One of two units at Vepco's Surry nuclear power plant near Newport News was shut down yesterday after a steam generator tube began leaking more than 19 gallons of slightly radio-active water an hour.

The unit will be out of operation for about three weeks while workers drain a portion of the reactor coolant system and test the steam generators to find out which tubes are leaking and repair them, according to Nuclear Regulatory Commission spokesman Kenneth Clark.

Clark said Vepso has experienced "a rather large number of leaks" in the Surry steam generator tubes, which have led the NRC to order the big electric company to shut down the plant on Jan. 2 for a special safety inspection.

Decause the plant had to be shut down anyway due to the leak, the inspection has been moved forward and is now taking place, a Vepco spokesman said.

"It was not an emergency-type situation," said the spokesman. He said the leaked water was contained within plant safety systems and there was "no adverse effect on the health or safety of the public."

The steam generator tubes are a heat-exchange system that work this way: superheated water from the reactor passes through them. The heat in the water is transferred from the tubes to water in a vessel surrounding them. This water in turn "flashes" into steam that drives generators.

Vepco has had so much trouble with these tubes that it is planning to replace them next year for about $77 million. Other plants across the country that have used similar tubes have had similar problems.

Virginia regulatory officials are not holding Vepco responsible for what appear to be manufacturing defects in the tubes. But the staff of the State Corporation Commission, which regulates utilities, has recommended that Vepco refund $5 million to its 1.2 million Virginia customers because of a previous leak in the Surry tubes. It went undetected by a Vepco inspection and caused an earlier shutdown.

The shutdowns are expensive because the company must produce expensive electricity from oil and coal fired plants in place of cheap nuclear-produced electricity. Surry provides more than 20 percent of Vepco's output.