05000423/LER-2007-001, Failure of Two Main Steam Safety Valves to Lift within the Acceptance Criteria
| ML071510080 | |
| Person / Time | |
|---|---|
| Site: | Millstone  |
| Issue date: | 05/23/2007 |
| From: | Price J Dominion, Dominion Nuclear Connecticut |
| To: | Document Control Desk, Plant Licensing Branch III-2 |
| References | |
| 07-0362 LER 07-001-00 | |
| Download: ML071510080 (7) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4232007001R00 - NRC Website | |
text
llA 11,11 41/ýY"'DominioW Dominion Nuclear Connecticut, Inc.
Millstone Power Station Rope Ferry Road, Waterford, CT 06385 MAY 2 3 2007 U.S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, DC 20555 Serial No.
MPS Lic/GJC Docket No.
License No.
07-0362 RO 50-423 NPF-49 DOMINION NUCLEAR CONNECTICUT. INC.
MILLSTONE POWER STATION UNIT 3 LICENSEE EVENT REPORT 2007-001 -00, FAILURE OF TWO MAIN STEAM SAFETY VALVES TO LIFT WITHIN THE ACCEPTANCE CRITERIA This letter forwards Licensee Event Report (LER) 2007-001-00 documenting a condition discovered at Millstone Power Station Unit 3, on April 05, 2007. This LER is being submitted pursuant to 10 CFR 50.73(a)(2)(i)(B), as a condition prohibited by the Technical Specifications.
If you have any questions or require additional information, please contact Mr. David W.
Dodson at (860) 447-1791, extension 2346.
Very truly yours, J.*
rice Sig ice President - Millstone
Serial No. 07-0362 Licensee Event Report 2007-001-00 Page 2 of 2 Attachments:
1 Commitments made in this letter: None.
cc:
U.S. Nuclear Regulatory Commission Region I 475 Allendale Road King of Prussia, PA 19406-1415 Mr. J. D. Hughey NRC Senior Project Manager Millstone Units 2 and 3 U. S. Nuclear Regulatory Commission, Mail Stop 8 B3 One White Flint North 11555 Rockville Pike Rockville, MD 20852-2738 Mr. S. M. Schneider NRC Senior Resident Inspector Millstone Power Station
Serial No. 07-0362 Docket No. 50-423 LICENSEE EVENT REPORT 2007-001-00.
FAILURE OF TWO MAIN STEAM SAFETY VALVES TO LIFT WITHIN THE ACCEPTANCE CRITERIA Millstone Power Station Unit 3 Dominion Nuclear Connecticut, Inc. (DNC)
Abstract
With the plant in MODE 1 at 100% power on April 5, 2007 set pressure "simmer" testing of Unit 3 (MPS 3) Main Steam Safety Valves (MSSVs) was conducted per plant procedures. This testing was conducted just prior to the recent refueling outage. During the conduct of testing, two MSSVs failed to lift within the (+/- 3%) acceptance criteria. Valve 3MSS*RV22B lifted at 1221.3 psig. (1.3 psig above the set pressure range, approximately 3.1%),
and 3MSS*RV22D lifted at 1232.8 psig. (12.8 psig above the set pressure range, approximately 3.8%).
3MSS*RV22B supports the B Steam Generator (SG) and valve 3MSS*RV22D supports the D SG. Both valves were subsequently adjusted/retested with the results within the required range of +/- 1%.
Based on information provided by Electric Power Research Institute Report report TR-1 13560 (Investigation of MSSV High First Lift Phenomenon in Dresser 3700 Series MSSVs, Sept. 2000), industry experience, MSSV test history at Millstone, and engineering judgment, the failure of the MSSVs to lift within the required set pressure range is attributed to a corrosive oxide locking action between surface layer materials of the disc-seat interface sometimes referred to as "oxide locking" or "micro bonding".
This condition is being reported pursuant to 10 CFR 50.73(a)(2)(i)(B) "Any operation or condition prohibited by the plant's Technical Specifications."
NRC FORM 366 (6-2004)
(If more space is required, use additional copies of (If more space is required, use additional copies of NRC Form 366A) (17) phenomenon. The EPRI report also concluded that "within the limits of detectability, no history of the micro bonding phenomena has been identified to date during actual plant transients, resulting in MSSV lifts". The MSSVs had not been tested since installed, and experienced micro bonding during the time period between overhaul and the MODE 1 scheduled "simmer" testing just prior to shutting down for the refueling outage in April of 2007.
3. Assessment of Safety Consequences
This condition is judged to be of very low safety significance. Five steam line safety valves are installed on each of the unit's four main steam lines with nominal lift settings increasing in 10 psi increments from 1185 to 1225 psig. The safety valves protect the SG and portions of the main steam [SB] and feedwater systems [SJ] from overpressure conditions. The valves also serve as a heat sink for the reactor coolant system if the main condenser [COND] is unavailable and the atmospheric steam dump valves cannot relieve pressure following a reactor trip or secondary system accident. The limiting FSAR events with respect to main steam and reactor coolant overpressurization are those presented in FSAR Section 15.2, which involve a decrease in heat removal by the secondary system. The accidents presented in FSAR Section 15.2 assume all MSSVs open at a lift setting 3% higher than their nominal set pressure. The as-found condition of the tested valves determined valves with the lowest lift setting on two main steam lines had a setting greater than the allowed +3% tolerance. 3MSS*RV22B was found with a lift setting 1.3 psig above the maximum allowed value, and 3MSS*RV22D was found with a lift setting 12.8 psig above the maximum allowed value. The average lift setting of all MSSVs tested was approximately 1% above the nominal lift setting, or approximately 2% lower than that assumed in the FSAR Section 15.2 Safety Analyses. Therefore, it is concluded that the overall MSSV response of the as-found MSSV condition is bounded by the MSSV response assumed in the FSAR Section 15.2 Safety Analyses.
Additionally, the micro bonding phenomenon has been limited to static, in-situ testing. The EPRI report indicates that under actual transient high pressure conditions, disk movement/flexure breaks the micro bonding allowing the MSSVs to lift uninfluenced by this phenomenon.
4. Corrective Action
Following the as-found lift, valve 3MSS*RV22B was subsequently "simmer" tested two additional times and the results were within the required as-left criteria of +/- 1% with no adjustments required. As a result of the initial lift outside of acceptable limits, for valve 3MSS*RV22B, two additional MSSVs were tested (3MSS*RV24B & 3MSS*RV25B). The as-found "simmer" tests for both of these valves were within the required range of +/- 3%.
Following the as-found lift, valve 3MSS*RV22D was subsequently "simmer" tested and the results were within the required as-found criteria of +/- 3%. However, one adjustment was necessary in order to meet the required as-left criteria of +/- 1%. As a result of the initial lift outside of acceptable limits for valve 3MSS*RV22D, two additional MSSVs were tested (3MSS*RV23A & 3MSS*RV23D). The as-found "simmer" tests for both of these valves were within the required range of +/- 3%.
Additional corrective actions to address micro bonding and the in-situ testing process will be evaluated in accordance with the station's Corrective Action Program.(1-2001)
LICENSEE EVENT REPORT (LER)
- 5.
Previous Occurrences
On September 30, 2005, during the performance of set pressure "simmer" testing of MSSVs, valve 3MSS*RV25C failed to lift within the +/- 3% acceptance criteria. The allowable as-found set pressure range is 1179-1251 psig. The valve lifted at 1252.9 psig. The valve was lifted two additional times and the results were within the required as-left criteria of +/- 1% with no adjustments required. Two additional MSSVs were set pressure tested, to meet the ASME/OM code requirement for test expansion. These valves (3MSS*RV23C and 3MSS*RV24C) were satisfactorily tested. The as-left "simmer" tests for both of these valves were within the required as-left criteria of +/- 1%.
Energy Industry Identification System (EIIS) codes are identified in the text as [XX].