The Palo Verde Nuclear Generating Station has cut power output at one of its reactors after a valve failed.

The Nuclear Regulatory Commission said a crew doing routine maintenance accidentally over-pressurized part of the steam valve’s controls, leaving one of the valves inoperative. Workers found the problem Wednesday afternoon, and the Unit 1 reactor was cut to 60 percent power.

An Arizona Public Service spokesman said it could take several days for the problem to be fixed. Engineers are already working on a tiny leak in the Unit 3 reactor that was found after the vessel was shut down for refueling in early October. Unit 3 is not expected to come back online until next month. Palo Verde is owned by APS and Salt River Project and provides power to more than 4 million customers around the Southwest.