United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 90-25: Loss of Vital AC Power With Subsequent Reactor Coolant System Heat-Up

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

                               April 16, 1990


Information Notice No. 90-25:  LOSS OF VITAL AC POWER WITH SUBSEQUENT 
                                   REACTOR COOLANT SYSTEM HEAT-UP


Addressees: 

All holders of operating licenses or construction permits for nuclear power 
reactors.

Purpose:

This information notice is being provided to inform addressees of a 
significant operating event which occurred at the Georgia Power Company's 
Vogtle Nuclear Plant on March 20, 1990.  The event took place while Unit 1 
was in mid-loop in operational mode 6 and demonstrates the need for operating 
personnel to be prudent in removing equipment from service or establishing 
atypical equipment alignments during shutdown.  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 March 20, 1990, a loss of vital ac power occurred while Vogtle Unit 2 was 
operating at 100 percent power and Unit 1 had been in a refueling outage for 
about 4 weeks.  Unit 1 was in mid-loop in operational mode 6 with plant 
equipment conditions as follows:

     o  The Unit 1 B reserve auxiliary transformer (RAT) was tagged out of 
        service for maintenance 
     o  The Unit 1 B emergency diesel generator (EDG) was tagged out of 
        service and disassembled for maintenance 
     o  The Unit 1 A RAT was supplying offsite power to the crosstied Unit 1 
        A and B vital buses
     o  The reactor coolant system (RCS) temperature was being maintained at 
        around 90 degrees Fahrenheit (F) via the train A residual heat 
        removal (RHR) pump, the train B pump was in standby
     o  The vessel head was in place with the bolts not fully tensioned 
     o  The pressurizer manway cover was removed






9004120133
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     o  The manways for steam generators 2 and 3 were partially bolted in 
        place and the manways for steam generators 1 and 4 were in place with 
        bolts fully tensioned 
     o  The inboard charging line check valve and an accumulator isolation  
        valve were open for inspection 
     o  The containment equipment hatch and the containment personnel hatch 
        were open

At about 9:20 a.m., a truck carrying fuel and lubricants in the plant low 
voltage switchyard backed into a support column for the feeder line supplying 
power to the Unit 1 A RAT and the Unit 2 B RAT.  The insulator for the C 
phase of the feeder line fractured and initiated a phase-to-ground electrical 
fault.  The fault resulted in a loss of power to the Unit 1 A RAT and the 
Unit 2 B RAT.  The Unit 2 B EDG started and loaded to the deenergized Unit 2 
B vital bus.  However, a Unit 2 turbine trip and reactor trip occurred 
because of an improperly connected (wrong tap) differential current 
transformer (DCT).  The DCT initiated the trip when the current surge 
associated with the phase-to-ground fault was sensed.  The Unit 2 trip was 
uncomplicated.  

Because both of the Unit 1 vital buses were crosstied and being supplied by 
the Unit 1 A RAT, the loss of the transformer deenergized both vital buses.  
Deenergizing these buses resulted in the loss of power to the operating RHR 
pump.  Since the Unit 1 B EDG was tagged out of service and disassembled for 
maintenance, the emergency power supply for the B vital bus was unavailable 
and the standby B RHR pump could not be started. 

The available Unit 1 A EDG started on bus undervoltage, but for unknown 
reasons, it shut down automatically after 1 minute and 20 seconds.  At 9:40 
a.m., plant operators declared a site area emergency.  A loss of all onsite 
and offsite ac power at Vogtle for more than 15 minutes is classified as a 
site area emergency.  The licensee made their declaration because all vital 
ac power was lost for greater than 15 minutes.  Approximately 18 minutes 
after the first start of the A EDG, the operators locally reset the load 
sequencer which automatically restarted the A EDG on undervoltage.  However, 
after 1 minute and 10 seconds, the diesel again shut down automatically.  At 
9:56 a.m., plant operators performed an "emergency" manual start of the 
diesel, which bypassed most of the diesel's protective trips.  The diesel 
started and loaded to the bus, the A RHR pump was restarted, and core cooling 
was reestablished to Unit 1.  With the start of the diesel, the site area 
emergency was downgraded to an "alert" at 10:15 a.m.  Containment integrity 
was established at 11:03 a.m. 

Plant personnel returned the Unit 1 B RAT to service after completing formal 
tagout removal procedures.  However, attempts to energize the transformer 
were delayed for several minutes because of a sticking mechanical interlock 
in the control circuitry for a motor-operated disconnect switch on the high 
side of the B RAT.  Power was restored to the B vital bus via the B RAT at 
11:40 a.m. At 12:38 a.m., core cooling was shifted to the B RHR train to 
facilitate subsequent electrical alignment changes. 

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                                                            IN 90-25 
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According to control room indication, RCS temperature increased from 90 to 
136 degrees F during the 36 minutes required to reenergize the A bus (1.3 
degrees F/min).   

Throughout the event, non-vital power was continuously provided to Unit 1 
from offsite sources, via backfeed through the main generator transformer.  
Also, the Unit 2 electrical distribution system remained energized (aside 
from the momentary loss of power before the reactor trip).  However, the 
Vogtle electrical system was not designed to permit easy interconnection of 
the Unit 1 vital buses to nonvital power or to the Unit 2 electrical buses.  
Therefore, there were no procedures in place to provide guidance on 
interconnecting the Unit 1 vital and non-vital buses or for interconnecting 
the Unit 1 electrical distribution system with the distribution system at 
Unit 2.  (There are no regulatory requirements that direct the licensee to 
develop interconnection procedures.)    

Discussion:

The NRC has previously identified concerns with the operability of necessary 
equipment in shutdown modes.  In Information Notice (IN) 80-20, "Loss of 
Decay Heat Removal Capability at Davis-Besse Unit 1 While in a Refueling 
Mode" an extended loss of decay heat removal capability was attributed to a 
number of factors, including inadequate procedures, inadequate administrative 
controls, and the conduct of extensive maintenance activities.  

Information Notice 84-42, "Equipment Availability for Conditions During 
Outages Not Covered by Technical Specifications," describes an event at the 
Palisades Nuclear Plant involving the total loss of all ac power.  The 
Palisades event was precipitated by personnel performing actions during a 
refueling outage without an appreciation for the effect of those actions on 
the plant as it was configured.  Many pieces of equipment were tagged out of 
service for maintenance.  Personnel intentionally interrupted offsite power.  
This action caused the loss of the only available service water pump.  As a 
result, cooling water was interrupted to the only available (and operating) 
diesel generator.  The diesel overheated and was manually tripped.  As a 
result, all station ac power was interrupted, causing the loss of fuel pool 
cooling.  (The reactor had been defueled, and its fuel was being stored in 
the spent fuel pool). 

The recent Vogtle event reemphasizes the need for careful planning of 
equipment outages during shutdown.  Licensees, in general, have considerable 
latitude in removing equipment from service and altering normal system 
lineups while in shutdown modes if only the minimum technical specification 
requirements are considered.  The desire to return nuclear units to service 
as quickly as possible can result in maintenance being conducted on 
redundant, safety-related systems, concurrently.  The appropriateness of 
conducting parallel maintenance activities should be considered in light of 
the availability of alternate equipment and procedures to mitigate the 
consequences of potential operational problems.  It is important that plant 
outage plans assure that an adequate complement of equipment capable of 
responding to potential events remains available throughout the outage.  
Particular care is needed when the reactor pressure boundary is not intact or 
the reactor coolant system is at mid-loop.   
.

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A special investigation of the Vogtle event is being conducted by an NRC 
Incident Investigation Team (IIT).  The information contained in this notice 
is preliminary and is subject to change, pending the findings made by the 
IIT.  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 
NRR project manager. 




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

Technical Contacts:  N. Fields, NRR
                     (301) 492-1173

                     E. Trager, AEOD
                     (301) 492-4496 

Attachment:  List of Recently Issued NRC Information Notices
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