Power output at one of Palo Verde Nuclear Generating Station's three reactors has been cut because of a problem with a main steam valve, plant operator Arizona Public Service said Thursday.
Operators at the plant 50 miles west of Phoenix discovered the problem Wednesday afternoon and the Unit 1 reactor was cut to 60 percent power, APS spokesman Jim McDonald said.
The problem occurred in a valve that is normally open on one of the reactor's two steam generators, Nuclear Regulatory Commission spokesman Victor Dricks said. The generators use water heated in the reactor to produce steam, which turns turbines connected to electrical generators. Each steam generator has two main steam valves, which must be able to close in less than 5 seconds in certain situations.
Dricks said a crew doing routine maintenance inadvertently over-pressurized a part of the valve control mechanism, rendering one valve inoperative.
NRC operating rules require a reactor shutdown if a main steam valve doesn't work, and APS was powering down the reactor when crews managed to close the valve. They kept the reactor at 60 percent power once the valve was closed and are allowed to run the plant at that setting as long as needed.
McDonald said it may take several days for the problem to be repaired.
Engineers are already working to fix a tiny leak in the reactor vessel at the plant's Unit 3 that was discovered after the reactor was shut down for refueling Oct. 5. That unit isn't expected to be back online until next month.
APS and another Arizona utility, Salt River Project, own the plant along with Public Service Co. of New Mexico, El Paso (Texas) Electric Co., Southern California Public Power Authority, Southern California Edison Co. and the Los Angeles Department of Water and Power. It supplies electricity to about 4 million customers in the four states.