OCEANSIDE -- As engineers worked Monday to unload massive nuclear components from a barge floating in Camp Pendleton's boat basin, critics said they remain concerned about plans to cut a 28-foot hole in the side of the two containment domes at the San Onofre Nuclear Generating Station.

"I try not to use the word 'scare' very often, but these two 28-foot-by-28-foot holes absolutely scare me," said Rochelle Becker, a member of the California-based Alliance for Nuclear Responsibility.

Southern California Edison, the station's majority owner and operator, brought two 640-ton steam generators into the Del Mar Boat Basin at Pendleton on Saturday morning. The company will soon move the units up the beach to the seaside nuclear power plant, where they will be installed in late September.

The 65-foot structures are too large to fit through access hatches built into San Onofre's twin concrete containment domes, so Edison must puncture each dome, temporarily removing 83 steel tendons that give the structures extra rigidity. Because of its location in earthquake-active Southern California, San Onofre's domes contain more bracing that those in other parts of the country.

After installing the new generators, Edison will repair the holes and install new tendons to replace the ones that were removed.

David Weisman, another Alliance for Nuclear Responsibility member, said in an e-mail that he is not convinced that punctured and patched domes will provide the same level of protection from aerial assault as they did before the operation.

"After cutting and patching these holes, how will resilience to airborne assault be any more robust?" Weisman said.

After the terrorist attacks of Sept. 11, 2001, many in the anti-nuclear community questioned whether San Onofre and other nuclear plants could withstand a direct hit from a large commercial aircraft.

Though Edison has repeatedly insisted that its engineers believe the domes and other structures at the plant are impregnable, critics such as Weisman point to a memo from the Nuclear Regulatory Commission, released in 2001, that acknowledges "the NRC did not contemplate attacks by aircraft" when the plants were designed.

Gil Alexander, a spokesman for Edison, said in an e-mail Monday that engineers have studied what effect cutting and patching the dome will have on its structural integrity.

"The tests and engineering studies included examining detensioning and removing the metal 'tendons' that pass through the whole area, adding to dome strength," Alexander said. "The analysis demonstrated that the process (Edison) plans to use will result in equal or greater dome strength."

Company officials will not say when the new generators will make their way up the beach from the boat basin to the plant, which is about 15 miles north of Oceanside. A large lifting machine with 16 axles and 256 wheels is being used to move the heavy units from barge to land.

Contact staff writer Paul Sisson at (760) 901-4087 or psisson@nctimes.com.