United States Nuclear Regulatory Commission - Protecting People and the Environment

Bulletin 79-27: Loss of Non-Class-1-E Instrumentation And Control Power System Bus During Operation

                                                            SSINS No.: 6820 
                                                            Accession No.: 

                                UNITED STATES
                          WASHINGTON, D. C. 20555 

                             November 30, 1979 

                                                  IE Bulletin No. 79-27 


Description of Circumstances: 

On November 10, 1979, an event occurred at the Oconee Power Station, Unit 3,
that resulted in loss of power to a non-class-1-E 120 Vac single phase power
panel that supplied power to the Integrated Control System (ICS) and the 
Non-Nuclear Instrumentation (NNI) System. This loss of power resulted in 
control system malfunctions and significant loss of information to the 
control room operator.  

Specifically, at 3:16 p.m., with Unit 3 at 100 percent power, the main 
condensate pimps tripped, apparently as a result of a technician performing 
maintenance on the hotwell level control system. This led to reduced 
feedwater flow to the steam generators, which resulted in a reactor trip due
to high coolant system  pressure and simultaneous turbine trip at 3:16:57 
p.m. At 3:17:15 p.m., the non-class-1-E inverter power supply feeding all 
power to the integrated control system (which provides proper coordination 
of the reactor, steam generator feedwater control, and turbine) and to one 
NNI channel tripped and failed to automatically transfer its loads from the 
DC power source to the regulated AC power source. The inverter tripped due 
to blown fuses. Loss of power to the NNI rendered control room indicators 
and recorders for the reactor coolant system (except for one wide-range RCS 
pressure recorder) and most of the secondary plant systems inoperable, 
causing loss of indication for systems used for decay heat removal and water
addition to the reactor vessel and steam generators. Upon loss of power, all
valves controlled by the ICS assumed their respective failure positions. The
loss of power existed for approximately three minutes, until an operator 
could reach the equipment room and manually switch the inverter to the 
regulated AC source. 

The above event was discussed in IE Information Notice No. 79-29, issued 
November 16, 1979. 

NUREG 0600 "Investigation into the March 28, 1979 TMI Accident" also 
discusses TMI LER 78-021-03L whereby the RCS depressurized and Safety 
Injection occured on loss of a vital bus due to inverter failure. 

Actions to Be Taken by Licensees 

For all power reactor facilities with an operating license and for those 
nearing completion of construction (North Anna 2, Diablo, Canyon, McGuire, 
Salem 2, Sequoyah, and Zimmer): 

IE Bulletin No. 79-27                                  November 30, 1979 
                                                       Page 2 of 3 

1.   Review the class-1-E and non-class 1-E buses supplying power to safety 
     and non-safety related instrumentation and control systems which could 
     affect the ability to achieve a cold shutdown condition using existing 
     procedures or procedures developed under item 2 below. For each bus: 

     a)   identify and review the alarm and/or indication provided in the 
          control room to alert the operator to the loss of power to the 

     b)   identify the instrument and control system loads connected to the 
          bus and evaluate the effects of loss of power to these loads 
          including the ability to achieve a cold shutdown condition. 

     c)   describe any proposed design modifications resulting from these 
          reviews and evaluations, and your proposed schedule for 
          implementing those modifications. 

2.   Prepare emergency procedures or review existing ones that will be used 
     by control room operators, including procedures required to achieve a 
     cold shutdown condition, upon loss of power to each class 1-E and 
     non-class 1-E bus supplying power to safety and non-safety related 
     instrument and control systems. The emergency procedures should 
     a)   the diagnostics/alarms/indicators/symptom resulting from the 
          review and evaluation conducted per item 1 above. 

     b)   the use of alternate indication and/or control circuits which may 
          be powered from other non-class 1-E or class 1-E instrumentation 
          and control buses. 

     c)   methods for restoring power to the bus. 

     Describe any proposed design modification or administrative controls to
     be implemented resulting from these procedures, and your proposed 
     schedule for implementing the changes. 

3.   Re-review IE Circular No. 79-02, Failure of 120 jolt Vital AC Power 
     Supplies, dated January 11, 1979, to include both class 1-E and 
     non-class 1-E safety related power supply inverters. Based on a review 
     of operating experience and your re-review of IE Circular No. 79-02, 
     describe any proposed design modifications or administrative controls 
     to be implemented as a result of the re-review. 

4.   Within 90 days of the date of this Bulletin, complete the review and 
     evaluation required by this Bulletin and provide a written response 
     describing your reviews and actions taken in response to each item. 

Reports should be submitted to the Director of the appropriate NRC Regional 
Office and a copy should be forwarded to the NRC Office of Inspection and 
Enforcement, Division of Reactor Operations Inspection, Washington, D.C. 

If you desire additional information regarding this matter, please contact 
the IE Regional Office. 

IE Bulletin No. 79-27                             November 30 1979   
                                                  Page 3 of 3 

Approved by GAO B180225 (R0072); clearance expires 7/31/80. Approval was 
given under a blanket clearance specifically for identified generic 
Page Last Reviewed/Updated Friday, May 22, 2015