SAN ONOFRE, Calif. -- A massive construction project to replace integral parts of Southern California's only nuclear generators will begin this month, Southern California Edison announced Monday.

The main phase of a $670 million project will create more than 1,000 temporary jobs and generate roughly $300 million in spending benefiting Southern California businesses, the utility announced.

The two nuclear generators perched on the side of the Pacific Ocean, at the San Diego-Orange county line, will each get two new steam generators inside the containment domes in the project. Each new generator is 65 feet tall, 22 feet in diameter and weighs 640 tons.

The generators absorb heat from the nuclear piles, and transfer it to water, flashing it into steam. That steam is then routed through special pipes to turn massive turbines and generate about 20 percent of the region's electricity on a normal day.

The four original steam generators are approaching the end of their service life, Edison said. The California Public Utilities Commission in 2005 approved a comprehensive cost-benefit analysis that concluded replacing steam generators would save utility customers as much as $1 billion during the next two decades, when compared to the likely cost of obtaining from other sources the 2,200 megawatts of power the plant generates.

Edison has had to take special steps to plan to slice into the containment domes, which are built with concrete that encases cables that were stretched tight to keep the structure intact. Cutting those cables, without jeopardizing the domes, required engineers to practice on a similar dome at a decommissioned power plant near Sacramento.

The largest pieces of the new generators arrived via barge at Oceanside last winter, and were creeped up the beaches of Camp Pendleton over a ponderous, months-long effort.

SCE plans to replace the two steam generators in the northern unit this fall and those in the southern unit during the fall of 2010. Each generator will be off line about 100 days for the changeovers.

The utility said San Onofre is one of the last plants in the U.S. nuclear power fleet to replace its steam generators. The process has been successfully carried out by 49 of the 58 U.S. plants of similar design without a serious safety incident, Edison said.

The construction project will be reviewed and monitored by the Nuclear Regulatory Commission, Edison said.