The broken steam pipe that closed down a nuclear power plant outside Rochester, N.Y., last week was more serious than originally thought, causing the plant to leak 700 gallons of radioactive cooling water a minute for the first few minutes after the accident.

When the ruptured pipe shut down the Ginna plant of Rochester Gas & Electric, officials of the Nuclear Regulatory Commission said the pipe had lost cooling water at 75 gallons a minute. NRC Regional Administrator Ronald C. Haynes corrected himself before Congress yesterday, declaring that the leak rate fell to 75 gallons from 700 gallons a minute when the water pressure of the steam boiler fell 1,000 pounds per square inch, from 2,250 psi to 1,250 psi, in less than four minutes.

"This suggests that what we saw here was the classic double-ended pipe break, where a tube was cut in half and leaking water out of both ends," explained Darrell Eisenhut, director of the NRC's licensing division. "It's highly unlikely that more than one tube broke because the leak was instantaneous, suggesting water pouring out of both ends of the same broken pipe at the same time."

None of this water escaped into the atmosphere although some slightly radioactive steam did.

Haynes told the House Interior Committee that the plant will stay closed at least three or four more weeks and possibly as long as two or three months. Technicians were scheduled to enter the plant today to examine the broken tube, then pull it out and inspect nearby tubes to see if they exhibit the same strains that led to the pipe rupture last week.

"If all they have to do is plug off this one leaking tube, then the plant could start up in three or four weeks," Haynes said. "If it's more than that, it could be a couple of months before they start up again."

Harold R. Denton, director of the NRC's office of nuclear reactor regulation, said nuclear power plant operators are required to inspect steam pipes every six months when they shut down to refuel and every three months if they display chronic tube corrosion.

"It's a fairly widespread problem," Denton said. "We have 31 plants of this type with problems that required plugging of tubes and four units at two plants Turkey Point in Florida and Surry in Virginia whose steam generator tubes had to be completely replaced."

Denton explained that steam generators are built with 10 percent more tubing than they need, so that when tube leaks or breaks occur they can be plugged and removed from service without losing any generating power.

A typical steam generator in a nuclear power plant has more than 3,000 steam tubes to remove heat from the uranium reactor and convey it as steam to power the turbine to generate electricity.

The tubes are three-quarters of an inch in diameter and each is 57 or 62 feet long, depending on whether it is U-shaped or straight. The tubes occupy an area the size of a football field.

In a telephone interview, Eisenhut said nuclear power plant operators have begun to "sleeve" defective tubes to keep them operating instead of plugging them. Sleeving involves the insertion of a narrower pipe into a tube that is thinning or cracking from corrosion.