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.