One of the three reactors at the nation's largest nuclear power plant has been shut down because of vibration in the unit's main emergency cooling line.

Arizona Public Service Co. had been operating the reactor at the Palo Verde Nuclear Generating Station at about one-third of capacity because of vibrations, but operators decided to shut it down Tuesday after the vibration rose above acceptable levels.

The unit could be restarted within two days, APS spokesman Jim McDonald said. Potential fixes could include adding shock absorbers, installing weights or heating the emergency cooling line.

The utility said a contributing factor to the increased vibration could be the unit's newly installed 800-ton steam generators and low-pressure turbines.

Palo Verde, about 50 miles west of downtown Phoenix, has been among the nation's top-performing plants since it opened in 1986 with its three reactors producing nearly 4,000 megawatts of electricity; however, problems over the past two years have caused more than a dozen shutdowns.

APS owns 29.5 percent of the plant and operates it for a consortium of utilities. It supplies electricity to about 4 million customers in Arizona, New Mexico, Texas and California.