Southern California Edison took one of the two large generating units offline Monday night at its San Onofre Nuclear Generating Station.
Edison (SCE) said the work is part of scheduled refueling, maintenance and upgrades for Unit 2.
Edison's Gil Alexander said the company is making other improvements while the unit is removed from service.
"We're going to make some additional technology upgrades that will make the plant more efficient, more cost-effective for customers and safer for our workers," said Alexander.
Alexander said the major upgrades include replacing the plant's two reactor heads.
"The design and construction of the new reactor heads, including the use of 'single forge' manufacturing which features fewer welds, will be even safer, while reducing customer costs by shortening maintenance outages," said Alexander. "The brand new reactor heads will provide us with more radiological protections for outage workers."
By outage he means when the generating units are removed from service for scheduled maintenance.
Alexander said retrofitting the high-pressure turbines with new components will increase efficiency and generating capacity.
"New high pressure turbines are going to allow us to produce a bit more power - 48 megawatts," said Alexander. "That's enough electricity in addition to serve all the needs of 31,000 homes."
He said the combined output of the two generating units, 2,200 megawatts, is enough electricity to power 1.4 million average Southern California homes.
Alexander said Unit 3 will also be removed from service for similar upgrades.
He said each maintenance outage is expected to take about two months.
Alexander said the work on both units will create about 1,700 temporary jobs at the nuclear plant.