The five nuclear plants ordered closed last week by the Nuclear Regulatory Commission because of computer design errors will stay closed for at least another two weeks while the NRC unravels the errors.

"I'm not going to guarantee any time limits on the closings," NRC Chairman Joseph M. Hendrie yesterday told the House Interior subcommittee on energy and the environment. "The only thing I will guarantee is that the NRC staff will work as hard as it can to make a judgment on the impact of the computer errors" that forced the closings.

The NRC closed the five plants -- two in Virginia and one each in Pennsylvania, Maine and New York -- when it discovered that computer codes that designed earthquake resistance into the piping were in error.

Hendrie said engineers at Stone & Webster, where the computer codes were designed, are working two 10-hour shifts a day, seven days a week, to reevaluate the impact of the code errors. He said it will take at least another two weeks to decide if the codes are in such error that supports must be built into the pipes of the five closed plants so they can withstand earthquakes.

"If there are only modest changes to be made here and there, then the commission might allow the plants to go back into operation immediately," Hendrie said. "However, if significant changes have to be made requiring alterations in the licenses granted to the plants then it might be a matter of months."

If it is a matter of months, said executives of the four electric ompanies whose plants were closed, their customers will have to pay for the extra oil and electricity the companies will have to buy to replace the lost nuclear power.

"We will need to buy over 1 million barrels of oil a month, which comes to more than $15 million," said John B. Randazza, assistant vice president of Central Maine Power Co., whose Maine Yankee atomic plant was closed by the NRC order. "That will mean an extra $12 million in power costs to our consumers, half of that for consumers in Maine who can't afford it."

Of the timing, Randazza said that improving spring weather means electrical demand will be down, but that power companies like his normally schedule plants for maintenance shutdowns now to make sure they're on line in the peak summer months.

"If the question is not resolved soon," Randazza testified, "then we'll have to shut the plant down again for its normal nuclear fuel reload right at the time our winter peak hits again. I think the commission should have considered these alternatives in making their decision."

Virginia Electric Power Co. estimated its oil replacement costs at $11 million to $12 million a month, as did the Power Authority of the State of New York. Duquesne Light Co. described itself as "pinched" by the closing of its power plant.

"We'll have to use oil," said Duquesne president S. G. Shaffer. "We'll have to buy electricity to meet the needs of Pittsburgh."

Much of yesterday's hearing focused on the possibilities of an earthquake striking the East Coast, where all the closed plants are located. Dr. Robert Wesson of the U.S. Geological Survey's Office of Earthquake Studies said that while damaging earthquakes are unlikely in the Northeast they are not unheard of.

"The four largest quakes in the East are all ancient, the last big one having struck the St. Lawrence River Valley in 1925," Wesson said. "But there was a quake in Attica, N.Y., in 1929 whose force exceeded the earthquake design at all five plants" closed by NRC.