Tennessee Valley Authority's Browns Ferry Nuclear Plant is near Athens, Ala.

The newest reactor at TVA's oldest nuclear plant automatically shut down Tuesday after power supply was temporarily interrupted to two recirculation pumps that supply water to the reactor core, TVA officials said today.

The trip of the two pumps at the Unit 3 reactor at the Browns Ferry Nuclear Plant caused the unit to cease power generation around 9:30 a.m. EDT on Tuesday. TVA spokesman Jim Hopson said today that feedwater pumps maintained water in the reactor core, but the unit automatically shut down as designed when water levels covering the fuel rods dropped by about 40 inches.

"We still had 13 feet over the reactor rods and the water was quickly restored to the previous level (of about 17 feet above the fuel rods)," Hopson said. "We continue to investigate the causes of this incident, but the reactor shut down automatically and there was no safety risk from this event."

TVA is keeping the reactor on standby until the investigation is completed, Hopson said.

Dave Lochbaum, senior scientist for the Union of Concerned Scientist and a former Browns Ferry and NRC employee, said previous TVA studies of the pumps that circulate water through the reactor core have indicated that an incident like what happened Tuesday should not have occurred.

"Apparently, Browns Ferry operated outside the bounds of its safety study," Lochbaum said. "Will TVA fix this dilemma before restarting Unit 3? And will TVA do anything to check whether Units 1 and 2 are equally impaired?"

Hopson said the problems on Unit 3 were corrected and are not believed to be similar to those of the other two Browns Ferry units.

"But we will continue to look at all of our equipment and procedures at all three units," Hopson said.

Lochbaum said the reactor scram was not of any safety concern but did interrupt some of the power generation at Browns Ferry and needs to be assessed to ensure such power interruptions don't continue to happen.

Browns Ferry has had only a limited number of reactor trips in recent years - far fewer than at the single unit Pilgrim Nuclear Plant in Massachussets, a similar boiling water reactor plant like Browns Ferry. The Pilgrim plant had six unplanned reactor shutdowns in 2013.