Just days after the Virginia Electric & Power Co. was forced to shut down two units of a Surry nuclear generating plant becuase of design errors, the company announced yesterday it has discovered cracks in the equipment in one unit of its nearly completed North Anna nuclear plant.

The cracked equipment, a steel plate in the cooling system of the North Anna 2 units in Louisa County, was designed by the same Boston engineering firm -- Stone and Webester -- blamed for design flaws that caused the Surry shutdown, a Vepco spokesman said.

Although both he Surry and North Anna plants were designed by the same firm, the two situations are unrelated, according to spokesmen for Vepco, Stone and Webster and the Nuclear Regulatory Commission (NRC).

The significance and effect of the cracks are unknown. Also unknown were the reasons for the crack and the time it will take to repair them, the Vepco spokesman said. Investigators from Nepco, the NRC and the Westinghouse Electric Corp. which supplied the equipment, are trying to determine whether the cracks will postpone the expected opening of the plant in July.

Vepco said it has "no reason to believe at this time" that similar cracks exist in the North Anna 1 unit, which began service last June. But the spokesman said "inspection may become necessary for that unit in the future."

The Vepco spokesman said he didn't know what effect the cracks would have had if the number 2 plant had been in operation.

The cracks were found in one of three steel plates designed to direct the flow of reactor coolant in the unit. The plat does not "affect the integrity of the main cooling pipe," Vepco said.

The cracks were discovered Wednesday during a routine inspection and were reported immediately to the NRC, Vepco said.

On March 13, five nuclear plants -- including the two Surry units -- Were ordered closed last week by the NRC when it found that the computer codes that designed earthquake resistance into the piping were in error. The NRC expressed fears that the flaws might cause the pipes that carry coolant to the nuclear units to rupture during an earthquake.

One of the Surry units, however, already had been shut down before the NRC order because one of its steam generators needed to be replaced.

Vepco estimated last week that it will spend $10 million more a month to generate power becuase of the Surry shutdown and that a power shortage this summer also may result.

Vepco said last week that its 1.3 million customers would have to pay 8 percent more in their bills to cover the costs of buying oil for conventional plants to generate power during the Surry shutdown. That would amount to an increase of about $2.75 a month on the average residential bill of $41.

Vepco officials yesterday said they could not determine whether the cracks at the North Anna plant would impose any added costs on consummers. Tuesday Vepco was awarded an $148.1 million rate increase that is expected to raise customers' bills by 11 percent.

NRC spokesman Frank Ingram said that the design codes used in the North Anna plant were different than the flawed design codes used in the Surry operations.

"I couldn't speculate if" the cracks at North Anna "are connected to the problems at Surry," Ingram said. "It seems to be another matter."

Ingram said the cracked plate had never been used and that he had "no assessment of the significance of it."