Federal regulators began examining a nuclear plant in northeastern Pennsylvania on Monday following three unexpected shutdowns, but the facility's operator said it is safe and stable.

A three-member team from the Nuclear Regulatory Commission arrived at the Susquehanna power plant near Berwick to look into operator performance, equipment reliability and decision-making, the agency said.

The move comes after three unplanned reactor shutdowns between Nov. 9 and Dec. 19. None of the incidents posed a safety threat, said Joe Scopelliti, a spokesman for Allentown-based PPL Corp., which operates Susquehanna.

"We did a thorough investigation of the causes of the shutdowns," said Scopelliti, noting the company is cooperating with the inspection.

The most recent shutdown occurred after a valve failed to open during plant startup, temporarily interrupting the water supply to one of the facility's two reactors.

The Nov. 9 shutdown was attributed to problems with a computer system that monitors water levels. A Dec. 16 malfunction happened during routine valve testing on Unit 2's main turbine system.

Neil Sheehan, a spokesman for the nuclear agency, said it's not clear how long the inspection will take. A report containing findings and recommendations is due 45 days after the inspection is completed.

Recommendations could include increased oversight, Sheehan said. He noted Susquehanna's Unit 1 reactor is already under a higher level of oversight because of a flooding event in 2010.