Neil Sheehan, an NRC Region I spokesman, said in an email Friday that one of two of the unit's main feedwater pumps shut automatically at about 4:40 pm EDT (2040 GMT) Thursday. This pump's shutdown, he said, "resulted in a recirculation flow runback, which in turn caused a power reduction to about 51-percent power."

"Troubleshooting has preliminarily identified the cause as an instrument air leak stemming from a 2-inch line failure that caused condensate booster pump valves to unexpectedly open," Sheehan said.