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
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IN 90-25
April 16, 1990
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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
April 16, 1990
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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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IN 90-25
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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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