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On August 1, 2001, Prairie Island Unit 1 e … On August 1, 2001, Prairie Island Unit 1 experienced an automatic reactor trip due to negative flux rate. Following the trip, several control rods initially failed to provide rod bottom position indication. This required the operating crew to enter emergency operating procedures and initiate boration to compensate for the apparent loss of shutdown. With the exception of the erroneous rod position indication, the plant response and operator actions subsequent to the trip were as expected.</br></br>The event was initiated by water intrusion into a control rod drive power supply cabinet. The water caused varistors in the 12 AC Control Rod Power Cabinet to malfunction. This, in turn, caused the Control Bank C2 control rods to drop into the core, resulting in the negative flux rate trip. The source of the water was spillage from an overhead room cooler drip tray. A partially plugged drip tray drain, combined with high ambient humidity, led to a condensation rate from the cooler coils that exceeded the capacity of the drain until the drip tray overflowed onto the control rod drive power supply cabinet below. Corrective actions include development of a preventative maintenance task to periodically inspect and clean the drip tray drains.</br></br>The control rod position indication error was caused by an unanticipated bias (equating to approximately 8 steps) that was introduced by a revised calibration procedure. The control rod position indications were recalibrated using the previously accepted methodology.using the previously accepted methodology. +
On August 1, 2001, Prairie Island Unit 1 e … On August 1, 2001, Prairie Island Unit 1 experienced an automatic reactor trip due to negative flux rate. Following the trip, several control rods initially failed to provide rod bottom position indication. This required the operating crew to enter emergency operating procedures and initiate boration to compensate for the apparent loss of shutdown. With the exception of the erroneous rod position indication, the plant response and operator actions subsequent to the trip were as expected.</br></br>The event was initiated by water intrusion into a control rod drive power supply cabinet. The water caused varistors in the 12 AC Control Rod Power Cabinet to malfunction. This, in turn, caused the Control Bank C2 control rods to drop into the core, resulting in the negative flux rate trip. The source of the water was spillage from an overhead room cooler drip tray. A partially plugged drip tray drain, combined with high ambient humidity, led to a condensation rate from the cooler coils that exceeded the capacity of the drain until the drip tray overflowed onto the control rod drive power supply cabinet below. Corrective actions include development of a preventative maintenance task to periodically inspect and clean the drip tray drains.</br></br>The control rod position indication error was caused by an unanticipated bias (equating to approximately 8 steps) that was introduced by a revised calibration procedure. The control rod position indications were recalibrated using the previously accepted methodology.using the previously accepted methodology. +
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