Three Mile Island nuclear reactor shuts down unexpectedly

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.

For Subscribers

FOR SUBSCRIBERSCBP leaders cut out of Trump ad buy decision

FOR SUBSCRIBERSAmerican passenger drafted goodbye texts to her family as Flydubai plane plummeted

FOR SUBSCRIBERSCornell rape allegation pushes embattled New York prosecutor into national spotlight

FOR SUBSCRIBERSAbdul El-Sayed tries to woo MAHA voters as Democrats seek inroads with RFK’s supporters

FOR SUBSCRIBERSKratom-like substances are popping up in a typically benign supplement

FOR SUBSCRIBERSGOP super PAC increases its spending for Sen. Jon Husted in tough Ohio race

FOR SUBSCRIBERSRecords reveal tumultuous marriage between slain New York Times executive and wife

FOR SUBSCRIBERSFrom clean energy warrior to ‘wall of shame,’ James Talarico reverses on a fossil fuel money pledge

FOR SUBSCRIBERSCongressional staff planned official trip with luxury stops in Italy and France weeks before election

FOR SUBSCRIBERSA harrowing ordeal aboard an Israel-bound plane

FOR SUBSCRIBERSOpenAI CEO Sam Altman to skip congressional hearing on rogue AI agents

FOR SUBSCRIBERSKey DOJ lawyers departed Trump ‘grand conspiracy’ probe amid pressure to produce, source says

FOR SUBSCRIBERSTrump’s new AI chatbot often links to a site launched under Clinton 26 years ago

FOR SUBSCRIBERSAt MAHA summit, industry CEOs the movement once vilified take the stage

FOR SUBSCRIBERSThis Paralympian has two goals: Win medals and give away $500,000 to her teammates

For Subscribers

FOR SUBSCRIBERSCBP leaders cut out of Trump ad buy decision

FOR SUBSCRIBERSAmerican passenger drafted goodbye texts to her family as Flydubai plane plummeted

FOR SUBSCRIBERSCornell rape allegation pushes embattled New York prosecutor into national spotlight

FOR SUBSCRIBERSAbdul El-Sayed tries to woo MAHA voters as Democrats seek inroads with RFK’s supporters

FOR SUBSCRIBERSKratom-like substances are popping up in a typically benign supplement

FOR SUBSCRIBERSGOP super PAC increases its spending for Sen. Jon Husted in tough Ohio race

FOR SUBSCRIBERSRecords reveal tumultuous marriage between slain New York Times executive and wife

FOR SUBSCRIBERSFrom clean energy warrior to ‘wall of shame,’ James Talarico reverses on a fossil fuel money pledge

FOR SUBSCRIBERSCongressional staff planned official trip with luxury stops in Italy and France weeks before election

FOR SUBSCRIBERSA harrowing ordeal aboard an Israel-bound plane

FOR SUBSCRIBERSOpenAI CEO Sam Altman to skip congressional hearing on rogue AI agents

FOR SUBSCRIBERSKey DOJ lawyers departed Trump ‘grand conspiracy’ probe amid pressure to produce, source says

FOR SUBSCRIBERSTrump’s new AI chatbot often links to a site launched under Clinton 26 years ago

FOR SUBSCRIBERSAt MAHA summit, industry CEOs the movement once vilified take the stage

FOR SUBSCRIBERSThis Paralympian has two goals: Win medals and give away $500,000 to her teammates

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.

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:

• Did Supreme Court justice tip hand on gay marriage?
• Mom who 'smooshed' son's alleged bully: No regrets
• Video: Thrill ride passengers get stuck 300 feet in the air
• Prosecutors give up fight for James Holmes' notebook
• Mom: Substitute teacher duct-taped kid's mouth shut

Follow US News from NBCNews.com on Twitter and Facebook