United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 91-64: Supplement 1: Site Area Emergency Resulting from a Loss of Non-Class 1E Uninterruptible Power Supplies

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

                                October 7, 1992

                                             FROM A LOSS OF NON-CLASS 1E
                                             UNINTERRUPTIBLE POWER SUPPLIES


All holders of operating licenses or construction permits for nuclear power


The U.S. Nuclear Regulatory Commission (NRC) is issuing this supplement to
provide additional information on replacement intervals for the control logic
back-up power battery packs associated with the Exide Electronics,
Incorporated (Exide) 75 KVA uninterruptible power supply (UPS) model
No. 575-60T3-120/208 and to provide additional simplified drawings of the
Exide UPS control logic power supply modifications that were performed by
Niagara Mohawk Corporation's Nine Mile Point, Unit 2, facility.  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
supplement are not NRC requirements, therefore, no specific action or written
response is required.  

Description of Circumstances

On August 13, 1991, the Nine Mile Point, Unit 2, facility was operating at
full power when a phase to ground electrical fault occurred on phase B of the
Unit 2 main power transformer.  The fault resulted in trips of the main
generator, main turbine, and reactor.  The fault also caused the voltage on
electrical distribution phase B buses to momentarily decrease to approximately
50 percent of its nominal value for approximately 200 milliseconds, after
which the  electrical distribution system voltage returned to the nominal

This momentary voltage degradation on the electrical distribution buses
resulted in the simultaneous loss of power output from each of the five
Non-Class 1E uninterruptible power supplies.  Exide's UPS units have internal
continuously charged back-up batteries to prevent a loss of control logic
power.  Exide's UPS control logic circuitry receives, processes, generates,
and sends electrical signals essential for proper UPS operation.  However, in
this incident the back-up power battery packs were apparently past their
useful life and were completely discharged.  


                                                       IN 91-64, Supplement 1
                                                       October 7, 1992
                                                       Page 2 of 3

The loss of power from the UPS units caused a loss of the control room
annunciators, the safety parameter display system computer, control rod
position indication, the plant process computer, the core thermal limits
computer, the feedwater control system, some of the lighting for the plant,
the plant radio and paging systems, some instrumentation for balance-of-plant
systems, and some instrument recorders.  Loss of control room annunciators
concurrent with the plant transient resulting from automatic tripping of the
main generator, main turbine, and reactor caused the licensee to declare a
site area emergency in accordance with the site emergency plan.  The loss of
control rod position indicators and other equipment losses burdened the 
operators in implementing the emergency procedures.  However, the operators
shut down the plant in accordance with emergency procedures.  About 13 hours
after the plant trip, the reactor was placed in a cold shutdown condition; and
approximately 1 hour later, the licensee ended the site area emergency.  


At the time of the August 13, 1991, event involving UPS losses, the 120 volt
(V) alternating current (ac) maintenance supply was the normal ac input power
source to the UPS control logic dc power supply (Attachment 1).  This power is
derived from the phase B bus electrical distribution system and is provided
through a regulating transformer.  It is not considered as reliable or as high
a quality of power as the power supplied from the inverter.  The incident
prompted the licensee to modify Exide's UPS internal wiring to give a normal
source of ac power to the UPS control logic power supply from the inverter
with the maintenance supply as the alternate source (Attachment 2).  This
modification makes the UPS less susceptible to plant electrical disturbances. 

Exide used Gate's Energy Products (Gates) rechargeable batteries for its
UPS control logic battery power application.  Therefore, the NRC staff
reviewed data sheets on the performance of Gates battery packs.  The
Gates data sheets contained battery characteristic curves and additional items
regarding actual battery performance.  NRC staff reviewed these data sheets
and the associated current values required for the control logic system and
determined that functional battery packs acting alone would support the
amperage consumption necessary for the UPS control logic to operate for longer
than 10 minutes.  The performance data sheets show that battery life
expectancy is a function of float voltage and temperature values with charge
and discharge cycle life being a function of the depth of discharge.  The data
sheets also show that greater float voltage or higher operating temperature
values decrease battery life.  For example, the performance data sheets
indicate that the battery could last slightly longer than 5 years with an
operating temperature of 30� Celsius (C) and a float voltage of 2.3 Vdc.  If
the operating temperature value is increased to 40�C with a float voltage
value of 2.3 Vdc, the battery life expectancy is indicated to be approximately
3 years.  If the float voltage value is increased to 2.4 Vdc the life
expectancy is indicated to be 2 years.  Further, if the operating temperature
value is increased to 50�C and the float voltage value is 2.4 Vdc, the.

                                                       IN 91-64, Supplement 1
                                                       October 7, 1992
                                                       Page 3 of 3

battery life expectancy decreases to approximately 1 year.  Therefore, high
float voltage values combined with excessively high operating temperature
values substantially decrease battery life.  Contributing factors to the event
were that the UPS back-up internal battery packs were completely discharged,
and the licensee had not performed adequate preventative maintenance for the
battery packs based upon actual in-service conditions that could adversely
affect battery life.  

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.  

                                      ORIGINAL SIGNED BY

                                   Charles E. Rossi, Director
                                   Division of Operational Events Assessment
                                   Office of Nuclear Reactor Regulation

Technical contacts:  Frank Ashe, NRR
                     (301) 504-2785

                     J. J. Petrosino, NRR
                     (301) 504-2979

1.  Simplified Diagram For UPS Control Logic Power Supply Design 
      Prior To Modification  
2.  Simplified Diagram For Modified UPS Control Logic Power 
      Supply Design 
3.  List of Recently Issued NRC Information Notices 

Page Last Reviewed/Updated Tuesday, November 12, 2013