Prairie Island’s unit 1 reactor faces additional scrutiny from federal regulators following a Nuclear Regulatory Commission finding related to backup batteries that would not recharge under certain conditions.
According to documents from the NRC, battery chargers installed in 1994 were shown to malfunction if incoming power dropped below a certain voltage. In that case, the chargers could be reset manually, and plant owner Xcel Energy argued its operators have long had procedures available to do so. Nonetheless, the NRC, upon discovering the condition last year, recently assigned it a white finding – the agency’s second-lowest adverse finding classification that indicates a low-to-moderate safety concern.
The battery packs can keep remotely operated safety equipment running in the event of a loss-of-power emergency. The NRC’s significance determination noted, “Prior to the NRC identifying the issue in 2010, the need for operator action to diagnose and restore a failed charger had not been identified as a time-critical operator action. The operator action was not directed by the emergency operating procedures and was not included in any operator training” for the complex scenarios that might precipitate charger malfunction.
The Minneapolis Star Tribune cited switching to backup diesel power as one situation that might reduce voltage to the chargers and cause them to “lock up.” The newspaper quoted Xcel as saying the equipment in question was replaced in May, and that a similar charger in unit 2 will be replaced during its next refueling. Prairie Island operates two Westinghouse pressurized water reactors generating roughly 550 megawatts each 28 miles southeast of Minneapolis, Minn.