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
"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.