United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 88-37: Flow Blockage of Cooling Water to Safety System Components

                                  UNITED STATES
                          NUCLEAR REGULATORY COMMISSION
                             WASHINGTON, D.C.  20555

                                  June 14, 1988

Information Notice No. 88-37:  FLOW BLOCKAGE OF COOLING WATER TO 
                                   SAFETY SYSTEM COMPONENTS


All holders of operating licenses or construction permits for nuclear power 


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 

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 

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 

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.


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 

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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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 
                                                            distribute or 
                                                            operate radio-
                                                            graphic exposure 
                                                            devices and source

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 
                                                            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
Page Last Reviewed/Updated Friday, May 22, 2015