ed did it "intentionally." Its employes didn't volunteer some information because they were uncertain about what it meant and were reluctant to cast it in the worst light, he said.

The congressional report, however, says the withholding was deliberate.

The utility has said that only starting up Unit One will save it from bankruptcy. The plant, very similar to Unit Two where the near-meltdown happened, was ready to begin operations the day the accident occurred. The NRC has kept it shut since then, first to undergo technical changes and then because of community fears and questions from citizens' groups about Met Ed's management.

Unit Two cleanup, which will continue well into 1985, could cost Met Ed's parent company, General Public Utilities Inc., $1 billion; the idle Unit One costs millions a month in carrying charges. Meanwhile Met Ed has to buy expensive power from other utilities to supply its customers.

Spokesmen for the industry insist that no technical reason keeps Unit One closed and note that three investigations, including Stello's, have found no evidence that Met Ed lied.

But the congressional inquiry, a staff report last year from the House Interior subcommittee on energy, chaired by Rep. Morris Udall (D-Ariz.), found that Met Ed had given "misleading" information more reassuring on plant conditions "than what the managers themselves believed and what was in fact the case."

In October, Gilinsky said he could not just let the issue go "because to do so is to send a message to every GPU employe, and indeed to the whole unclear industry . . . that any nuclear utility can get away with deciding for itself when, and to what extent, it will give information about vital safety matters to state and federal officials responsible for public protection."

NRC Chairman Nunzio J. Palladino has spoken out strongly in recent weeks on the issue of managerial competence in the nuclear industry, calling for a rededication to safety and quality construction. Managerial integrity has become an issue in the case of the Diablo Canyon reactor site in California, where the NRC is checking into allegations that Pacific Gas & Electric Co. officials reviewed and altered an allegedly independent consultant's report on the plant.

At the hearing yesterday, NRC staff members revealed yet another problem at TMI: Some of the unused Unit One reactor's steam generator tubes are leaking. They estimated that it would take two months to find the cause and to check the other tubes, possibly extending the estimated March date by which the reactor could be physically ready to start if the NRC gives it the go-ahead, the staff said.

A special licensing board on Unit One's case recently issued a two-part report approving Met Ed's managerial abilities and operational plans and raising no objection to startup. But NRC may accept, modify or ignore the board's view in its own decision, due in February, on the unit's future.