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A reactor at the Three Mile Island nuclear power plant shut down unexpectedly this afternoon because of a cooling problem, a month after it went offline because of a leak in the cooling system, the Nuclear Regulatory Commission says.

Update at 4:54 p.m. ET: The NRC says a cooling pump for Unit One stopped working at 2:16 p.m. ET, triggering the automatic shutdown, as the system is designed to do. An agency inspector was at the plant at the time.

The same unit shut down automatically Aug. 22 as it was being taken offline to fix a coolant leak.

Here's some of what the NRC's Preliminary Notification update explained when Unit One went back online Sept. 5:

The plant reached cold shutdown on August 23, 2012, and the source of RCS leakage was confirmed to be from micro-cracks in the alloy 600 diaphragm for the upper pressurizer heater bundle. This heater bundle was subsequently replaced with a bundle that contained a stainless steel, non-alloy 600 heater diaphragm. An NRC specialist inspector, who was deployed to the site, confirmed the source of the RCS leakage and monitored pressurizer heater replacement activities. The licensee subsequently conducted extent-of-condition inspections on the other two pressurizer heater bundles and no indications of leakage were identified. The inspectors determined that the licensee's post-installation and extent-of-condition inspections were acceptable.

It's not yet clear whether the same pump or part failed today.

Original post: The Three Mile Island nuclear power plant shut down unexpectedly this afternoon, releasing steam containing radiation "below detectable levels," a utility spokesman told WHTM-TV.

Exelon spokesman Ray DeSantis said that the plant shut down automatically at 2:20 p.m. ET and that the cause was not immediately known. He did not identify which of the plant's two reactors was involved.

DeSantis said the shutdown "presents no risks to public health or safety, and electric customers will not be affected," WHTM writes.

Residents near the plant, outside Harrisburg, Pa., the state capital, reported hearing a loud bang, the station says. A similar noise was heard Aug. 22, TMI's Unit One shut down automatically while being taken offline manually for to repair a heater element on the plant's pressurizer tank, Exelon said at the time.

According to a Nuclear Regulatory Commission report, the shutdown resulted from a leak of reactor coolant. Unit Two went back online Sept. 5.

The plant's March 1979 accident was the worst involving a commercial U.S. reactor. Here's background on that accident from the NRC and Wikipedia.

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