Information Notice No. 88-37: Flow Blockage of Cooling Water to Safety System Components
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
June 14, 1988
Information Notice No. 88-37: FLOW BLOCKAGE OF COOLING WATER TO
SAFETY SYSTEM COMPONENTS
Addressees:
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose:
This information notice is being provided to alert addressees to a potentially
generic problem involving flow blockage in safety-related piping interconnec-
tions due to biofouling. This condition may occur and not be detected due to
stagnant water in system interconnecting piping which is not routinely flushed
or flow tested. The event described highlights the importance of maintaining
these lines free of clams, corrosion, and other foreign material. It is ex-
pected 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 9, 1988, Catawba Unit 2 tripped from 20 percent power as the unit was
returning to full-power operation following the first refueling outage. As
the feedwater regulating valves were being placed in automatic, the B steam
generator (SG) feedwater regulating valve failed open. This caused a
feedwater swing, and, after the operator assumed manual control of the B steam
generator feedwater regulating valve, a high-high level in the D SG, which
resulted in a main turbine trip, a main feedwater isolation, and a main
feedwater pump trip.
The Catawba auxiliary feedwater (AFW) system is safety grade, designed to
start automatically in the event of loss-of-offsite power, trip of main
feedwater pumps, a safety injection signal, or low-low SG level. Three AFW
pumps are provided, powered from separate and diverse power sources. Two
motor-driven pumps are powered from two separate trains of emergency onsite
electrical power, each normally supplying two SGs. A turbine driven AFW pump
is driven by steam generated from either of two SGs. The condensate storage
system is utilized for normal AFW supply and includes the main condenser
hotwell, two upper surge tanks per unit, and a shared AFW condensate storage
tank (CACST). However, the condensate storage system is not safety grade.
Therefore, the safety grade nuclear service water (NSW) system provides an
assured backup
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water supply. The NSW system is supplied from either Lake Wylie or the
standby nuclear service water pond (SNSWP). In the event of partial or
complete loss of the condensate storage supply, as sensed by two out of three
indications of low suction pressure from any train, automatic swapover to the
NSW system is provided. The NSW system also provides assured makeup to
several other systems, including the fuel pool cooling and component cooling
systems.
Both motor-driven auxiliary feedwater (AFW) pumps automatically started upon
loss of the main feedwater pumps. An A train AFW pump low suction pressure
resulted in the automatic swapover of train A AFW pump suction from condensate
grade water to nuclear service water (NSW) causing valves RN 250A and CA 15A
to open. The attached Figure 1 provides a schematic of the AFW flow paths
prior to suction flow swapover, while Figure 2 provides the flow paths after
swapover. The A train AFW pump normally supplies AFW to SGs A and B.
Meanwhile, levels in SGs A and B were dropping. The low-low level set point
was reached in SG A and a reactor trip occurred. Approximately 13 minutes
into the event, the operator noted that the NSW valve to the AFW system (RN
250A) had opened. The valve was closed immediately. However, at that time it
was not noted that the A train AFW pump suction line valve from the NSW system
(CA 15A) also had opened and that NSW had therefore been pumped into SGs A and
B.
Approximately 20 minutes into the event, it was noted that the SG B level was
decreasing. Initial AFW flow had been normal (300 gpm per SG), but had de-
graded to approximately 200 gpm to SG A and 100 gpm to SG B. The flow control
valve to SG A was fully open at this time. During shift turnover,
approximately 35 minutes into the event, the oncoming shift found that valve
CA 15A had also opened. The main feedwater pumps were then restarted and the
AFW system placed on standby. Actions were initiated to disassemble and
inspect the AFW flow control valves. When the A and B SG AFW flow control
valves were disassembled, it was determined that the valves were clogged with
Asiatic clam shells.
Discussion:
Fouling of service water systems has been recognized for some time as having
the potential for compromising system operability. Information Notice 81-21,
"Potential Loss of Direct Access to Ultimate Heat Sink," and Bulletin 81-03,
"Flow Blockage of Cooling Water to Safety System Components by Corbicula Sp.
(Asiatic Clam) and Mytilus Sp. (Mussel)," issued by the NRC Office of
Inspection and Enforcement (IE) addressed the potential for fouling of
safety-related heat exchangers by Asiatic clams, mussels, and debris from
other shell fish. Information Notice 86-96, "Heat Exchanger Fouling Can Cause
Inadequate Operability of Service Water Systems," addressed similar fouling
problems from buildup of mud, silt, and corrosion in NSW piping. The AFW flow
control valves at Catawba have an anti-cavitation trim that can collect
particles of clam shells and may thus exacerbate flow degradation from
fouling.
Duke Power Company has in place several programs and practices designed to
verify NSW flow to various systems and components. These include (1) periodic
flow balancing of the NSW system, (2) testing to verify heat transfer
capability of essential heat exchangers served by the NSW system, (3) periodic
cleaning of
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June 14, 1988
Page 3 of 3
heat exchangers based on differential pressure indications and, 4) visual
examination of heat exchangers and related piping during cleaning for the
presence of clams and unusual fouling conditions. Previous inspection of the
NSW system dead leg piping for clams consisted of spot radiographic (RT)
inspection of low spots in the piping between the NSW and AFW systems. None
of the RT inspections revealed any clams. The valves in the NSW/AFW suction
swapover lines had been periodically stroke tested, but no flow through the
lines was established.
As a result of the Catawba Unit 2 AFW swapover from condensate to NSW and the
introduction of raw NSW containing clams into the AFW system, Duke Power
Company initiated a program of flushes and inspections of dead legs between
the NSW system and various safety-related systems. The NSW system flushes and
inspections performed included the NSW backup supply to AFW system piping, NSW
emergency makeup to component cooling system piping, and NSW backup to
penetration valve injection piping. Flushing also was planned on the NSW
emergency makeup to spent fuel pool cooling piping. Spot radiographic
inspection indicated the presence of clams in the piping to the component
cooling and spent fuel pool cooling piping prior to flushing. These locations
were not previously radiographed.
No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact one of the techni-
cal contacts listed below or the Regional Administrator of the appropriate NRC
regional office.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contacts: Thomas Peebles, RII
(404) 331-4196
Brian Bonser, RII
(404) 331-4198
W. Lefave, NRR
(301) 492-0862
Attachment: List of Recently Issued NRC Information Notices
. Attachment
IN 88-37
June 14, 1988
Page 1 of 1
LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
_____________________________________________________________________________
Information Date of
Notice No._____Subject_______________________Issuance_______Issued to________
88-36 Possible Sudden Loss of RCS 6/8/88 All holders of OLs
Inventory During Low Coolant or CPs for PWRs.
Level Operation
88-35 Inadequate Licensee Performed 6/3/88 All holders of OLs
Vendor Audits or CPs for nuclear
power reactors.
88-34 Nuclear Material Control 5/31/88 All holders of OLs
and Accountability of or CPs for nuclear
Non-Fuel Special Nuclear power reactors.
Material at Power Reactors
87-61, Failure of Westinghouse 5/31/88 All holders of OLs
Supplement 1 W-2-Type Circuit Breaker or CPs for nuclear
Cell Switches power reactors.
88-33 Recent Problems Involving 5/27/88 All Agreement
the Model Spec 2-T States and NRC
Radiographic Exposure licensees
Device authorized to
manufacture,
distribute or
operate radio-
graphic exposure
devices and source
changers.
88-32 Promptly Reporting to 5/25/88 All NRC material
NRC of Significant licensees.
Incidents Involving
Radioactive Material
88-31 Steam Generator Tube 5/25/88 All holders of OLs
Rupture Analysis or CPs for
Deficiency Westinghouse and
Combustion
Engineering
designed nuclear
power plants.
88-30 Target Rock Two-Stage 5/25/88 All holders of OLs
SRV Setpoint Drift or CPs for nuclear
Update power reactors.
_____________________________________________________________________________
OL = Operating License
CP = Construction Permit
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