Workers have removed the four old generators and are positioning the new ones to be welded into place. Work began in early February and will finish in early April.

“This project has posed some major challenges for us,” said plant manager Jim Becker. “It’s a construction project and is unique in the history of the plant.”

In a normal refueling shutdown, 1,000 temporary workers are brought in to replace a third of the fuel in the reactor and do maintenance work. Now, an additional 1,400 workers were brought in to replace the steam generators, for a total of 2,400 extra employees.

Some of these workers are employed by SGT West, a French-American company that specializes in jobs such as this and has replaced more than 100 steam generators at more than 30 other nuclear plants in the country, said Pete Resler, a spokesman with Diablo owner Pacific Gas and Electric Co.

The two-month outage is the first of a two-part, $700 million effort to replace all of the plant’s eight steam generators. Four generators for the plant’s other nuclear reactor will be installed early next year.

The steam generators measure 70 feet long and contain thousands of water tubes that transfer heat from the reactor to the electrical generators. After more than two decades of use, the walls of the old tubes have thinned and some have had to be plugged.

The old generators wouldn’t have lasted to the end of the plant’s operating licenses in 2024 and 2025, so PG&E decided to replace the generators rather than try to repair them or shut the plant down early, Becker said.

The new steam generators are expected to last 50 years. This could allow PG&E to apply to renew the plant’s operating licenses by an additional 20 years, a step that many other nuclear plants around the country are taking.

PG&E is studying whether it will apply for license renewal. Becker said replacing the generators is a cost-effective move no matter what the utility eventually decides.