July 27 The U.S. Nuclear Regulatory Commission on Friday alerted nuclear power plant operators to a potential design vulnerability that could affect key safety equipment and requested additional information about power system designs.
The regulator's notice comes after Exelon Corp's 1,136-megawatt unit 2 at Byron nuclear power station automatically shut on Jan. 30 due to unbalanced voltage entering the plant's power system from the transmission network.
The Byron 2 outage lasted about a week in early February.
"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," the NRC said.
"The degraded offsite power source potentially could have damaged the plant's emergency core cooling system," the NRC said in a statement.
NRC regulations require reliable offsite and onsite power systems with sufficient capacity and capability to operate safety-related systems, the regulator said.
Loss of offsite power was identified by the NRC as an important issue to be addressed in the wake of the devastating earthquake and tsunami that led to the meltdown of reactors at Japan's Fukushima nuclear plant in March 2011.
The NRC will use the information it receives to determine if further regulatory action is needed.
The NRC issued a notice March 1 to inform licensees of recent experience involving loss of one of three phases of the offsite power circuit, including the Byron event.
Operators have to provide information on their electric system designs within 90 days. Both operating commercial power reactors and the four combined licenses for new reactors issued earlier this year must respond, the NRC said.