SONGS Reports Vibration Sensor Issue With Implications for Backup Power During Earthquakes

Engineers at Southern California Edison’s San Onofre nuclear plant have reported that vibration sensors meant to protect bearings in emergency diesel generators could conceivably shut them down during an earthquake.

While actual reactor accident conditions would cause emergency systems to bypass the sensor and keep the diesels running, the plant nonetheless disabled the electronics in question and reported the discovery to the Nuclear Regulatory Commission as an unanalyzed condition.

You can download a copy of Southern California Edison’s report to the NRC here.

According to the filing, workers were reviewing the replacement of a vibration sensor on March 16 and questioned whether the generator’s high-vibration trip could spuriously actuate during a concurrent earthquake and loss of outside power. Neither regulators nor equipment documentation had apparently explored the question, so SONGS began a formal analysis of the sensor’s response to earthquake vibration, and workers disabled high vibration trips on the diesels March 17.

The sensor is part of a non-critical engine trip circuit to protect components within four General Motors Electromotive Division generators procured from Stewart & Stevenson Services and installed in 1981, according to Southern California Edison. If activated, the vibration monitor automatically resets and allows operators to restart the generator from either the control room or the generator itself. The utility noted that backup systems at the plant would provide four hours for operators to restart diesel power in the event of a station blackout.

Non-critical generator trips, like those triggered by the vibration sensors, are also bypassed automatically in the event of a safety injection actuation signal. Therefore, the plant reported, there was no safety significance for a design basis accident requiring actuation of emergency safety systems.

When the sensor question was raised, both SONGS units were offline as the plant and the NRC address premature tube wear in steam generators.

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