Photo shows the main generators at the Diablo Canyon Nuclear Power Plant. A hiccup in the hydrogen cooling system of one of these massive generators forced the Unit 2 reactor to be shut down, even as Unit 1 is down for refueling.

The Diablo Canyon Nuclear Power Plant is completely shutdown with one unit undergoing a scheduled refueling but the other shut down unexpectedly.

Unit 1 was shutdown successfully on Oct. 3 for what plant owner Pacific Gas & Electric said was “a scheduled and planned refueling and maintenance outage,” according to a news release from PG&E spokeswoman, Suzanne Hosn.

Unit 2, which ordinarily would be kept online while the other unit is refueling, was shutdown Thursday, Oct. 15 “in order to perform maintenance on the main generator,” Hosn said. “There is no impact to the health and safety of the public or plant personnel.”
Hosn characterized the Unit 2 shutdown as “unscheduled maintenance.” Hosn said early in the morning of Oct. 15, “operators received a monitoring indication that the rate of hydrogen consumption was increasing on Unit 2’s main electrical generator cooling system, indicating a flow of hydrogen out of the system.”

The unit was shutdown safely and work on the generator was begun. “The components under review,” Hosn said, “are part of a cooling system for that electrical generator, on the non-nuclear side of the plant. These maintenance activities cannot be performed while the unit is operating.”

She said the main generator didn’t necessarily “fail.” “The components under review are part of the cooling system for the Unit 2 main generator. Many power plants (nuclear, fossil, etc.) use hydrogen as an efficient and effective cooling method to transfer heat away from power generating turbines. At Diablo Canyon, hydrogen cooling systems are used to cool the electrical generators, which are located on the non-nuclear side of the plant.”

How serious was the damage? “Though our inspections are ongoing,” she said, “the equipment is intact and in place.”
What will the unscheduled maintenance entail? “We are currently conducting inspections on the component and will safely and methodically work through necessary maintenance or repair activities before returning the unit to service,” she said, as apparently they need to do a deep dive into what went wrong.

The Nuclear Regulatory Commission was notified, as is the case with most all unexpected events at the plant. “We have been in communication with our Nuclear Regulatory Commission Resident Inspectors regarding this unplanned outage on Unit 2, and our Senior Resident Inspector has been on-site,” she said. “This maintenance issue in no way impacts or impedes our commitment or ability to protect the health and safety of the public and plant personnel. “

With both Diablo Canyon units down, it removes some 2,300 megawatts of electricity — about 10% of the State’s total — from the power grid, as temperatures continue in the high 90s across much of California.