For almost 15 years, the operator of the Prairie Island, MN, nuclear power plant knew a safety-related backup battery charger would stop working during some emergencies, but they just replaced the faulty device last month after federal regulators learned about the problem.

Workers at the plant discovered the problem in 1996 during an accident simulation. The charger would lock up and stop recharging one of four battery packs when the plant’s outside power source cut off and the plant switched to diesel generators.

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U.S. Nuclear Regulatory Commission (NRC) inspectors, in preliminary findings earlier this month, faulted plant owner Xcel Energy for how it handled the problem. Instead of fixing or replacing the charger, the utility for years turned off the charger during simulation tests so it wouldn’t lock up, the NRC said.

Power from the batteries operates valves, controls and other systems in U.S. plants. To cope with a charger failure during a crisis, Xcel intended to send a worker into the plant to manually restart it. Though no such accident has happened at Prairie Island, NRC inspectors feared workers trying to cope with a crisis might overlook the faulty charger, allowing batteries to discharge and potentially compromise safety.

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The problem illustrates how a subtle shortcoming in nuclear safety can exist for years. Xcel and the NRC disagree on the problem’s severity.

“We thought that the chargers worked most of the time and if they tripped off-line, we will put them on-line — that logic was wrong,” said Mark Schimmel, Xcel’s site vice president for the Prairie Island plant, who noted Xcel self-reported the problem to the NRC.

Even so, Schimmel disagreed with NRC’s preliminary finding the charger problem should be rated as “white,” meaning it had low to moderate safety significance. The NRC rates safety issues with a color-coded system that classifies findings as green, white, yellow or red, in increasing order of significance.

A white finding would trigger enhanced safety regulatory oversight, and would be the fifth such finding for the plant in a decade, according to the NRC. The plant has not had more severe lapses. Xcel plans to contest the NRC’s latest white rating at a meeting next month.

Backup batteries got the spotlight early in Japan’s Fukushima Daiichi nuclear disaster because they lasted only a few hours. Diesel generators didn’t work after the tsunami and earthquake in March.

In May, Xcel replaced the charger that first failed in 1996, and said it intends to replace three others like it on other battery units during a future plant shutdown.

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