United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 92-53: Potential Failure of Emergency Diesel Generators due to Excessive Rate of Loading

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                            WASHINGTON, D.C.  20555

                                 July 29, 1992

                               GENERATORS DUE TO EXCESSIVE RATE OF LOADING


All holders of operating licenses or construction permits for nuclear power


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice
to alert addressees of the possibility that emergency diesel generators (EDGs)
could fail during a loss of offsite power (LOOP) if certain electrical loads
are automatically started (i.e., after breaker close permissive interlocks have
been met) at the same time as other loads that are sequenced onto the emergency
buses.  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 March 17, 1992, during an NRC electrical distribution system functional
inspection (EDSFI) of the Calvert Cliffs Nuclear Power Plant, Units 1 and 2,
the EDSFI team found that loads which are required to be supplied by the EDGs
in a specific sequence, could be started at an inappropriate time because of
the design of the load sequencer logic.  Specifically, the load sequencer sends
a close permissive signal to the breakers for certain loads.  Additional
process signals are needed to close the breakers and start these loads.  At
Calvert Cliffs, Units 1 and 2, the NRC determined that under certain accident
conditions, such as a small break loss-of-coolant accident (LOCA) in a pipe
with a diameter in the range of 4 inches, concurrent with a loss of offsite
power, certain loads such as containment spray pump and some heating,
ventilation, and air conditioning (HVAC) loads could be started at the same
time as other loads that are automatically sequenced onto the emergency buses. 
Loading the EDG more rapidly than its designed capability could cause it to
degrade in voltage or stall.  This condition could also occur during a loss of
offsite power without a concurrent LOCA.  

The above problem was also found at the Salem Nuclear Generating Station, the
Kewaunee Nuclear Power Plant, and the Palisades Nuclear Plant.


                                                           IN 92-53
                                                           July 29, 1992
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Most EDG installations require that the loads be sequenced or connected to the
EDG in a way that allows the engine and voltage regulator to recover nominal
speed and voltage after each large motor load is started and before the next
load is connected.  Normal design practice is to use a timing device that
starts the loads on a predetermined schedule based on the required accident
loading sequence and the EDG's loading capabilities.  Some safety features
actuation system (SFAS) loads will not start until obtaining both a signal from
the load sequencer and a process signal.  Examples of this type of load are (1)
chiller units for HVAC systems, which only start the centrifugal compressor
when the chilled water output reaches a temperature setpoint; (2) containment
spray pumps that are actuated by a pressure setpoint; and (3) emergency service
water pumps that start from a temperature setpoint.  The simultaneous addition
of significant loads onto the EDG could fail the EDG.  Sequencer time drift or
errors in timing could also cause this problem.  Either situation could result
in an inoperable EDG.

The Baltimore Gas and Electric Company, licensee for the Calvert Cliffs Nuclear
Power Plant, Units 1 and 2, solved the problem for the containment spray pumps
by modifying the design to start the spray pumps at a predetermined sequence
step.  However, the associated containment spray valves will open only when the
containment pressure setpoint is reached.  

In summary, the ability of the EDG to supply the required voltage and current
could be degraded or lost if the rate of loading on the EDG exceeds its design

This information notice requires no specific action or written response.  If
you have any questions about the information in this notice, please contact one
of the technical contacts 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 contacts:  Roy Mathew, RI
                     (215) 337-5194

                     Eugene Lazarowitz, RI
                     (215) 337-5392

                     Thomas Koshy, NRR 
                     (301) 504-1176

Attachment:  List of Recently Issued Information Notices.
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