The US Nuclear Regulatory Commission has issued a Bulletin to all nuclear power plant licensees requesting information about their electric power system designs and alerting them to a potential design vulnerability that could affect the operation of key safety equipment.

On 30 January, Byron Station, Unit 2, shut down automatically due to unbalanced voltage entering the onsite power distribution system from the transmission network. The plant’s electric power system’s protection scheme was not designed to sense the loss of one of three power phases and automatically trip circuits to isolate the degraded outside power source and switch to emergency backup power. Plant operators diagnosed the problem eight minutes later and manually tripped the necessary circuits.

“The degraded offsite power source potentially could have damaged the plant’s emergency core cooling system,” NRC said in a statement.

“NRC regulations and plant technical specifications require reliable off-site and onsite power systems with sufficient capacity and capability to operate safety-related systems,” it continued.

“Therefore, the NRC staff is seeking information to verify compliance with existing regulations and plant-specific licensing bases.”

The staff will use the information it receives to inform the Commission and determine if further regulatory action is needed, NRC said.

Licensees have been asked to provide information on their electric system designs within 90 days. The Bulletin applies to the 104 currently operating commercial power reactors in the United States and the four combined licenses for new reactors issued earlier this year.

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