Technicians are expected to enter the shut-down Ginna nuclear power plant later this week, but it will be at least three weeks before the plant can resume generating electricity.

The plant, 16 miles east of Rochester, was shut down Monday morning when a steam pipe broke, venting radioactive steam to the air and spilling an estimated 11,000 gallons of radioactive water into the sump on the floor of the plant's concrete containment building.

"The plant is in cold shutdown," an official of Rochester Gas & Electric Co. said late today. "We think our workers can enter the plant and begin to assess damage by Saturday."

"Cold shutdown" means that technicians have managed to lower the temperature of the water covering the plant's uranium core to below 200 degrees Fahrenheit, far cooler than the plant's normal operating temperature of 575 degrees. It also means the core's cooling water is below its boiling point, making it unnecessary to pressurize the core, and making it safe for technicians to enter the concrete containment building where the core rests inside a stainless steel reactor vessel.

"The next thing to do is to drain the steam generator that has a broken tube and get people into that generator to inspect and remove the ruptured tube," Ronald C. Haynes, regional administrator of the Nuclear Regulatory Commission, said today. "If only one tube was broken in a straightforward way, the plant could be back on line in three weeks. If more than one tube is ruptured or the break is complicated, it may be a couple of months."

The upstate New York utility's first job is to clean up the radioactive spill, so that technicians can work on the damaged steam generator where the ruptured pipe or pipes are. Rochester Gas Vice President John Oberlies said he hoped that the cleanup could begin later this week and that the plant could be back in operation "in a few weeks."

Harold R. Denton, director of the Office of Nuclear Regulation for the NRC, said the radiation that escaped into the air Monday was measured at the plant boundary at three millirems, which is about one-seventh as much as a patient might get from a chest X-ray. Denton said he did not know how much radiation was in the water on the plant floor but suggested it was not much more than instruments found in the air.

"It might be expensive for the operator to clean up," Denton said, "but in terms of public health consciousness, it was not very serious."

Federal and utility officials said they suspected the leak was caused by a rupture in a tube or tubes that had corroded during the 12 years the plant has been in operation. The NRC's Haynes said most leaks of this type are caused by corrosion.

"I would say," Haynes went on, "that it would not be unexpected to have a few more steam generator occurrences of this kind."