RICHLAND, Wash. (AP) — Energy Northwest has unexpectedly shut down its nuclear power plant near Richland after operators received an indication that a system used to cool equipment wasn't working.

The Tri-City Herald reports the Columbia Generating Station was shut down Monday after operators were alerted to problems with the system that uses water to provide cooling to heat exchangers and pumps, including those that control the power level of the reactor.

Energy Northwest spokesman Mike Paoli says it seems that a water system valve may not have been in the right position, but that an investigation is ongoing.

He says the plant will be restarted once all systems can operate to standards.

The last time the plant had an unplanned shutdown was in November 2009, when there was a hydraulic fluid leak.