Careless workers may have caused the steam pipe rupture that shut down the Ginna nuclear power plant outside Rochester, N.Y., Jan. 25 and has kept it closed ever since.

A spear-shaped, six-inch-long piece of boilerplate found wedged among previously damaged tubes that had been plugged and taken out of use was tentatively identified yesterday by the Nuclear Regulatory Commission as the culprit in the accident that closed the Ginna plant of the Rochester Gas & Electric Corp. The piece of boilerplate matches a plate at the top of the steam generator that was cut out with torches and replaced by workmen as long ago as 1975.

The NRC study concluded, however, that the rupture posed no significant risk to public health and safety and that there was no immediate need to take action on the situation.

As Timothy Martin, director of the NRC's division of engineering and technical programs, reconstructed it for the commission, the "foreign object," when it was cut out, could have fallen among the thousands of pipes that make up the steam generator. It then could have become wedged between two plugged pipes near the pipe that ruptured and closed the plant in January. The spear-like chunk of metal could have rattled between the two pipes and sheared off their ends, which is how they were found when inspected by technicians last month.

"It looks as if the wedged piece of metal wore away one end of the plugged tube," Harold R. Denton, director of regulation for the NRC, explained in an interview. "The sharp ends at the bottom of the plugged tube were then free to rattle against the tube that eventually ruptured."

Whatever happened, the tube ruptured without warning, leaking water at the rate of 760 gallons a minute for almost five minutes. By the time plant operators had the leak under control, the plant's cooling system had lost more than 18,000 gallons of water.

Despite the seriousness of the leak, the NRC concluded that the risk from exposure to radiation was low.

"Comparing the risk from exposure to radioactive material released from Ginna with the risk from the normal incidence of cancer fatalities and genetic abnormalities in the general population, the risk to the public health and safety from this exposure is insignificant," the report said.

When the plant will start up again is unknown. One complicating factor is the possibility that the reactor vessel housing the uranium fuel bundles may have undergone what engineers call "thermal shock" during the cooldown process, in which the nuclear chain reaction was stopped and the plant shut down.

The loop that carried water to the damaged steam generator underwent a sudden 200-degree drop in temperature less than an hour after the accident, which could have meant that a rush of cold water had reached the reactor vessel and put a sudden stress on the walls of the reactor vessel. This could have cracked the reactor vessel.

The prevailing belief is that this did not happen, that temperature detectors near the cold water inlet valve sensed the sudden drop in temperature because the water in the loop feeding the damaged generator was not mixing and had turned stagnant.

"This is an area we are going to watch very closely," Denton told the commission. "It's a key issue to be resolved before the plant can restart."