05000366/LER-2007-001, Regarding Main Steam Isolation Valves Fail Local Leak Rate Testing Due to Out of Specification Internal Tolerances
| ML071020496 | |
| Person / Time | |
|---|---|
| Site: | Hatch  |
| Issue date: | 04/11/2007 |
| From: | Madison D Southern Nuclear Operating Co |
| To: | Document Control Desk, NRC/NRR/ADRO |
| References | |
| NL-07-0707 LER 07-001-00 | |
| Download: ML071020496 (6) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii) |
| 3662007001R00 - NRC Website | |
text
D. I.
Wadiiron (DonRir)
SouU~om Nucluf Vim Resident - Hatch O p m t i ~ g collpuy, IDC.
Plant Edwin I. Hatch 11028 Hatch Parkway. North Baxley. Georgia 31513 Tel912.537.5859 April 1 1,2007 Fax 912.366.2077 Docket No.:
5@366 U. S. Nuclear ReguIatoty Commission ATTN: Document Control Desk Washington, D. C. 20555-0001 Edwin 1. Hatch Nuclear Plant - Unit 2 Licensee Event Report 2-2007-001 Main Steam Isolation Valves Fail Local Leak Rate Testing D
k u
Ladies and Gentlemen:
In accordance with the requirements of 10 CFR 50.73(a)(2)(ii), Southern Nuclear Operating Company (SNC) is submitting the enclosed Licensee Event Report concerning main steam isolation valves that failed local leak rate testing due to out-of-specification internal tolerances.
This letter contains no NRC commitments. If you have any questions, please advise.
Sincerely,
/ D. R. Madison Vice President - Hatch Edwin I. Hatch Nuclear Plant 1 1028 Hatch Parkway North Baxley, GA 3 1513 cc:
c y
Mr. J. T. Gasser, Executive Vice Resident Mr. D. H. Jones, Vice President - Engineering RTYPE: CHA02.004 U. S. Nuclear Redatow Commission Dr. W. D. Travers, Regional Administrator Mr. R. E. Martin, NRR Project Manager - Hatch Mr. D. S. Simpkins, Senior Resident Inspector - Hatch
LICENSEE EVENT REPORT (LER)
(See reverse for required number of Valves (MSIVs). These tests determined, for both the 'B' penetration (2B21-F022B (inboard) and 2B21-F028B (outboard)) and the 'C' penetration (2B2 1-F022C (inboard) and 2B2 1-F028C (outboard)), that the minimum pathway leakage for each of these penetrations exceeded the values specified in the plant's Technical Specifications. The maximum leakage rate allowed for all of the main steam lines is 250 standard cubic feet per hour (scfh). The as-found measured LLRT minimum pathway leakage for the 'B' penetration was 294 scfh, and for the 'C' penetration was 280 scfh. The total minimum pathway leakage through all four main steam lines was The most likely direct causes of the MSIV minimum pathway leakages exceeding the values specified in the Technical Specifications were out-of-specification internal valve tolerances and dimensions. It was determined that the probability of the MSIVs seating leak-tight will be reduced if the clearances between the in-body valve guides and the main disc are too large, or if anomalies in the seating areas for these valves exist, such as incorrect seating angle, width of the seating surface, or high or low spots in the seating surfaces. Internal inspection of the MSIVs determined that one or more of these conditions existed in each of the four referenced MSIVs. Each of the anomalies identified were corrected by machining or by installing an oversize disc that reduced the clearances in the valve. Review of the plant's maintenance practices determined that checking these clearances was not part of the plant's normal maintenance activities. The applicable plant procedure has been revised to address this issue.
NRC FORM 386 (6-2004)
PLANT AND SYSTEM IDENTIFICATION
General Electric - Boiling Water Reactor Energy Industry Identification System codes appear in the text as (EIIS Code XX).
DESCRIPTION OF EVENT
On February 12,2007 Hatch Unit 2 was in the refueling mode for its 19th Refueling Outage with fuel in the vessel and the reactor cavity flooded for refueling operations. At that time, engineers and technicians were performing local leak rate testing (LLRT) on the Main Steam Isolation Valves (MSIVs, EIIS Code SB).
These tests determined, for both the 'B' penetration (2B2 1 -F022B (inboard) and 2B2 1 -F028B (outboard))
and the 'C' penetration (2B2 1 -F022C (inboard) and 2B21-F028C (outboard)), that the minimum pathway leakage for each of these penetrations exceeded the values specified in the plant's Technical Specifications surveillance requirement (SR) 3.6.1.3.1 1. This SR addresses the leakage restrictions through the MSIVs.
The MSIVs have specific leakage rates established in the plant's Technical Specifications to ensure that the assumptions of the safety analysis are met. The maximum leakage rate allowed for all of the main steam lines (MSLs) is 250 standard cubic feet per hour (scfh). The as-found measured LLRT minimum pathway leakage for the 'By penetration was 294 scfh, and for the 'C' penetration was 280 scfh. Additionally, outboard MSIVs 2B21 -F028A and 2B21 -F028D, for the 'A' and 'D' penetrations respectively, were found to exceed the leakage limits specified for an individual valve; however, the inboard MSIVs for these penetrations met their Technical Specification leakage limits. The total minimum pathway leakage through all four MSLs was 574 scfh.
As a result of these MSIV LLRT failures, an event recovery team was assembled, including a representative from the valve manufacturer, to determine the causes for each of the valve failures and to ensure that adequate corrective actions were taken to restore the valves to a condition that would provide reliable service.
A fault tree was constructed to determine the most likely cause of the LLRT failures. As-found Air Operated Valve (AOV) diagnostics and various tests were performed to determine if the MSIV actuators were a likely cause or contributor to the MSIV failures. It was concluded that the most likely direct causes of the MSN failures were out-of-specification internal valve tolerances and dimensions. The as-found conditions of the valves were determined by performing internal valve inspections that were focused on areas of potential leakage. MSNs have a main disc that has a seat in the main valve body and a stem disc that has a "pilot" seat in the main disc. This design establishes four seating surfaces where an anomaly could cause internal valve leakage. Additionally, other plants were contacted that use Rockwell International MSIVs to gain insight from their experience. From discussions with these plants and the vendor representative, it was determined that if the clearances between the in-body valve guides and the main disc are too large the probability of MSIVs seating leak-tight is reduced.
A review of the plant's maintenance practices determined that checking these clearances was not part of the normal maintenance activities. The diametral clearances between the valve body guides and the main disc need to be checked and maintained within limits in order to assure leak-tightness of the valves. These clearances are small and difficult to obtain when profiling the valves to determine the actual as-found conditions. Additionally, it was observed that the vendor manuals reviewed did not contain any specific (If more space is required, use additional copies of NRC Form 3664)
ADDITIONAL INFORMATION
Other Systems Affected:
No systems other than those already mentioned in this report were affected by this event.
Failed Components Information
Master Parts List Number: 2B21 -F022B, F028B, EIIS System Code: SB 2B2 1 - F022C, F028C Manufacturer: Rockwell International Reportable to EPM: Yes Model Number: 1 6 12 JM MNTY Root Cause Code: X Type: Valve, Shutoff EIIS Component Code: SHV Manufacturer Code: R344 Commitment Information:
This report does not create any permanent licensing commitments.
Previous Similar Events
LER 2-2005-001 documents a similar event for the MSIVs where both the inboard and outboard valves on an MSL failed the LLRT testing. Corrective actions for that event did not take into account the potential impact of not maintaining the internal tolerances and dimensions identified in this event.