FRENCHTOWN TWP. (WWJ/AP) – DTE Energy Co. says it has shut down the Fermi 2 nuclear power plant after a water leak was detected within the dry well that encloses its reactor.

Crews were shutting the plant down around 7 a.m. Thursday morning when a monitoring system took over as a safety measure and the plant shut down automatically, DTE spokesman Guy Cerullo told the Monroe News.

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Cerullo said Saturday that officials are still working to determine an exact cause. He said the leak — which was contained — was found to be from a system that cools heat exchanges in the plant and not the reactor, which remains safe.

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The plant is along Lake Erie in Monroe County’s Frenchtown Township.

Fermi has been in operation since Jan. 1988, after a construction period that spanned 20 years. Its operating units include Detroit Edison, an electric utility serving 2.1 million customers in Southeastern Michigan; MichCon, a natural gas utility serving 1.2 million customers in Michigan; and other non-utility, energy businesses focused on gas storage and pipelines, unconventional gas production, power and industrial projects, and energy trading.

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