The Nuclear Regulatory Commission has ended its intensive monitoring of a situation at the Prairie Island nuclear power plant.
Xcel Energy declared an "unusual event" at 6:24 a.m. Tuesday. At that time, plant workers were getting an indication that cooling water levels were decreasing in the Unit 2 reactor. Since then, other equipment has indicated the levels are actually stable.
NRC spokeswoman Viktoria Mitlyng says a possible explanation is that some instruments were not working properly.
"If there's an issue with instrumentation you could get a signal that's not based on actual plant conditions," she said.
Mitlyng says the agency will analyze possible causes of the event. Both the NRC and Xcel Energy say there were no releases of radiation and the public was not in danger. Unit 2 has been down for refueling for about two weeks.
Xcel Energy says Tuesday's incident is not related to an episode during the Unit 2 shutdown process two weeks ago. At that time, operaters received an alert that prompted them to shut it down immediately rather than gradually.
Xcel said that issue is still being evaluated, but it was related to the unit's feed-water heater level control. The company says it will probably address that problem by revising operating procedures to avoid operating at a specific power level for an extended period of time.
Meanwhile, the company has decided not to appeal an NRC ruling last month that found a security-related problem that is prompting the agency to do additional inspections at the Prairie Island plant.