Information Notice No. 91-22: Four Plant Outage Events Involving Loss of AC Power or Coolant Spills
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
March 19, 1991
Information Notice No. 91-22: FOUR PLANT OUTAGE EVENTS INVOLVING LOSS
OF AC POWER OR COOLANT SPILLS
All holders of operating licenses or construction permits for nuclear power
This information notice is intended to alert addressees to the potential for
equipment failures resulting from loss of power or loss of coolant inventory
that could affect the adequacy of decay heat removal during plant outages.
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:
During plant outages, non-routine plant configurations may result from
refueling and maintenance/surveillance activities going on at the same time.
As permitted by plant technical specifications, equipment may be taken out
of service that would otherwise be required to be operable during power
operation. The following four events occurred during a one-week period in
March 1991. Two events involve losses of ac power sources with redundant
trains out of service. The other two events involve reactor coolant spills
from improper valve manipulations. The four events demonstrate the
importance of careful planning to ensure the coordination of planned outages
of equipment, tests of systems and components, and plant conditions.
DIABLO CANYON UNIT 1--MARCH 7, 1991
Diablo Canyon Unit 1 suffered a loss of offsite power when, while the unit
was shut down for refueling with 35 feet of water above the core, the boom
of a mobile crane came too close to the 500 kV transformer lead, shorting it
and tripping the transformer.
Electrical power for Unit 1 operations was being back-fed through the main
transformer to the unit auxiliary transformers. The main generator had been
disconnected from the main transformer, so that the backfeed was possible.
March 19, 1991
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Unit 1 standby startup transformer, the normal source of power during a
shutdown, had been taken out of service for maintenance.
When the main transformer tripped and no longer supplied power, the unit's
three emergency diesel generators started automatically and picked up
emergency loads as designed. Offsite power was restored to the Unit 1
auxiliary buses five hours after the initiating event by cross-tying the
Unit 2 standby startup transformer into the Unit 1 startup bus.
The mobile crane was improperly brought to within 2 or 3 feet from the power
lines, permitting arc-over. The licensee's Accident Prevention Rules
indicate that 27 feet is the minimum required clearance between mobile
cranes and 500 kV transmission lines.
OCONEE 3--MARCH 8, 1991
Oconee Unit 3 was 24 days into its refueling outage with the reactor coolant
system (RCS) in a reduced inventory condition when shutdown cooling was lost
for about 18 minutes. Shutdown cooling was lost when the operating
low-pressure injection (LPI) pump cavitated following a rapid loss of water
from the RCS. Approximately 14,250 gallons of primary water drained into
the sump during stroke testing of the reactor building emergency sump
suction valve. Approximately 9,750 of that 14,250 gallons came from the
reactor system; and about 4,500 came from the borated water storage tank
(BWST). Apparently, the blind flange which should have been placed between
the sump suction valve and the sump, was placed on the wrong line. Hence,
when the sump suction valve was exercised, the water drained from that line
into the sump. The water level in the reactor vessel dropped from the
vicinity of the reactor vessel head flange to the bottom of the hot leg.
The operators entered Abnormal Operating Procedures for "Loss of Low
Pressure Injection System" which called for suction to be taken from the
BWST. This action caused 4,500 gallons of BWST water to be dumped into the
sump, and 5,250 gallons to flow to the reactor vessel restoring the level in
the reactor vessel. Once the operators terminated the loss of reactor
coolant, adequate net positive suction head was reestablished and the
LPI pumps were vented and restarted.
OYSTER CREEK -- MARCH 9, 1991
Oyster Creek had been shut down for refueling since February 16, 1991. The
reactor vessel head had been removed and the refueling cavity filled with
water such that there were 60 feet of water above the core and 30 feet of
water above the spent fuel rack. The main transformer had been taken out of
service for maintenance so no back feed was available to the 4.16kV safety
buses. Also, the licensee was performing refueling outage maintenance on
its "C" 4.16 kV emergency safety bus and on the EDG that powers the "C" bus.
The available 4.16 kV emergency safety bus, the "D" bus, was being energized
from off site through the startup transformer. These conditions had put the
plant in a Technical Specifications ACTION statement that required weekly
testing of the standby EDG for the "D" bus.
March 19, 1991
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During a weekly surveillance on March 9, 1991, the standby EDG for the "D"
bus failed its test and was declared inoperable. Water was leaking into one
cylinder. This put the plant in a condition in which there were no operable
EDGs, and only one startup transformer was available. This condition
existed until offsite power to the "C" bus was restored in approximately
33 hours. During that time period offsite power to the startup transformer
provided the only ac power source.
CALVERT CLIFFS UNIT 2-- MARCH 12, 1991
On March 12, 1991, Calvert Cliffs Unit 2 was in MODE 5, nearing the end of
an extended outage when approximately 1900 gallons of borated reactor
coolant was inadvertently discharged into the containment through the
containment spray header.
Decay heat removal was being provided by a low-pressure safety injection
(LPSI) pump and two shutdown cooling (SDC) heat exchangers. The licensee
was lining up the valves to fill one of the safety injection tanks (SITs).
This fill activity required that the discharge cross-connect valve for both
of the SDC heat exchangers be shut and the discharge cross-connect isolation
valve between the LPSI and the containment spray pumps be opened. Another
step in this fill procedure was to verify that the containment spray header
was isolated by independently verifying that the containment spray isolation
valve was closed.
The reactor coolant discharged because operators deviated from the
established SIT valve line up and verification procedure, allowing a
flowpath to be established from the LPSI pump to the motor-operated
isolation valve of the containment spray header. Flow was noted through the
containment spray header when the inboard containment spray isolation valve
was observed to be leaking grossly.
The NRC staff sent augmented inspection teams (AITs) to determine the facts
associated with these events at Diablo Canyon, Oconee, and Oyster Creek. A
special Region I inspection team was sent to Calvert Cliffs. The results of
these inspections will be factored into the NRC staff's ongoing evaluation
of safety risks during shutdown and low-power operation. This information
notice will be supplemented, as necessary, as additional information becomes
available. Information Notice No. 90-55: "Recent Operating Experience
of Loss of Reactor Coolant Inventory While in a Shut-down Condition"
describes similar events that occurred at Catawba Unit 1, Maine Yankee and
Braidwood Unit 2.
March 19, 1991
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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
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contacts: Robert A. Benedict, NRR Angie Young, NRR
(301) 492-1169 (301) 492-1167
Thomas A. Greene, NRR John W. Thompson, NRR
(301) 492-1175 (301) 492-1171
Attachment: List of Recently Issued NRC Information Notices
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