Workers have shut down a nuclear plant in southern New Jersey after a leak was detected in the containment building.

The Nuclear Regulatory Commission says the reactor was shut down at the Salem Unit 1 power plant in Lower Alloways Creek Township after slightly radioactive water was discovered leaking at a rate of 4 gallons a minute Thursday night.

The NRC says the spill was confined to the plant's containment building and there was no threat to the public or workers.

The initial investigation has determined the leak is coming from packing on a pressurizer spray valve used in the reactor coolant system.

The valve was last replaced during a refueling outage in that began in April.

Officials say workers are waiting to isolate the valve to determine repairs.