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Maintenance work on the ammeter of a 4160 V bus cut power to two pumps at the Monticello nuclear plant Tuesday, leading to an automatic reactor shutdown from full power, officials said.

The shutdown occurred at 10:44 a.m. when the bus locked out, removing power from a reactor feed pump and a reactor recirculation pump, according to a Nuclear Regulatory Commission (NRC) event report. The water level in the reactor fell, then began to rise to +48 inches, the reactor-water-level-high setpoint. The main turbine and another feed pump tripped, as designed, and the reactor shut down automatically.

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The “reactor water level began to drop and C.4.A Abnormal Procedure for SCRAM was used to restart 11 reactor feed pump and recover water level. (The) minimum water level reached was -26 inches,” the report said.

A low-level signal triggered group 2 primary containment isolation. The emergency core cooling system did not activate, and safety relief valves did not come into play.

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“All plant safety systems functioned as designed, and the situation posed no danger to the public or plant workers,” plant owner Xcel Energy said. “Plant operators are investigating the cause of the shutdown, which is not expected to be lengthy.”

This is not the first time the Monticello plant shutdown as a leaking gasket on a structure that houses the reactor forced a shutdown in the middle of August. After making repairs, the plant began operating at full power in late August.

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