A malfunctioning pump at the site of the worst nuclear power plant accident in US history has triggered an automatic shutdown.
The shutdown at Pennsylvania's Three Mile Island was the second in as many months. Government regulators and plant officials say it poses no threat to public health or safety.
Operator Exelon Generation said no detectable levels of radiation escaped.
The failure of a coolant pump tripped a computerised system that shuts downs the reactor.
The Nuclear Regulatory Commission said once the reactor has cooled, plant workers can access the containment building and troubleshoot.
The plant also automatically shut down on August 22 while operators were manually shutting it down for repairs.
The plant is named after the island where it is located in the Susquehanna River. It had a partial meltdown of a reactor in 1979.