Bulletin 81-03: Flow Blockage of Cooling Water to Safety System Components by Corbicula Sp. (Asiatic Clam) And Mytilus Sp. (Mussel)

                                                       SSINS No.: 6820 
                                                       Accession No.: 
                                                       IEB 81-03 

                               UNITED STATES 
                           WASHINGTON, D.C. 20555 
                               April 10, 1981 

                    SP. (MUSSEL) 

Description of Circumstances: 

On September 3, 1980, Arkansas Nuclear One (ANO), Unit 2, was shut down 
after the NRC Resident Inspector discovered that Unit 2 had failed to meet 
the technical specification requirements for minimum service water flow rate
through the containment cooling units (CCUs). After plant shutdown, Arkansas
Power and Light Company, the licensee, determined that the inadequate flow 
was due to extensive plugging of the CCUs by Asiatic clams (Corbicula 
species, a non-native fresh water bivalve mollusk). The licensee 
disassembled the service water piping at the coolers. Clams were found in 
the 3-inch diameter supply piping at the inlet to the CCUs and in the cooler 
inlet water boxes. Some of the clams found were alive, but most of the 
debris consisted of shells. The size of the clams varied from the larvae 
stage up to one inch. The service water, which is taken from the Dardanelle 
Reservoir, is filtered before it is pumped through the system. The strainers 
on the service water pump discharges were examined and found to be intact. 
Since these strainers have a 3/16-inch mesh, much smaller than some of the 
shells found, it appears that clams had been growing in the system. 

Following the discovery of Asiatic clams in the containment coolers of Unit 
2, the licensee examined other equipment cooled by service water in both 
Units 1 and 2. Inspection of other heat exchangers in the Unit 2 service 
water system revealed some fouling or plugging of additional coolers (seal 
water coolers for both redundant containment spray pumps and one 
low-pressure safety injection pump) due to a buildup of silt, corrosion 
products, and debris (mostly clam shell pieces). The high-pressure safety 
injection (HPSI) pump bearing and seal coolers were found to have 
substantial plugging in the 1/2-inch pipe service water supply lines. The 
plugging resulted from an accumulation of silt and corrosion products. 

Clam shells were found in some auxiliary building room coolers and in the 
auxiliary cooling water system which serves non-safety-related equipment. 

Flow rates measured during surveillance testing through the CCUs at ANO-2 
had deteriorated over a number of months. Flushing after plant shutdown 
initially resulted in a further reduction in flow. Proper flow rates were 
restored only after the clam debris had been removed manually from the CCUs.

The examination of the Unit 1 service water system revealed that the "C" and
"D" containment coolers were clogged by clams. Clams were found in the 
3-inch inlet headers and in the inlet water boxes. However, no clams were 

                                                           IEB 81-03 
                                                           April 10, 1981 
                                                           Page 2 of 5 

in the "A" and "B" coolers. This fouling was not discovered during 
surveillance testing because there was no flow instrumentation on these 

The service water system in Unit 1 was not fouled other than stated above, 
and the licensee attributed this to the fact that the service water pump 
suctions are located behind the main condenser circulating pumps in the 
intake structure. It was thought that silt and clams entering the intake 
bays would be swept through the condenser by the main circulating pumps and 
would not accumulate in the back of the intake bays. In contrast, Unit 2 has 
no main circulating pumps in its intake structure because condenser heat is 
rejected through a cooling tower via a closed cooling system. As a result of
lower flowrates of water through the Unit 2 intake structure, silt and clams
could have a tendency to accumulate more rapidly in Unit 2 than in Unit 1. 
During the September outage, clams and shells were found to have accumulated
to depths of 3 to 4-1/2 feet in certain areas of the intake bays for Unit 2.

The Asiatic clam was first found in the United States in 1938 in the 
Columbia River near Knappton, Washington. Since then, Corbicula sp. has 
spread across the country and is now reported in at least 33 states. The 
Tennessee Valley Authority (TVA) power plants also have experienced fouling 
caused by these clams. They were first found in the condensers and service 
water systems at the Shawnee Steam Plant in 1957. Asiatic clams were later 
found in the Browns Ferry Nuclear Plant in October 1974 only a few months 
after it went into operation. This initial clam infestation at Browns Ferry 
was enhanced by the fact that, during the final stages of construction, the 
cooling water systems were allowed to remain filled with water for long 
periods of time while the systems were not in use. This condition was 
conducive to the growth and accumulation of clams. Since that time, the 
Asiatic clam has spread across the Tennessee Valley region and is found at 
virtually all the TVA steam-electric and hydroelectric generating stations. 

Present control procedures for Asiatic clams vary from station to station 
and in their degree of effectiveness. The use of shock chlorination during 
surveillance testing as the only method of controlling biofouling by this 
organism appears to be ineffective. The level of fouling has been reduced to
acceptable levels at TVA stations by using continuous chlorination during 
peak spawning periods, clam traps, and mechanical cleaning during station 

The results of a series of tests on mollusks performed at the Savannah River
facility showed that mature Corbicula sp. had as much as a 10 percent 
survival rate after being exposed to high concentrations of free residual 
chlorine (10 to 40 ppm) for up to 54 hours. When the clams were allowed to 
remain buried in a couple of inches of mud, their survival rates were as 
high as 65 percent. 

In studies on shelled larvae, approximately 200 microns in size, TVA 
reported preliminary results indicating that a total chlorine residual of 
0.30 to 0.40 ppm for 96 to 108 hours would be required to achieve 100 
percent control of the Asiatic clam larvae. 

                                                           IEB 81-03 
                                                           April 10, 1981 
                                                           Page 3 of 5 

Corbicula sp. has also shown an amazing ability to survive even when removed
from the water. Average times to death when left in the air have been 
reported for low relative humidity as 6.7 days at 30C (86F) and 
13.9 days at 20C (68F) and for high relative humidity as 8.3 days 
at 30C and 26.8 days at 20C. 

Corbicula sp. on the other hand, has shown a much greater sensitivity to 
heat. Tests performed by TVA resulted in 100 percent mortality of clam 
larvae, very young clams, and 2mm clams when they were exposed to 47C 
(117F) water for 2 minutes. Mature clams, up to 14mm, were also tested 
and all died at 47C following a 2 minute exposure. A statistical 
analysis of the 2 minute exposure test data revealed that a temperature of 
49C (120F) was necessary to reach the 99 percent confidence level 
of mortality for clams of the size tested. 

To date, heat has been shown to be the most effective way of producing 100 
percent mortality for the Asiatic clam. At ANO, the service water system was
flushed with 77C (170F) water obtained from the auxiliary boiler 
for approximately one half hour; 100 percent mortality was expected. 

A similar problem has occurred with mussels (Mytilus sp.). Infestations of 
mussels have caused flow blockage of cooling water to safety-related 
equipment at nuclear plants such as Pilgrim and Millstone. Unlike the 
Asiatic clam, mussels cause biofouling in salt water cooling systems. 

The event at ANO is significant to reactor safety because (1) the fouling 
represented an actual common cause failure, i.e, inability of safety system 
redundant components to perform their intended safety functions, and (2) the
licensee was not aware that safety system components were fouled Although 
the fouling at ANO-2 developed over a number of months, neither the licensee
management control system nor periodic maintenance or surveillance program 
detected the failure. 


Holders of Operating Licenses: 

1.   Determine whether Corbicula sp. or Mytilus sp. is present in the 
     vicinity of the station (local environment) in either the source or 
     receiving water body. If the results of current field monitoring 
     programs provide reasonable evidence that neither of these species is 
     present in the local environment, no further action is necessary except
     for items 4 and 5 in this section for holders of operating licenses. 

2.   If it is unknown whether either of these species is present in the 
     local environment or is confirmed that either is present, determine 
     whether fire protection or safety-related systems that directly 
     circulate water from the station source or receiving water body are 
     fouled by clams or mussels or debris consisting of their shells. An 
     acceptable method of confirming the absence of organisms or shell 
     debris consists of opening and visually examining a representative 
     sample of components in potentially affected safety systems and a 
     sample of locations in potentially affected 

                                                           IEB 81-03  
                                                           April 10, 1981 
                                                           Page 4 of 5 

     fire protection systems. The sample shall have included a distribution 
     of components with supply and return piping of various diameters which 
     exist in the potentially affected systems. This inspection shall have 
     been conducted since the last clam or mussel spawning season or within 
     the nine month period preceding the date of this bulletin. If the 
     absence of organisms or shell debris has been confirmed by such an 
     inspection or another method which the licensee shall describe in the 
     response (subject to NRC evaluation and acceptance), no further action 
     is necessary except for items 4 and 5 of actions applicable to holders 
     of an operating license. 

3.   If clams, mussels or shells were found in potentially affected systems 
     or their absence was not confirmed by action in item 2 above, measure 
     the flow rates through individual components in potentially affected 
     systems to confirm adequate flow rates i.e., flow blockage or 
     degradation to an unacceptably low flow rate has not occurred. To be 
     acceptable for this determination, these measurements shall have been 
     made within six months of the date of this bulletin using calibrated 
     flow instruments. Differential pressure (DP) measurements between 
     supply and return lines for an individual component and DP or flow 
     measurements for parallel connected individual coolers or components 
     are not acceptable if flow blockage or degradation could cause the 
     observed DP or be masked in parallel flow paths. 

     Other methods may be used which give conclusive evidence that flow 
     blockage or degradation to unacceptably low flow rates has not 
     occurred. If another method is used, the basis of its acceptance for 
     this determination shall be included in the response to this bulletin. 

     If the above flow rates cannot be measured or indicate significant flow
     degradation, potentially affected systems shall be inspected according 
     to item 2 above or by an acceptable alternative method and cleaned as 
     necessary. This action shall be taken within the time period prescribed
     for submittal of the report to NRC. 

4.   Describe methods either in use or planned (including implementation 
     date) for preventing and detecting future flow blockage or degradation 
     due to clams or mussels or shell debris. Include the following 
     information in this description: 

     a.   Evaluation of the potential for intrusion of the organisms into 
          these systems due to low water level and high velocities in the 
          intake structure expected during worst case conditions. 

     b.   Evaluation of effectiveness of prevention and detection methods 
          used in the past or present or planned for future use. 

5.   Describe the actions taken in items 1 through 3 above and include the 
     following information: 

     a.   Applicable portions of the environmental monitoring program 
          including last sample date and results. 

                                                           IEB 81-03 
                                                           April 10, 1981 
                                                           Page 5 of 5 

     b.   Components and systems affected. 

     c.   Extent of fouling if any existed. 

     d.   How and when fouling was discovered. 

     e.   Corrective and preventive actions. 

Holders of Construction Permits: 

1.   Determine whether Corbicula sp. or Mytilus sp. is present in the 
     vicinity of the station by completing items 1 and 4 above that apply to
     operating licenses (OL). 

2.   If these organisms are present in the local environment and potentially
     affected systems have been filled from the station source or receiving 
     water body, determine whether infestation has occurred. 

3.   Describe the actions taken in items 1 and 2 above for construction 
     permit holders and include the following information: 

     a.   Applicable portions of the environmental monitoring program 
          including last sample date and results. 

     b.   Components and systems affected. 

     c.   Extent of fouling if any existed. 

     d.   How and when fouling was discovered. 

     e.   Corrective and preventive actions. 

Licensees of facilities with operating licenses shall provide the requested 
report within 45 days of the date of this bulletin. Licensees of facilities 
with construction permits shall provide the report within 90 days. 

Provide written reports as required above, signed under oath or affirmation,
under the provisions of Section 182a of the Atomic Energy Act of 1954. 
Reports shall be submitted to the Director of the appropriate Regional 
Office and a copy forwarded to the Director, Office of Inspection and 
Enforcement, NRC, Washington, D.C. 20555. 

This request for information was approved by GAO under a blanket clearance 
number R0072 which expires November 30, 1983. Comments on burden and 
duplication should be directed to Office of Management and Budget, Room 
3201, New Executive Office Building, Washington, D.C. 20503. 

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