Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
July 2, 1992
NRC INFORMATION NOTICE 92-49: RECENT LOSS OR SEVERE DEGRADATION OF
SERVICE WATER SYSTEMS
Addressees
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to recent operating experience problems involv-
ing the loss or potential loss of safety-related heat transfer capability in
service water systems. 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 are not NRC requirements; therefore, no specific
action or written response is required.
Description of Circumstances
Nine Mile Point, Unit 1, February 21, 1992. When performing post-maintenance
testing while the reactor was shut down, the licensee, Niagara Mohawk Power
Corporation, inadvertently isolated the ultimate heat sink by closing all
gates to the SWS inlet bay. Because one SWS pump and two circulating water
pumps were running, the water level in the bay rapidly decreased. For about
6 minutes, the level was below that assumed in the licensing basis and below
the minimum level necessary to maintain net positive suction head for any of
the SWS pumps in the bay. The running SWS pump cavitated; the licensee
started the emergency SWS pump as required by procedures, but then had to
stop it because of low discharge pressure (NRC Augmented Inspection Team
(AIT) Report 50-220/92-80).
The licensee had aligned the gates in the intake SWS bay in an off-normal
configuration for reverse flow to allow post-maintenance testing of the gate
D opening circuit. The licensee uses the reverse flow configuration to
prevent icing during winter months. The maintenance included the removal of
an undocumented electrical jumper used to bypass the mechanical tension
overload protection switch in the drive motor circuit. The licensee did not
know if the gate could be opened or closed during reverse flow operation with
the jumper removed. After closing the gate, the licensee could not then
reopen it. Within 2-3 minutes the level decreased to a point where neither
the normal SWS pumps nor the emergency SWS pumps could maintain adequate
suction.
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The licensee promptly opened gate D by jumpering the tension overload switch
and also opened one of the normal lineup inlet gates. The gates take about
5 minutes to fully open. The intake bay level returned to normal within
6 minutes and both emergency SWS pumps were successfully started within
another 3 minutes.
The AIT concluded that the root causes of this event were: the failure to
follow the established work control process, inadequate management oversight,
inadequate communications within and among organizations participating in the
work activities, and an insensitivity to shutdown risk among multiple
licensee organizations.
Arkansas Nuclear One, Unit 2, April 16, 1991. The licensee, Arkansas Power
and Light Company, declared both loops of the safety-related SWS inoperable
with the reactor in startup conditions. Debris from the lake, the normal
supply of cooling water, had bypassed the screens at the pump suctions and
clogged the pump discharge strainers of both operating loops. Fortunately,
the standby SWS pump was not operating at the time and its discharge strainer
remained free of debris. The licensee switched the suction of the standby
SWS pump to its emergency source and started the pump within about 3 minutes.
The licensee restored the clogged loops to operable status within about
2 hours (LER 50-368/91-12).
The loss of both SWS trains meant that cooling was not available to
engineered safety features equipment and component cooling water heat
exchangers, which cool systems carrying fluids that may be radioactive. This
condition resulted when maintenance personnel performed sections of the
procedure for rotating the traveling screens out of sequence and, thus,
allowed screen rotation without wash water flow. Consequently, the flow of
debris bypassed the traveling screens and entered the suction of the two
operating SW pumps. Had the standby SW pump discharge strainer become
clogged, the event would have been much more severe.
Ineffective communication between operations and maintenance personnel
prevented a complete understanding of the operation in progress at the time.
The licensee took steps to strengthen management control and the training of
personnel in this procedure and in communications.
Fitzpatrick, October 19, 1990. The licensee, Power Authority of the State
of New York, manually scrammed the reactor from 45 percent of full power
because the fouling rate for the circulating water traveling screens exceeded
the cleaning rate of the screen wash system. A shift in wind direction
contributed to an unusually large debris accumulation on the screens. Shear
pins on the two operating screens failed. As the screens bowed inward
because of the high differential pressure, some of the debris floated around
the screens. The licensee scrammed the reactor to mitigate this degrading
condition (LER 50-333/90-23).
In this event, while performing maintenance on one of the traveling screens,
personnel unintentionally disabled the screen differential pressure alarm
system, which would have provided early indication of fouling. The licensee
determined the root cause of the event to be that the applicable operating
and. IN 92-49
July 2, 1992
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maintenance procedures did not specify the need to isolate the differential
pressure instrument system from only the specific intake bay that is drained.
Millstone Unit 1, October 4, 1990. The licensee, Northeast Nuclear Energy
Company, manually tripped the reactor from 45 percent of full power because
of circulating water system and service water system fouling that resulted
in degraded SWS cooling, which resulted in increased containment temperature
and pressure. Storm-induced high winds and seas caused an excessive amount
of seaweed to accumulate on the traveling screens of the circulating water
system. After first questioning the off scale indication of the differential
pressure instruments, the licensee stopped two of the four operating
circulating water pumps to relieve stress on the screens, but the relief was
insufficient to prevent three of the five screens from collapsing. The two
operating pumps reduced the water level in the intake structure bays, which
caused the operating SWS pumps to cavitate. This condition decreased
SWS pressure, degraded the performance of the reactor building closed cooling
water heat exchanger, increased the containment temperature and pressure, and
decreased the main condenser vacuum (LER 50-245/90-16).
The SWS provides cooling to the turbine and the reactor building closed
cooling water heat exchangers and to the heat exchangers for the diesel
generators. The emergency SWS provides long-term cooling to the suppression
pool during a loss-of-coolant accident (LOCA). The licensee noted that the
concurrent loss of these systems with a LOCA is outside the design basis for
Millstone Unit 1.
The licensee concluded that if all the circulating water pumps had been
tripped on increasing differential pressure, the three damaged screens might
not have been breached, and SWS performance might not have been degraded.
The licensee delayed its decision to trip circulating water pumps because
control room personnel had not been informed that plant equipment operations
personnel had disabled all the screens for manual cleaning. The control room
personnel did not trip all circulating water pumps as required by the
applicable operating procedure.
Discussion
"Operating Experience Feedback Report - Service Water System Failures and
Degradations in Light Water Reactors," NUREG-1275, Volume 3, November 1988,
summarized and discussed service water system (SWS) events from 1980 to
early 1987. Generic Letter 89-13, "Service Water System Problems Affecting
Safety-Related Equipment," July 18, 1989, requested specific licensee actions
to resolve SWS problems.
The Nine Mile Point Unit 1 event shows that personnel errors and failure to
follow procedures can cause the safety-related SWS to become inoperable.
The other 3 events are examples in which intake debris, caused by adverse
environmental conditions, together with personnel errors, either caused or
could have caused the safety-related SW system to become inoperable. All
four events illustrate that recovery strongly depends on human action,
particularly with respect to following procedures and accurately
communicating information.. IN 92-49
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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 Office of
Nuclear Reactor Regulation (NRR) project manager.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical contacts: John Thompson, AEOD
(301) 492-8091
Vern Hodge, NRR
(301) 504-1861
James Tatum, NRR
(301) 504-2805
Attachment: List of Recently Issued NRC Information Notices
.
