Diablo Canyon Unit 2 shut down for refueling. Get an inside look at the nuclear power plant

Diablo Canyon nuclear power plant is in the midst of a routine refueling and maintenance outage — and that means the facility is buzzing with more activity than usual.

On April 7, PG&E shut down Unit 2 to refuel the reactor, a process that lasts about a month, according to PG&E spokesperson Suzanne Hosn. The process also includes maintenance work on the massive turbine that generates power for millions of California homes.

Meanwhile, Unit 1 continues to generate electricity at the plant, Hosn said.

PG&E refuels each of the twin reactors once every 18 to 20 months, but this outage is unique, as PG&E is also performing extra inspections on the reactor as part of its license renewal process.

“This outage is three outages worth of work,” director of outage planning and execution Erik Werner said.

The U.S. Nuclear Regulatory Commission is reviewing PG&E’s application to renew the plant’s operating license for another 20 years, a process that could take about two years. The California Public Utilities Commission allowed PG&E to keep operating the plant five years past its originally scheduled closure dates because not enough clean energy had been procured in the state to replace it.

The power plant generates 9% of California’s electricity and 17% of the state’s carbon-free electricity.

PG&E temporarily hired about 1,000 extra workers to service the plant during the outage, bringing Diablo’s staff size to 2,300 employees, Hosn said.

PG&E hired local staff where possible, “however, some short-term projects require special training, qualifications and skill sets – in those instances, traveling teams are brought in from out of the area,” Hosn said in a statement.

What happens during a refueling outage?

The plant is powered by its two nuclear reactors, but it generates electricity through much more mundane means: by heating water to produce steam, which spins the turbines that drive the rotating shaft that generates electricity that can be sent to the grid.

During a refueling outage, all 193 fuel assemblies in the Unit 2 reactor are “removed from the reactor vessel and transported to a spent fuel pool,” Hosn said. “These pools are 40 feet deep and are concrete and steel-lined structures built into bedrock. Multiple safety systems ensure cooling is continuously provided during operations.”

While the reactor is empty, PG&E performs routine maintenance on Unit 2 — from the reactor to the turbine building.

That includes inspecting a wide variety of equipment and replacing parts as needed, according to Hosn.

This refueling outage came with extra work, however, because PG&E must perform about 200 license renewal inspections of the plant during the refueling outage, Werner said.

“Inspections include ultrasonic and visual inspections of tank interiors, welds and components. We also perform cable insulation testing and visual inspections of normally inaccessible concrete,” Hosn said.

Once the maintenance is complete, two-thirds of the old fuel assemblies re-installed in the reactor along with one-third new fuel.

“We’re refueling the gas tank,” Werner said.

The next refueling outage is scheduled for Unit 1 in spring 2025, Hosn said.

This is Unit 2’s 24th refueling outage, the same number that Unit 1 has undergone, too, Hosn said.