ML19284A996
| ML19284A996 | |
| Person / Time | |
|---|---|
| Site: | Arkansas Nuclear  |
| Issue date: | 10/21/1980 |
| From: | Michelson C NRC OFFICE FOR ANALYSIS & EVALUATION OF OPERATIONAL DATA (AEOD) |
| To: | Harold Denton, Stello V NRC OFFICE OF INSPECTION & ENFORCEMENT (IE), Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML19248B746 | List: |
| References | |
| FOIA-81-362, TASK-AE, TASK-E016, TASK-E16 AEOD-E016, AEOD-E16, NUDOCS 8011060029 | |
| Download: ML19284A996 (5) | |
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MEMORANDUM FOR:
Harold R. Denton, Director Office of Nuclear Reactor Regulation Victor Stello, Jr., Director C" ice of Inspection and Enforcement FROM:
Carlyle Michelson, Director Office for Analysis and Evaluation of Operational Data
SUBJECT:
FLOW BLOCKAGE IN ESSENTIAL EQUIPMENT AT AN0 CAUSED BY CORBICULA sp. (ASIATIC CLAMS) e Recently, AE00 has initiated a case study to gather information and develop recommendations to prevent potential flow blockage in redundant safety-related cooling water systems resulting from tt e grcwth of Asiatic Clams in these systems. Members of my staff along with a fisheries biologist from the Environmental Engineering Branch of NRR have visited ANO to discuss the problems encountered with clams in their Service Water System (SWS).
The purpose of this memo is to report our findings to date so that they may be used by the responsible program offices as appropriate. The plant specific information presented herein was gathered by informal discussion with the licensee and should be regarded as preliminary.
The AEOD study on this issue is still ongoing.
BACKGROUND Corbicula species (sp.), or the Asiatic Clam as it is commonly called, is a non-native bivalve (two hinged shells) mollusc.
It was first found in the U.S. in 1938 on the northern shore of the Columbia River near Knappton, Washington.
Since that time, Corbicula sp. have rapidly spread across the country and are now reported in at least 27 states. The species of Asiatic Clam introduced into this country is tyoically found in freshwater but there are several other species. found in Asia, that prefer brackish water. There is presently some disagreement among experts as to which species is present e
in the United States.
The possibility also exists that what is typically -
called the Asiatic Clam in this country may represent several species.
The fresh water clams, such as have been found at AN0 and Browns Ferry, are monoecious (bisexual). The adult clam reportedly releases planktonic larvae ranging in size from 200-240 microns.
In a relatively short time, the larvae cease to drift in the water and settle on the substrate (river bed, pipe, etc.).
Within a month they begin to attach themselves to the substrate by means of a secreted byssus or threadlike holdfast. As the clams mature this byssus dissolves.
1,f the substrate is muddy or silty, the clams will reside in the top 2 to 3 inches but they are thought to be otherwise unattached and free to be swept along in the flow of water.
There seems to be some controversy over this point, however, since nature clams have been found " attached" to the upper inside surface of horizontal pipes even after the pipes were drained.
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i i Corbicula sp. reach sexual maturity within the first year. The peak spawning season occurs when.the water temperature is between 62 F and 75*F, typically in liay and September at AN0 and Browns Ferry.
One adult clam can release many thousands of larvae in one season at a rate of 300 to 400 per day during the peak. Corbicula sp. have a life expectancy of 2-4 years, can grew up to 60mm in length and have proven to be very hardy.
Studies pe-formed on these clams have shown them to be resistant to chlorination.
The results of a series of tests performed at the Savannah River Facility showed that mature Corbicula sp. had as much as a 10 percent survival rate af ter being exposed to high concentrations of free residual chlorine (10-4Cppm) for up to 54 hours6.25e-4 days <br />0.015 hours <br />8.928571e-5 weeks <br />2.0547e-5 months <br />. When the clams were allowed to remain buried in a couple inches of mud, their survival rates were as high as 65 percent.
In studies 01 shelled larvae, approximately 200 microns in size, TVA reported preliminary results which indi-cited that a total chlorine residual of 0.30-0.40 ppm for 96 to 108 hours0.00125 days <br />0.03 hours <br />1.785714e-4 weeks <br />4.1094e-5 months <br /> would be required to achieve 100 percent control of the Asiatic Clam 'arvae.
Nuclear power plants generally chlorinate intermittently accordinL tc a predeter-mined schedule (e.g., Monday, Wednesday and Friday for 15 minutesi. Sodium hypo-chlorite, typically used for chlorination, is injected at the intake structure until a total free chlorine level of 1 ppm is measured at the system discharge.
In view of the information above, it appears that the chlorination procedures presently followed by nuclear power plants are ineffective in controlling Asiatic Clans.
Corbicula sp. has also shown an amazing ability to survive even when renoved from the water.
Average times to death when left in the air have been reported for low relative humidity as 6.7 days at 30*C (86 F) and 13.9 days at 20 C (68*F) and for high relative humidity as 8.3 days at 30 C and 26.8 days at 20 C.
Corbicula sp. on the other hand has shown a much greater sensitivity to heat.
Tesis performed by TVA resulted in 100 percent mortality of clan larvae, very young clams and ?mm clams when they were exposed to 47 C (117 F) water for two minutes. Mature clams, up to lamm, were also tested and all died at 47*C follow-ing a two minute exporure. A statistical analysis of the two minute exposure test data revealed that a temperature of 49*C (120 F) was necessary to reach the 99 percent confidence lavel of mortality for clams of the size tested.
To date, heat has been shown to be the most effective way of producing 100 percont mortality for the Asiatic Clam. At ANO, the SWS was flushed with 77 C (170 F) water obtained from the auxiliary boiler for approximately one half hour,100 percent mortality was expected.
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f 9 AND ANO Unit 2 was shutdown on September 4,1980 af ter failing to meet the minimum service water flow-rate through the containment air coolers specified in the Technical Specifications. The Unit 2 containment air coolers are redundant to
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the containment spray system and function both during normal operation and following the LOCA to remove heat from the containment.
Two supplies of cooling water are provided to these coolers, chilled water is used during normal operations and service water is used following a LOCA.
The two sources of cooling water are supplied to independent cooling coils and during normal plant operation there is no service water flow through the coils served by the SWS.
In order to ascertain the reason for low service water flow in the Unit 2 containment coolers, the licensee disassembled the service water piping on the supply and return side of the containment coolers.
The piping on the return side was found clean and no clams were found in the outlet of the coolers. On the inlet side, clams were found in the 3" service water headers that supply the coolers as well as in the cooler inlet waterboxes.
Clams were removed from the A, C, and D containnent coolers.
The B containment cooler was removed from service on March 17, 1980 and has been blank flanged.
The clams found in the containment coolers were comprised of some live clams but most of the debris was shells. The size of the clams was about 15 to 16mm j
(approximatley 5/8 inch). The strainers on the service water pump discharges were examined and found to be intact.
Since these strainers are 3/16" mesh, much smaller than some of the shells found, it indicates that clams have 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 Unit 1 and Unit 2.
In Unit 2, clam shells were also found in the seal water coolers of both redundant containment spray pumps and the seal water cooler of one of the low pressure safety injection pumps. Clam shells and some live clans were found in the Auxiliary Cooling Water System ( ACWS) which serves non-safety related equipment in the turbine building. The ACWS is a continuously running system as contrasted to the SWS in which most components are isolated during normal power operation. The "B" high pressure injection pump seal water coolers, while free from clams, were found to be clogged with silt. The piping to the "B" high pressure injection pump coolers,1/2" nominal pipe size, was all e
replaced as a result of the silt problem, It was noted that the high pressure injection pumps are horizontal pumps with the impeller between the shaf t bearings. These pumps have four coolers, piped in parallel, that are supplied by service water, 2 bearing oil coolers and 2 i
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'I seal water coolers.
Each high pressure injection pump is provided with inttrumentation to measure differential pressure across the four coolers, how-ever, the pressure taps are located such that the indicated pressure drop is that across the four coolers in parallel rather than each individual cooler.
Being vertical pumps, the low pressure injection pumps and the containment spray pumps each have only a single seal cooler. These seal coolers are also provided with instrumentation to measure differential pressure.
i It was stated by the licensee that the differential pressure across the seal and bearing coolers on these pumps is not recorded as part of the monthly sur-veillance program.
I The examination of the Unit 1 SWS revealed that the CSD containment coole were clogged by clams.
Clams were found in the 3 inch inlet headers and in the inlet waterboxes, however, no clams were found in the A&B coole s.
i This was not discovered during surveillance testing since there is no flow instru-mentation on these coolers.
Further investigation revealed that the service water strainer serving the A&B coolers was intact while the one serving the C&D coolers was broken. The licensee reasoned that the clams found in the C&D coolers did not grow in the system but were swept in through the broken strainer.
The SWS of Unit 1 was free of clams other than stated above and the licensee attributed this to the fact that the service water pum.,
uctions 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.
The situation was worse at Unit 2 and the licensee attributed this to the fact that Unit 2 having a cooling tower, has no main circulating pumps in its intake structure, therefore, silt and clams could have a tendency to accumulate more rapidly than in Unit 1.
In addition, it appeart u.at only 2 of the 3 Unit 2 intake bays were cleaned within the last They ars normally scheduled to be cleaned each refueling outage, how-year.
ever, AND-2 is in their first fuel cycle.
During this outage, the intake bays were cleaned and an estimated 150 to 500 cubic feet of clams and silt was removed.
As at Unit 2, the Unit 1 intake bays are scheduled to be cleaned during every refueling outage.
PRELIMINARY RECOMMENDATIONS 4
In view of the fact that clams and silt are a potential mechanism for common mode failure, this office recommends that consideration be given to the install-ation of flow instrumentation for each essential component supplied with service water where such does not already exist.
Although we have not performed a com-plete systems review, examples of components for whic'h ficw indication should be considered are the Unit 1 containment air coolers and the individual seal and w
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bearing coolers on the Unit 2 high pressure injection pumps. Once provided, it is also recommended that this flow indic tion be periodically monitored and a
included in the surveillance requirements specified in the plant Technical Specifications.
It has been noted that, in some instances, the reporting of events by licensees contain the end effect and not the cause. Thir is particularly true in cases where low flow is due to system fouling; either by marine growth (e.g., clams or mussels), buildup of corrosion products, or accumulation of silt, to name some of the more common fouling mechanisms. This lack of data has hindered the evaluation of the scope and magnitude of this problem that may face many operating reactors. To fill this gap in information, it is requested that the responsible program offices obtain from the licensees, by whatever means appropriate, their operating history regarding problems L cutages caused by fouling of cooling water systems and whether biological monitoring has ever revealed the presence of clams, mussels or other potentially troublesome marine growth in either the source or receiving waterbody.
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Of ice for Analysi(s and Evaluation C
le Michelson Jirector of Operational Data Contact E. V. Imbro J. M. Giannelli cc:
E. Jordan E. Blackwood G. Lainas J. Olshinski G. Holahan D. Pickett W. Johnson R. Martin G. Vissing E. Adensam W. Dircks w
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