Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
January 30, 1992
NRC INFORMATION NOTICE 92-09: OVERLOADING AND SUBSEQUENT LOCK OUT OF
ELECTRICAL BUSES DURING ACCIDENT CONDITIONS
Addressees
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to a particular scenario identified by a licensee
whereby electrical buses could be overloaded during accident conditions,
deenergized, and prevented from being reenergized. It is expected that
recipients will review the information for applicability to their facilities
and consider actions, as appropriate, to avoid similar problems. However,
suggestions contained in this information notice are not NRC requirements;
therefore, no specific action or written response is required.
Description of Circumstances
On November 21, 1991, the Power Authority for the State of New York, the
licensee for Indian Point Nuclear Generating Unit 3, reported that the vital
buses could become overloaded during a loss-of-coolant-accident (LOCA) as a
result of emergency operating procedures directing operators to restore non-
essential loads to the buses if offsite power is not lost. The procedure
would result in both emergency and non-emergency loads being powered
concurrently from the same bus. The licensee's recent calculations reveal
that performing this procedure could have overloaded buses. The resulting
overcurrent condition could lock out a bus (i.e., deenergize the bus and
prevent it from being reenergized from any source including the associated
emergency diesel generator).
Discussion
A LOCA is generally analyzed as the most limiting condition in power demand
for onsite electrical systems. When non-essential loads are added to the
power system under this most demanding condition, electrical buses and
breakers could be overloaded. The electrical protection system recognizes
such an overloaded condition as an electrical fault on a bus and locks out
the bus. The dc-powered breaker controls retain the faulted condition, and
the emergency diesel generator will not be able to energize the bus until
protective relays are manually reset. The scenario could result in
disabling redundant trains of safety-related equipment.
9201240235
.
IN 92-09
January 30, 1992
Page 2 of 2
This information notice requires no specific action or written response. If
you have any questions about the information in this notice, please contact
the technical contact listed below or the appropriate Office of Nuclear
Reactor Regulation (NRR) project manager.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical contact: Thomas Koshy
(301) 504-1176
Attachment: List of Recently Issued NRC Information Notices
.