A pipe ruptured at a nuclear power plant 16 miles east of Rochester, N.Y., yesterday, triggering a nuclear "emergency" that shut down the plant and forced the Nuclear Regulatory Commission to activate its Incident Response Center outside Washington.

The "emergency" was downgraded to an "alert" last night.

The pipe broke at the Rochester Gas & Electric Co.'s Ginna plant on Lake Ontario just before 9:30 a.m., forcing radioactive steam into the air in five-second bursts for about three minutes. The ruptured pipe caused a sudden drop in water pressure that set off the plant's alarm system. The chain reaction inside the plant's reactor vessel was stopped and the plant shut down within minutes.

"We don't know yet whether the shutdown was automatic or called manually by the control room operators," Harold R. Denton, director of the NRC's Office of Nuclear Reactor Regulation, said in a telephone interview. "Whichever way it was done, it seems to have been handled just right."

Radioactive steam escaped into the air when safety valves opened to prevent the steam from leaking out of the plant's primary cooling loop into its backup loop, which had just taken over the task of cooling the uranium core. The safety valves are built to vent steam into the air if a pipe break occurs in either of the two main cooling lines.

When it broke, the ruptured pipe also leaked radioactive water onto the floor of the concrete containment building at a rate of about 75 gallons a minute for almost two hours before the pipe was sealed off. Technicians estimated that about 8,000 gallons of radioactive cooling water spilled onto the floor before being emptied into a sump tank.

"The radioactivity outside the building was measured at about one millirem per hour," which is less than permissible under NRC regulations, Denton said. "A few hours after the incident, radioactivity was down to normal background cosmic and solar radiation levels."

Officials said last night that five plant employes had been treated for minor exposure to radiation.

Despite the apparent quick response of the nuclear power plant's operators, the NRC activated its Incident Response Center "at the request" of its regional administrator to provide technical guidance to the plant's operators as they brought the plant to what nuclear engineers call a "cold shutdown."

This was only the fourth time the NRC had done this after a "loss of coolant" accident shut down a nuclear power plant. The first was in 1979 when the Three Mile Island accident took place, the second also in 1979 when Wisconsin's Prairie Island plant suffered a pipe break, and the third last year when the St. Lucy plant in Florida had a loss of coolant accident.

At no time during yesterday's accident was the hot uranium core of the Ginna (pronounced Gih-NAY) plant threatened with a loss of cooling water, Denton said. When the pipe broke in the main steam generator line, the plant's emergency cooling system, which floods the reactor core with water, came on and brought the water pressure back up to its normal 2,000 pounds per square inch.

Denton said it would be at least two days before technicians could enter the containment building to inspect the ruptured pipe and another few days to remove the ruptured section of pipe for analysis. He said it might be at least two weeks before the broken pipe can be replaced and preparations made to start up the plant once again.

At the plant site on the frozen shores of Lake Ontario, steam could be seen pouring out of vents at the top of the containment building as technicians continued to bring the plant to a cold shutdown. Except for a few passing motorists who stopped to ask why the steam was being pumped out of the reactor, there was little excitement among the residents of the neat stone farmhouses that surround the plant's property.

"It's been steaming like this since the first thing this morning," said Jinny Loomis, the wife of a local doctor who has lived across the street from the plant since it opened in 1970. "If there were any real emergency here, I'm sure they would have told us we had to evacuate."

Denton said there are 3,000 pipes, each about seven-eighths of an inch in diameter, in each of the two cooling loops at the plant. In the last 12 years, he said, Rochester Gas has plugged more than 100 pipes in each loop that had been found to be cracked or thinning from wear.

"My experience has been that when one of these pipe accidents happen, they are random and unpredictable," Denton said. "All three that failed before this one in the last few years seemed to be adequate at the time of failure."

Meanwhile, the Vermont Yankee nuclear plant in Montpelier, Vt., was shut down yesterday to repair a radioactive steam leak in a moisture separation system that lowers the water content of the steam generated by the plant before it reaches the plant's turbines. Plant officials noticed a "wisp of steam" escaping from a pipe a few days ago and decided to shut the plant down before the leak got worse.

And in Middletown, Pa., officials said that leaking steam generator tubes probably will delay the restart of TMI's Unit 1 reactor for at least six months.

TMI officials had expected the undamaged unit to be ready for restart by the end of February, subject to NRC permission.

The Unit 1 reactor has been shut down since March 1979, when its sister reactor was involved in the nation's worst commercial nuclear power accident.