05000456/FIN-2008004-04
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Finding | |
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| Title | Bryozoan Infestation at the lake screenhouse circulating water forebays |
| Description | On September 2, 2008, during the performance of 1BwVSR 5.5.8.SX-1 ASME Surveillance Requirements for 1A Essential Service Water Pump, the 1A SX pump discharge strainer experienced high differential pressure. Shortly after the high differential pressure alarm annunciated, the licensee noted differential pressure across the discharge strainer exceeded twenty psid (normally less than six psid). Additionally, the SX system discharge header pressure dropped by 40 psig (normally 100 -105 psig) and system flow had decreased more than 3000 gpm. The licensee suspended the surveillance and entered TS LCO 3.7.8 Action Statement, A.1 for inoperable SX system. A licensee investigation into this issue identified that bryozoan colonies had accumulated in the circulating water forebays during the summer through deposition and subsequent growth. Bryozoans or moss animals are colonial organisms consisting of many similar connecting zooids, each with its independent food-gathering structure, mouth, digestive tract, muscles, nervous system and reproductive ability. They reproduce asexually creating a cyst like structure called a statoblast, which serves to reseed and spread these colonies. Most bryozoan colonies occur as flat encrustations or grow in upright arborescent patterns; however, a species of bryozoan found in Midwest lakes and the Mississippi River (Pectinatella Magnifica) secretes a gelatinous ball, which grows bigger as the colony increases its number. These massive colonies form jelly-like balls that can exceed 60 centimeters in diameter. The licensee indentified that the greatest accumulation of both dead and living bryozoans colonies on the floor of the forebays were downstream of the traveling screens and decreased significantly toward the north side of the forebays (closer to the circulating water pump intake). In Unit 1, the bryozoan colonies were between two to three feet deep downstream of each traveling screen in the forebay (east and west side). This was consistent on two of the forebays on Unit 1 (1B and 1C). The 1A forebay had a lower amount of bryozoan colonies, approximately one to six inches in depth. On Unit 2, bryozoan colonies were also two to three feet deep downstream of the east travel screens, but, significantly lower downstream of the west traveling screens, approximately two to six inches. The licensee approximated that the total biomass of bryozoan colonies on Unit 2 was lower than Unit 1 by 30 percent. During performance of 1BwVSR 5.5.8.SX-1, the control switch for the SX strainer backwash was placed in the OFF position. The SX strainer backwash function normally occurs automatically every eight hours or on high strainer differential pressure. The strainer backwash motor and isolation drain valve power supplies and control circuits are not safety-related or seismically qualified and following a seismic event these components could lose power. Specifically, the SX strainer motor is powered through an electrical panel with an automatic/manual control switch mounted within the SX pump room and if damage to internal electrical control panel circuits occurs during a seismic event, the SX strainer motor may not function. Weeks prior to the 1A SX pump surveillance test, operators noted indications of abnormal flow indication through the lake screenhouse forebays. Specifically, on August 17, 2008, Braidwood Unit 1 experienced a reduction of approximately two percent in circulation water flow. On August 22, 2008, Unit 2 experienced a nine percent reduction in circulating water flow. Shortly thereafter, Unit 1 experienced an additional six percent reduction in circulating water flow. The following day the Unit 1 circulating water flow return to normal and the Unit 2 began a slow trend toward the normal circulating water flow rate. On August 29, 2008, Unit 2s slow trend to normal flow rate stopped and circulating water flow rate remained four percent below normal. On August 25 and 31, 2008, while in its normal system alignment (pump ON and backwash strainer in automatic) the 1B SX system pump discharge strainer high differential pressure alarm annunciated. The automatic backwash feature was able to clear the high differential pressure (dp) condition experienced by the strainer in approximately 30 minutes. On August 26, 2008, the 2A SX pump discharge strainer high differential pressure alarm annunciated, again. The systems automatic backwash feature was able to clear the high dp condition. Subsequently, on September 4, 2008, the licensee attempted to run the 2A SX pump with the discharge strainer backwash motor out of service and unavailable. Following the initially increase in differential pressure (greater than four psid) the licensee attempted to manually backwash the discharge strainer using licensee procedure BwMP 3301, Manual Backwash Operations. The operators and mechanical maintenance staff were unsuccessful in performing the manual backwash of the strainer. The licensee swap from the 2A SX pump to the 2B SX pump after the discharge strainer differential pressure had quickly risen to 10.1 psid. The licensee declared the 2A SX pump inoperable and unavailable. The Inspectors reviewed the licensees CAP for previous Bryozoa-related issues and ascertained that in October of 2005; the licensee discovered the abnormal growth of an unnamed species of Bryozoa in the circulating water intake forebay locations. Shortly after the 1A circulating water pump was secured on October 2, 2005, the 2A and 2B SX pump discharge header pressure and the 2A SX strainer differential pressure high annunciated in the control room. Following the 2005 event, the licensee performed an equipment apparent cause evaluation (EACE) to address this issue. The EACE concluded that the apparent cause for the abnormal growth of bryozoans in the circulating water forebays was indeterminate. The EACE concluded that the impact of bryozoa to the station raw water systems (circulating water, non-safety related service water, fire protection and SX) had been minimal. This conclusion was because the monitored SX strainer backwashes operated normally, experienced normal differential pressures and performed SX strainer backwash cycles as required and as designed. The licensee risk assessment also concluded that based on the Unit 1 and Unit 2 SX system performance, as monitored via the Adverse Condition Monitoring Plan, the licensee assessed the consequences of bryozoan growth in the forebays as a low risk plant operations. The risk assessment also concluded that there was not a regulatory impact cased by the abnormal growth of bryozoans in the circulating water forebays. At the conclusion of the inspection period, the licensee completed inspections and cleanup of each lake screenhouse forebay. The licensee declared both the 1A and the 2A SX pump operable. The inspectors were reviewing the adequacy of the corrective actions identified following the 2005 Bryozoan colonies infestation. Additionally, the inspectors were evaluating the overall impact of the Bryozoan infestation on both trains of the essential service water system for both units. Pending completion of this review, the issue will remain an open URI. (URI 05000456/2008004-04, 05000457/2008004-04) |
| Site: | Braidwood  |
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| Report | IR 05000456/2008004 Section 1R22 |
| Date counted | Sep 30, 2008 (2008Q3) |
| Type: | URI: |
| cornerstone | Mitigating Systems |
| Identified by: | NRC identified |
| Inspection Procedure: | IP 71111.22 |
| Inspectors (proximate) | A Garmoe B Dickson J Heath M Mitchell M Munir M Perry R Ng R Skokowski R Winterb Dickson A Garmoea Garmoe B Dickson M Holmberg M Mitchell M Perry M Thorpe _Kavanaugh R Jones R Ng R Skokowski T Hartmanm Munir R Daley R Langstaff R Winter L Jones |
| INPO aspect | |
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Finding - Braidwood - IR 05000456/2008004 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (Braidwood) @ 2008Q3
Self-Identified List (Braidwood)
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