05000260/LER-2009-002
| Browns Ferry Unit 2 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded |
| 2602009002R01 - NRC Website | |
I. PLANT CONDITION(S)
Unit 2 was in Mode 4, performing a pressure vessel hydrostatic test with reactor pressure approximately 525 psig. Units 1 and 3 were operating in Mode 1 at 100 percent RTP (3458 megawatts thermal). Units 1 and 3 were unaffected by the event.
II. DESCRIPTION OF EVENT
A. Event:
On May 31, 2009, at approximately 1210 hours0.014 days <br />0.336 hours <br />0.002 weeks <br />4.60405e-4 months <br /> Central Daylight Time (CDT), during the performance of the Unit 2 pressure vessel hydrostatic test, ASME Section XI System Leakage Test of the Reactor Pressure Vessel and Associated Piping, 2-SI-3.3.1.A, TVA identified a leak on the Residual Heat Removal (RHR) [BO] Shutdown Cooling root valve [RTV] that could not be isolated. Following the confirmation that the leak was a through wall leak associated with an ASME Class I pressure boundary the test was terminated. Operations maintained the reactor in Mode 4 in accordance with Technical Requirements Manual (TRM) Action Statement 3.4.3.A.1.
On May 31, 2009, TVA initially reported this to the NRC under 10 CFR 50.72(b)(3)(ii)(A) as a leak in a weld between a 0.75-inch diameter leak-off line attached to the RHR Shutdown Cooling Root Valve (2-SHV-074-049). However, following the removal of the insulation, additional inspections determined that the leak-off line had been previously removed and a tapered plug was installed in its place. A through-wall leak was found at the interface weld between the tapered plug and the valve body. On June 2, 2009, TVA provided NRC with a revision to the May 31, 2009, 10 CFR 50.72(b)(3)(ii)(A) report.
TVA is submitting this report in accordance with 10 CFR 50.73(a)(2)(ii)(A) as an event or condition that resulted in the nuclear power plant, including principal safety barriers, being seriously degraded.
B. Inoperable Structures, Components, or Systems that Contributed to the Event:
None.
C. Dates and Approximate Times of Major Occurrences:
May 31, 2009� 1210 hours0.014 days <br />0.336 hours <br />0.002 weeks <br />4.60405e-4 months <br /> CDT� Identified leak.
May 31, 2009 1639 hours0.019 days <br />0.455 hours <br />0.00271 weeks <br />6.236395e-4 months <br /> CDT� TVA made an 8-hour non-emergency report per 10 CFR 50.72(b)(3)(ii)(A).
June 2, 2009� 1739 hours0.0201 days <br />0.483 hours <br />0.00288 weeks <br />6.616895e-4 months <br /> CDT� TVA provides an update to May 31, 2009, 8-hour non-emergency report.
D. Other Systems or Secondary Functions Affected
None.
E. Method of Discovery
TVA identified the leak through visual inspection during a scheduled performance of 2-SI-3.3.1.A.
F. Operator Actions
None.
G. Safety System Responses
None.
III. CAUSE OF THE EVENT
A. Immediate Cause
The immediate cause for the through wall leak was a flaw in the interface weld between the tapered plug and the 2-SHV-074-049 body.
B. Root Cause
A weld, performed in February of 1992, on the valve body and the tapered plug contained a defect. The root pass was made using the gas-tungsten process. This process was the industry standard in 1992. TVA postulates that a small amount of moisture that was present during the weld process caused a defect in the weld that was not identified during non destructive examination (NDE). The flaw slowly propagated and resulted in the failure. The leak was detected on May 31, 2009, during the performance of 2-SI-3.3.1.A.
C. Contributing Factors
None.
IV. ANALYSIS OF THE EVENT
In February of 1992, NA shut down Unit 2 due to excessive leakage in the drywell. The follow-up inspection identified a leak in the weld between a test pipe nipple and 2-SHV-074-049 valve body.
The test nipple was removed and a tapered plug was installed in the valve body. During the repair, water intrusion between the valve and the tapered plug prevented proper moisture-free weld site preparation. To combat the moisture issue, TVA installed an o-ring on the tapered plug. The o-ring was intended to reduce the leakage long enough to allow the completion root pass. TVA completed the root pass of the weld using the gas-tungsten process. This process was the industry standard at the time the repair was made. Following successful inspection of the weld root pass, the weld was completed and final inspections performed.
On May 2, 2009, NA repaired the weld made in February 1992. After excavating the original weld, a root pass was performed using the shielded-metal arc process. This process is less susceptible to moisture intrusion than the gas-tungsten and is now the industry standard. The completion of the weld activity was followed by successful completion of 2-SI-3.3.1.A.
V. ASSESSMENT OF SAFETY CONSEQUENCES
The safety consequences of this event were not significant. Plant Technical Specifications (TSs) require monitoring of reactor coolant leakage. When leakage limits are exceeded, the TSs require the reactor be placed in Mode 4. During the previous operating cycle, reactor coolant leakage had been less than the TS limits.
The visual inspection, performed as part of 2-SI-3.3.1.A, identified the through-wall leak at the tapered plug and the valve body welded interface. BFN entered TRM Condition 3.4.3, Structural Integrity, Condition A.1, which requires either immediate restoration of structural integrity of the affected component or maintaining the reactor in Mode 4, or the reactor coolant system less than 50 degrees F above the minimum temperature required by nondestructive testing considerations, until each indication has been investigated and evaluated. Until completion of repairs, BFN maintained the reactor in accordance with these requirements. Therefore, TVA concludes that this event did not affect the health and safety of the public.
VI. CORRECTIVE ACTIONS
A. Immediate Corrective Actions
Once it was determined that the leak was part of the ASME Class I pressure boundary, Operations maintained the reactor in Mode 4.
B. Corrective Actions to Prevent Recurrence (1) WA repaired the weld. The completion of the weld repair was followed by successful completion of 2-SI-3.3.1.A.
VII. ADDITIONAL INFORMATION
A. Failed Components
None.
B. Previous LERs on Similar Events None.
C. Additional Information
Corrective action document for this report is Problem Evaluation Report 172551.
D. Safety System Functional Failure Consideration:
This event is not considered a safety system functional failure according to NEI 99-02.
E. Scram With Complications Consideration:
This event did not result in a complicated scram according to NEI 99-02.
VIII. COMMITMENTS
None.
TVA does not consider this corrective action a regulatory requirement. TVA will track the completion of this action in the Corrective Action Program.