OCEANSIDE -- A small fishing boat, headed out of Oceanside Harbor at about 7:30 a.m. Saturday encountered a sight odd enough for its skipper to call out: "You don't see something like that every day."

By "that" he meant two ocean tugs muscling a heavily laden barge into the harbor channel. Sitting atop the barge, like a pair of gigantic, sideways milk bottles, were two 640-ton steam generators, vital components in what will be major surgery for the San Onofre Nuclear Generating Station located 15 miles up the coast.

Southern California Edison, San Onofre's operators and majority owner, commissioned the 65-foot-long, 22-foot-wide structures -- and two more just like them that have not yet arrived -- in 2005 at a cost of $680 million.

Ross Ridenoure, Edison's chief nuclear officer, said Saturday that delivery of the giant components was a milestone for San Onofre.

"This is the biggest project we've undertaken since construction took place in the 1970s and early 1980s," Ridenoure said, adding that replacing the components will extend the plant's life by decades.

The generators were built by Mitsubishi Heavy Industries in Kobe, Japan, and were carried across the Pacific Ocean by the heavy-lift ship Happy Ranger, arriving at a dock in Long Beach Harbor in late December. From there they were loaded onto a barge for the trip south to the Del Mar Boat Basin at Camp Pendleton, arriving during a 7-foot high tide on Saturday morning.

Ridenoure said the generators will be loaded onto a special tracked "crawler" that will transport them about 15 miles north to San Onofre. Citing security concerns, he declined to say exactly when or by what route the trip will occur. However, a statement released by the company in 2006 indicates that the crawler will travel eight miles up the beach before moving onto a frontage road that runs past the seaside plant.

The generators contain thousands of thin metal tubes and function like a car's radiator. According to previous Edison statements, the plant's existing steam generators must be replaced because their internal tubes are cracking. Tubes inside the new generators are made of a tougher alloy that is more resistant to the extreme heat generated by San Onofre's sustained nuclear reaction.

Getting the new parts into San Onofre's iconic concrete reactor domes will be no easy matter.

Ridenoure said it will be necessary to cut a 28-foot hole in the side of each dome because existing access hatches are too small.

While similar operations have occurred at other nuclear reactors throughout the nation, the operation will be a little different at San Onofre.

Each dome contains a latticework of tensioned steel cables called tendons, which give the domes extra rigidity. San Onofre was built with extra tendons in order to make it more resistant to earthquakes. Ridenoure said it will be necessary to "de-tension" and remove about 100 of the tendons in order to cut an access hole.

According to a draft environmental impact report filed with the California Public Utilities Commission in 2005, the operation will be somewhat unique.

"De-tensioning tendons of the type at (San Onofre) has never been attempted at another operating nuclear plant," the report states."Most of the tendons are not designed to be de-tensioned or removed."

Since the report was written, generators have been replaced at the Diablo Canyon plant in Northern California. Unlike San Onofre, Diablo Canyon's reactors have four steam generators each. Ridenoure explained that San Onofre's access hole will need to be larger, because it has fewer generators, but they're bigger.

Installation will take between 60 and 70 days, and will not occur until a scheduled refueling and maintenance outage planned for September.

Some anti-nuclear activist groups have worried over the last few years that something could go wrong during the cutting and de-tensioning operation, though Edison has shown confidence that it call pull the job off with no ill effects.

Company engineers traveled to the Rancho Seco Nuclear Generating Station near Sacramento in 2005 to practice its plans on a similar reactor dome that was shut down in 1989. Golden said in 2005 that the plant has "identical" tendons.

"This operation has been performed at, I believe, 28 other plants across the country," Ridenoure said. "We are confident that it will go smoothly."

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