Jonathan Ernst / Reuters File Three Mile Island's steam towers are visible for miles around the nuclear plant, which is located 12 miles south of Harrisburg, Pa.

A reactor at Three Mile Island, the site of the nation’s worst nuclear accident, shut down unexpectedly on Thursday afternoon when a coolant pump tripped and steam was released, the Nuclear Regulatory Commission told NBC News.

The system tripped when "the pump stopped operating and created a power/flow imbalance," said NRC spokesman Neil Sheehan.

The plant responded as designed and is stable with no impact on public health or safety, added NRC spokeswoman Diane Screnci.

If any radiation was in the released steam, Screnci said, it would be below detectable levels.

Exelon, the plant operator, said in a statement that "during the shutdown, steam was released into the atmosphere, creating a loud noise heard by nearby residents."

A NRC inspector based at the plant "responded to the control room immediately after the reactor trip to independently assess control room operators' response to the event and ensure safety systems were functioning as designed," Sheehan said. "He did not identify any immediate concerns with operator or equipment performance."

Plant operators were not yet sure what caused the problem.

"Once the reactor is sufficiently cooled down, plant personnel will be able to access the containment building and troubleshoot the problem," Sheehan added.

March 28, 1979: NBC's David Brinkley, Gerald Harrington, Steve Delaney and Carole Simpson report on the Three Mile Island accident.

Located about 12 miles south of Harrisburg, Pa., Three Mile Island in 1979 saw a partial meltdown of one of its nuclear reactor cores. Small amounts of radiation were released into the environment when the reactor core lost cooling water, exposing the highly radioactive fuel rods.

A presidential commission later said the accident was "the result of a series of human, institutional and mechanical failures."

Several thousand people claimed they had suffered ill health effects from radiation, but their lawsuit was rejected by a federal court in 1996 with the judge concluding they had not proved their case.

Various assessments by the government and nuclear industry have concluded no radiation-related deaths or illnesses resulted from the accident.

NBC's Anne Thompson and Tom Costello, as well as The Associated Press, contributed to this report.

More content from NBCNews.com:

Follow US News from NBCNews.com on Twitter and Facebook



