United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 88-55: Potential Problems caused by Single Failure of an Engineered Safety Feature Swing Bus

                                UNITED STATES
                           WASHINGTON, D.C.  20555

                                 August 3, 1988 

                                   OF AN ENGINEERED SAFETY FEATURE SWING BUS 


All holders of operating licenses or construction permits for nuclear power 


This information notice is being provided to alert addressees to potential 
problems caused by single failure of an engineered safety features swing bus. 
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 do not 
constitute NRC requirements; therefore, no specific action or written response 
is required.

Description of Circumstances:

On January 28, 1988, while Robinson Unit 2 was operating at full power, 
Carolina Power and Light reported to the NRC Operations Center, in compliance 
with 10 CFR 50.72, that a single failure in the system supplying electrical 
power to the three safety injection pumps could cause two of them to fail to 
perform their intended function.  On the following day, the licensee reported 
to the NRC Operations Center that Robinson Unit 2 was being shut down in 
compliance with its technical specifications.  

As shown in Figure 1, each of two class 1E buses supplies power to a safety 
injection pump.  One of these buses also supplies power to a third safety 
injection pump via a swing bus.  The swing bus is normally supplied with power 
by bus E1; however, if bus E1 fails to supply power to the swing bus, then the 
swing bus is automatically transferred to bus E2.  An interlock prevents the 
swing bus from being supplied simultaneously from both of its supply buses.  
As originally designed, a safety injection signal causes all three safety 
injection pumps to start automatically.  

During the licensee's review of the emergency electrical distribution system, 
including dc control power, which was done in response to an NRC request, the 
licensee identified several postulated single failures that could result in 
loss of two of the three safety injection pumps.  For example, loss of dc 
control power for train B would cause loss of safety injection pumps B and C. 
However, the analysis of record for the loss-of-coolant accident (LOCA) 

.                                                               IN 88-55
                                                               August 3, 1988
                                                               Page 2 of 2

assumed that a single failure would cause only one safety injection pump to 
fail.  Pending resolution of this problem, the auto-start capability for the 
swing pump was defeated, the plant was derated to 60 percent of full power, 
and the allowable power-peaking factor was reduced from 2.32 to 2.26. 

Subsequently, the licensee performed a new LOCA analysis assuming that only 
one safety injection pump operates to mitigate the consequences of a LOCA.  
Calculation of emergency core coolant flow was based on system test 
performance degraded by 5 percent.  Using acceptable models, the licensee 
found that peak cladding temperature, local metal-water reaction, and total 
metal-water reaction would be within the acceptance criteria of 10 CFR 50.46 
with the reactor operating at 100 percent power with the power peaking factor 
at 2.32 before the accident.  On the basis of these results, NRC permitted the 
licensee to resume full power operation after removing the auto-start 
capability of the swing pump and after the limiting conditions for operation 
were changed to require that two safety injection pumps be operable, each 
capable of automatic initiation from a separate emergency bus.  

No specific action or written response is required by this information notice.
If you have questions about this matter, please contact one of the technical 
contacts listed below or the Regional Administrator of the appropriate 
regional office. 

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

Technical Contacts:  Roger Woodruff, NRR
                     (301) 492-1180

                     Peter Kang, NRR
                     (301) 492-0812 

Attachments:   1.  Figure 1 - Robinson 2, Power Supply for Safety Injection 
               2.  List of Recently Issued NRC Information Notices 

.                                                            Attachment 2
                                                            IN 88-55 
                                                            August 3, 1988 
                                                            Page 1 of 1

                             LIST OF RECENTLY ISSUED
                            NRC INFORMATION NOTICES 
Information                                  Date of 
Notice No._____Subject_______________________Issuance_______Issued to________

88-54          Failure of Circuit Breaker    7/28/88        All holders of OLs
               Following Installation of                    or CPs for nuclear
               Amptector Direct Trip                        power reactors. 

88-53          Licensee Violations of NRC    7/28/88        All manufacturers 
               Regulations, Which Led to                    and distributors 
               Medical Diagnostic                           of radio- 
               Misadministrations                           pharmaceuticals 
                                                            for human use, 
                                                            pharmacies, and 
                                                            medical licensees.

88-52          Failure of Intrauterine       7/27/88        Medical licensees.
               Tandem of Fletcher 
               Applicator Brachytherapy 
               Devices During Patient 

88-46,         Licensee Report of            7/26/88        All holders of OLs
Supplement 1   Defective Refurbished                        or CPs for nuclear
               Circuit Breakers                             power reactors. 

88-51          Failures of Main Steam        7/21/88        All holders of OLs
               Isolation Valves                             or CPs for nuclear
                                                            power reactors. 

88-50          Effect of Circuit             7/18/88        All holders of OLs
               Breaker Capacitance                          or CPs for nuclear
               on Availability of                           power reactors. 
               Emergency Power 

88-49          Marking, Handling,            7/18/88        All holders of OLs
               Control, Storage and                         or CPs for nuclear
               Destruction of Safe-                         power reactors and
               guards Information                           all other licensed
                                                            involving a 
                                                            formula quantity 
                                                            of special nuclear

88-48          Licensee Report of            7/12/88        All holders of OLs
               Defective Refurbished                        or CPs for nuclear
               Valves                                       power reactors. 
OL = Operating License
CP = Construction Permit 
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