Malfunctioning sensor adds to list of problems at San Onofre nuclear power plant

Grant Slater/KPCC

The San Onofre Nuclear Power Plant is seen on April 6, 2012. The operators of the San Onofre nuclear power plant have told federal regulators they're looking into problems with a potentially malfunctioning vibration sensor.

The operators of the San Onofre nuclear power plant have told federal regulators they're looking into problems with a potentially malfunctioning vibration sensor.

The sensor is not part of the ongoing problems with steam generator tubes that have kept the facility shut down since Jan. 31.

Jennifer Manfrè with Southern California Edison said the utility submitted a required written report to the Nuclear Regulatory Commission noting that SCE personnel identified a vibration sensor at San Onofre Nuclear Generating Station that might incorrectly initiate a shutdown of the emergency diesel generators when a loss of offsite power happens at the same time as an earthquake.

The sensor mechanism is designed to shut the diesel generators down during excessive vibration, said Manfrè, which would indicate mechanical engine damage in the diesel generators.

"We want to make sure if the diesel generators kicked on due to a loss of offsite power, and there is an earthquake at the same time, that the vibration sensor would not trip and cause the generator to turn off," said Manfrè.

She said there is a system designed to bypass the sensor and keep the generators operating in the event of an offsite power outage or emergency at the plant.

Manfrè said SCE engineers are continuing to analyze the vibration sensor and have not determined if the sensor would actually cause a shutdown during an earthquake.

She said multiple backup safety mechanisms are part of federal requirements at the nuclear plant.

The facility has been completely shut down since Jan. 31 due to unusual wear in steam generator tubes.

Manfrè said inspectors continue to analyze data to determine the cause of the wear.

"There is no timeline for restarting the plant until we fully understand the reasons for the accelerated tube wear and then look at possible remediation or repairs," said Manfrè.

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