United States Nuclear Regulatory Commission - Protecting People and the Environment

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

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF NUCLEAR REACTOR REGULATION
                           WASHINGTON, D.C.  20555

                               October 9, 1991


NRC INFORMATION NOTICE 91-64:  SITE AREA EMERGENCY RESULTING FROM A LOSS OF 
                               NON-CLASS 1E UNINTERRUPTIBLE POWER SUPPLIES


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 problems resulting from the common mode 
failure of uninterruptible power supplies used in nonsafety-related 
applications at Unit 2 of the Nine Mile Point Nuclear Station.  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 August 13, 1991, Nine Mile Point Unit 2 was operating at full power when 
a fault occurred on phase B of the main transformer which caused the 
generator, turbine, and reactor to trip.  Station loads were transferred 
automatically from the normal station service transformer, which receives 
power from the generator, to the reserve station transformer, which receives 
power from the grid.  During the transient which lasted for about 12 cycles, 
voltage on the station's phase B buses decreased to approximately 50 percent 
of normal value before returning to normal. 

The voltage transient resulted in loss of the power output from five of 
eight nonsafety-related uninterruptible power supplies.  Loss of these power 
supplies caused the loss of control room annunciators, indication of control 
rod positions, the core thermal limits computer, the process computer, the 
safety parameter display system computer, the feedwater control system, some 
instrumentation for balance-of-plant systems, some instrument recorders, the 
plant radio and paging systems, and some of the lighting for the plant.  
Some of the instrument recorders that were lost failed "as is."  For 
example, the average power range monitors continued to indicate 100% after 
the reactor tripped.  Nevertheless, control room operators were able to 
verify from average power range meters and local power range lights mounted 
on back panels and from other indications that the reactor actually had 
tripped and was shut down.  


9110030270
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                                                            IN 91-64
                                                            October 9, 1991
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Because of the loss of control room annunciators and the reactor transient 
resulting from automatic tripping of the generator, turbine, and reactor, 
Niagara Mohawk Power Corporation (the licensee) declared a site area 
emergency in conformance with the emergency plan.  The operators shut down 
the plant in accordance with emergency operating procedures.  Thirty-four 
minutes after the trip, plant personnel had restored power output from the 
five nonsafety-related uninterruptible power supplies using the alternate 
maintenance power sources.  Thirteen hours after the trip, the reactor was 
in the cold shutdown condition, and an hour later, the licensee ended the 
site area emergency.  

As shown in Attachment 1, each uninterruptible power supply cabinet receives 
3-phase 600-Vac power from the plant's electrical distribution system.  This 
system is connected to either the generator or the grid.  Inside the 
cabinet, the 600-Vac input power passes through an ac-to-dc converter to 
obtain approximately 140-Vdc power which then passes through an inverter to 
generate quality 3-phase 208/120-Vac power that is within specified limits 
for voltage amplitude, frequency, and phase.  The output from the cabinet is 
distributed to critical loads, many of which are sensitive to the voltage 
irregularities that are sometimes present with normal ac power.  To prevent 
the interruption of power to these loads when the normal supply of power is 
lost, a 125-Vdc battery is connected to the uninterruptible power supply 
between the ac-to-dc converter and the dc-to-ac inverter.  

The uninterruptible power supply cabinet also receives three-phase 
208/120-Vac maintenance power.  This power is provided through a voltage 
regulator and is used to supply critical loads when the supply of uninter-
ruptible power from the inverter is not available.  Within the cabinet, a 
control logic system selects either inverter or maintenance power for 
distribution to the critical loads.  

Prior to and at the time of the event, the control logic power supply 
normally received power from phase B of the maintenance power source.  If 
maintenance power was not available, the input for the control logic power 
supply automatically transferred to the output of the inverter.  However, on 
August 13, 1991, the control logic tripped the uninterruptible power supply 
in response to the transient on phase B of the maintenance power source.  
The licensee had expected that the control logic power supply would transfer 
to the output of the inverter and, when inplant electrical distribution 
system voltages returned to normal, would transfer back to the maintenance 
source.  Had this happened, the uninterruptible power supply would have 
continued to supply the critical loads.

If the control logic power supply had been wired to receive its normal power 
from the inverter and backup power from the maintenance source, the control 
logic would have had a continuous source of power and the loss of power 
output from the uninterruptible power supplies would not have occurred.  The 
control logic power supplies for the three nonsafety-related and the two 
safety-related uninterruptible power supplies that were not lost during the 
event normally receive dc power for control logic through a dc-to-dc 
converter.

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                                                            IN 91-64
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                                                            Page 3 of 4


Six battery packs, each consisting of 3 small rechargeable lead-acid cells 
within the uninterruptible power supply, provide power to the control logic 
and the indicator lights when no other power is available.  If the battery 
packs had been functional during the transient, they would have provided 
sufficient power to the control logic for the necessary time to have 
prevented loss of the uninterruptible power supplies.  After the event, the 
licensee found that the electrolyte in the lead-acid cells was dry.  

Although the preventative maintenance section in the vendor's manual does 
not address the battery packs, another section of the manual states that 
they should be replaced every 4 years.  The vendor had installed the battery 
packs in the units in 1984.  Subsequently, the licensee determined that the 
battery packs should be replaced more frequently because of the 
environmental conditions within the cabinets.

The licensee obtained the uninterruptible power supplies from Exide 
Electronics in 1981.  In 1985, the licensee obtained a revised manual for 
the uninterruptible power supplies from Exide Electronics which recognized 
the importance of reversing the connections of normal and backup power for 
the control logic power supplies.  New units supplied by Exide Electronics 
did have the connections reversed.  However, Exide Electronics did not 
specifically advise the licensee of this change.  The licensee had obtained 
the uninterruptible power supplies as commercial grade products.  

The licensee has modified the uninterruptible power supplies to provide 
normal input power to the control logic power supplies from the output of 
the inverter and alternate power from the maintenance source.  The licensee 
has also replaced the battery packs for the control logic and has 
established an annual replacement schedule.  The licensee intends to 
evaluate the performance of the switching circuitry for supplying input 
power to the control logic under various conditions and to process 
appropriate changes to the vendor's manual.

Discussion

The transformer fault caused the generator, turbine, and reactor trips.  The 
transformer fault also led to a momentary degradation of voltage on the 
plant's electrical distribution system and, in turn, the common mode failure 
of the five uninterruptible power supplies.  Because of the loss of control 
rod position indicators due to loss of the uninterruptible power supplies, 
the operators could not immediately verify that all control rods had fully 
inserted into the reactor core.  This resulted in some difficulty in 
implementing the emergency procedures.  However, the reactor was in a safe 
condition at all times, and operators were able to monitor all safety 
parameters, except for control rod positions, throughout the event.  The two 
safety-related uninterruptible power supplies were not lost.  Had a design 
basis accident occurred in conjunction with this event, power would have 
been available for the instrumentation and control components in the 
required safety systems.

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                                                            IN 91-64
                                                            October 9, 1991
                                                            Page 4 of 4


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 project manager.




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


Technical contacts:  Jack E. Rosenthal, AEOD
                     (301) 492-4440

                     Roger Woodruff, NRR
                     (301) 492-1152


Attachments:
1.  Uninterruptible Power Supply Before Design Change
2.  List of Recently Issued NRC Information Notices
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Page Last Reviewed/Updated Tuesday, November 12, 2013