ML070370192
| ML070370192 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 01/31/2007 |
| From: | Scherer A Southern California Edison Co |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| Download: ML070370192 (7) | |
Text
SOUTHERN CALIFORNIA A. Edward Scherer EDfISON 0Manager of Nuclear Regulatory Affairs An EDISON INTERNATIONAL Company January 31, 2007 Document Control Desk U.S. Nuclear Regulatory Commission Washington, DC 20555-0001
Subject:
Docket Nos. 50-361 and 50-362 Inspection and Mitigation of Alloy 82/182 Pressurizer Butt Welds San Onofre Nuclear Generating Station, Units 2 and 3
Dear Sir or Madam:
In October 2006, while performing inspections of its pressurizer Alloy 82/182 butt welds in accordance with Material Reliability Program (MRP)-139, another licensee of a pressurized water reactor discovered several circumferential indications in its pressurizer surge, safety, and relief nozzles. Because of the potential importance of this issue, SCE is submitting this letter to notify you of actions taken and planned for inspecting and mitigating Alloy 600/82/182 butt welds on pressurizer spray, surge, and safety nozzles at San Onofre Nuclear Generating Station (SONGS), Unit 2 and Unit 3.
NOZZLE DESIGN, INSPECTION AND MITIGATION INFORMATION The SONGS Unit 2 and Unit 3 pressurizer design provided five large bore nozzles including one spray line nozzle, two active and one spare safety valve nozzles and the surge line nozzle. The design of these nozzles incorporate a forged stainless steel safe end for the spray line nozzle, and cast stainless steel safe ends for the remaining safety valve and surge line nozzles.
The carbon steel pressurizer vessel and its adjoining Alloy 182 weld "butter" associated with these large bore nozzles received post weld heat treatment prior to attachment of the stainless steel safe end. A post weld heat treatment was not performed after attaching the stainless steel safe end using the alloy 82/182 weld.
A performance demonstration initiative (PDI) compliant inspection meeting MRP-139 requirements was performed on the spray line nozzle during the Cycle-14 refueling outages in January and October 2006 for SONGS Units 2 and 3 respectively. No indications were identified during those inspections. In addition, a full structural weld overlay was implemented on the spray line nozzles during Cycle-14 refueling outages.
As a result, the SONGS Units 2 and 3 spray line has been successfully mitigated from primary water stress corrosion cracking (PWSCC).
P.O. Box 128 San Clemente, CA 92672 949-368-7501
/ 0 Fax 949-368-7575
Document Control Desk January 31, 2007 The three safety valve nozzles and the surge line nozzle dissimilar metal welds (DMWs) were also volumetrically examined during the Cycle-14 refueling outage. Six small, linear, axially orientated indications were identified in two of the SONGS Unit 2 pressurizer safety valve nozzles. Eddy current examination of those indications demonstrated that there was no surface connection and therefore were not PWSCC.
No other indications were identified during these safety valve and surge line volumetric examinations. In addition to the inspection performed on safety valve and surge line nozzles, a full structural weld overlay was successfully applied to the three safety valve nozzles on both SONGS Units 2 and 3. As a result, the pressurizer safety valve nozzles have been successfully mitigated from PWSCC.
An MRP-139 compliant inspection of the safety valve and surge line nozzles was not possible because volumetric examination into the cast stainless steel safe end material has not been qualified. The technique used to inspect these nozzles was PDI qualified for the existing geometry except for the cast material. The presence of the cast safe end component precluded a qualified examination within the casting, and limited the extent of axial scanning into the Alloy 82/182 from the safe end side. Axial scanning from the vessel side of the weld (for detection of circumferentially oriented planar flaws),
and circumferential scanning in both directions on the vessel nozzle and Alloy 82/182 weld and "butter" surface was accomplished within the qualified PDI procedure requirements. Therefore, the entire PWSCC sensitive Alloy 82/182 material up to the safe end fusion line (approximately three fourths of the required volume) was successfully examined. This does not meet MRP-139 minimum volume requirement of 90%.
A full structural weld overlay of the SONGS Unit 2 surge line has been planned for the Cycle-1 5 refueling outage currently scheduled to begin in November 2007. A full structural weld overlay was also planned and initiated on the SONGS Unit 3 surge line during the Cycle-14 refueling outage in October, 2006. Those mitigation plans would have satisfied the remaining MRP-139 requirement applicable to pressurizer DMWs in 2007. Unexpected defects were identified however, within the initial weld overlay layer on the SONGS Unit 3 pressurizer surge line. Weld procedure modifications failed to resolve the defects during a second weld overlay attempt. In both cases, the weld defects adversely affected underlying material. Based on the confidence gained from the inspection effort that susceptible surge line materials were free of PWSCC, a decision was made to defer further attempts at performing a weld overlay on SONGS Unit 3 until the cause of the welding defects was understood and corrected. This decision would cause SONGS Unit 3 not to be in full conformance with MRP-139 requirements for pressurizer DMWs until the Fall of 2008.
ASSESSMENT FOR CONTINUED SERVICE DURING CYCLE-14 As outlined above, the only remaining pressurizer nozzle that is susceptible to PWSCC failure is the surge line nozzle for each unit. For comparative purposes, both SONGS Units 2 and 3 will have accumulated approximately nineteen and one half effective full
Document Control Desk January 31, 2007 power years upon completion of Cycle-14 in November 2007 and October 2008 respectively.
A review of fabrication records has revealed that a repair weld was performed at the safe end to vessel nozzle weld joint on SONGS Unit 2 surge line. A review of the SONGS Unit 3 fabrication records did not identify any weld repairs made during fabrication of the pressurizer surge line. Fabrication records of "in process" repair of these welds were not required, and the possibility of such repairs can not be fully ruled out by the document review. There has been no in service repairs made to the surge line DMW of either SONGS Unit 2 or Unit 3.
As described above, the volumetric examination of the surge line nozzles performed during the Cycle-14 refueling outages provides reasonable assurance that branching PWSCC defects are not present within the susceptible material of the surge line DMWs.
United States Nuclear Regulatory Commission (NRC) evaluations of the potential consequences from surge line cracks similar to those recorded at Wolf Creek were presented during the December 20, 2006 public meeting on this subject. Those evaluations indicate that the time interval between initiation of a through wall leak and piping failure would be at least 7 months. This is ample time for an effective leakage monitoring program to detect and correct defects before piping failure might occur.
LEAKAGE MONITORING AND RESPONSE PLAN The current reactor coolant system (RCS) leakage monitoring plan at San Onofre includes formal and informal elements. RCS Operational leakage measurements are required by Technical Specification Surveillance Requirement 3.4.13.1 every 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> except during periods of transient operation when the interval can be extended up to 120 hours0.00139 days <br />0.0333 hours <br />1.984127e-4 weeks <br />4.566e-5 months <br />. Surveillance interval extensions are rare, but are necessary to accommodate periods where large volumes of water must be added for reactivity and volume control. (Leakage calculations are based on integration of water inventories and are susceptible to small errors in charging and letdown flow measurements when large integrated volumes involved).
Under existing procedural requirements, steady state RCS leakage is measured and compared to Technical Specification limits (zero allowable pressure boundary leakage, 1.0 gpm unidentified leakage). The measurement is also compared to a procedural limit of 0.15 gpm total unidentified leakage. An investigation is initiated when this 0.15 gpm unidentified leakage limit is exceeded. The intent of this investigation is to assess whether a degraded condition may exist. The Corrective Action Program expectations for timeliness of the investigation and followup corrective actions are applicable.
Document Control Desk January 31, 2007 The 0.15 gpm limit is based on historical data and is designed to be sensitive to potentially significant changes. The standard deviation associated with measurements taken during stable plant conditions is approximately 0.02 gpm. Normal plant conditions, such as charging pump packing wear, occasionally result in leakage that exceeds this limit. The 0.15 gpm threshold for investigating leakage can be changed based on recognized leakage sources. Guidance is provided for the conduct of the investigation; however criteria for plant shutdown are not prescribed. Engineering procedures also provide guidance for supplemental monitoring and investigation of RCS leakage.
Monitoring and investigation of RCS leakage inside containment is also achieved through a combination of containment radiation monitors, sump levels, volume control tank levels and humidity and temperature indications. These indications are continuously available to control room personnel and are routinely monitored.
Development of an RCS leak during the period between required 72 hour8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> Surveillances may be identified using these instruments depending on its location and magnitude.
Southern California Edison (SCE) is participating with the Pressurized Water Reactor Owners Group effort in developing enhanced RCS leakage monitoring strategies.
As described above, timeliness requirements following recognition of a potential RCS leak inside containment are established by the SONGS Corrective Action Program.
PWSCC cracking identified at Wolf Creek nuclear plant and the failure analyses performed on those indications has indicated a larger potential safety significance for small RCS leaks than had been previously recognized. Effectively, this information has resulted in a changed expectation for timeliness of leak detection, identification and completion of corrective actions.
COMMITMENT TO MRP-139 SONGS Unit 2 will shutdown prior to December 31, 2007 for a refueling outage and to meet the requirements of Generic Letter 2004-02 "Potential Impact of Debris Blockage on Emergency Recirculation during Design Basis Accidents at Pressurized-Water Reactors." SCE will not operate SONGS Unit 2 after December 31, 2007 until SONGS Unit 2 is in compliance with the MRP-1 39 guidance.
SCE is confident in the inspections performed to date at SONGS Unit 3 and in the structural integrity of the SONGS pressurizer DMWs. Nevertheless, SCE decided for commercial reasons that a mid-cycle outage during 2007 will be scheduled for SONGS Unit 3. SCE will not operate SONGS Unit 3 past December 31, 2007, until SONGS Unit 3 is in compliance with the MRP-139 guidance.
Document Control Desk January 31, 2007
SUMMARY
Details concerning the locations inspected and mitigated are summarized in the attached tables. SCE has concluded that based on the inspections performed during the completed Cycle-14 refueling outages, both SONGS Units 2 and 3 can be safely operated to their next scheduled refueling outages.
Full structural weld overlays have been completed on the pressurizer spray and safety valve nozzles during the Cycle-14 refueling outages. Those modifications have mitigated susceptibility to PWSCC failure at those locations. SCE is planning to implement a full structural weld overlay modifications on the pressurizer surge line nozzle of both SONGS Units 2 and 3. These design modifications mitigate susceptibility to PWSCC related piping failure and will result in full conformance with MRP-139 requirements for 2007.
SCE is committing that neither SONGS Unit 2 nor Unit 3 will operate past December 31, 2007, without meeting the MRP-1 39 requirements for pressurizer DMWs.
SCE is actively participating with the industry to improve analytical and mitigation technologies that will further reduce PWSCC related challenges to RCS integrity and will revise plans as necessary to ensure continued safe operation.
The NRC will be informed if SCE revises the current plans to have all pressurizer DMWs mitigated with a full structural weld overlay prior to operation after December 31, 2007.
Our staff is available to meet with the NRC to discuss any of the information in this letter. If there are any questions, please contact Mr. Thomas Raidy at (949) 368-7582.
Sincerely, cc:
B. S. Mallett, Regional Administrator, NRC Region IV N. Kalyanam, NRC Project Manager, San Onofre Units 2 and 3 C. C. Osterholtz, NRC Senior Resident Inspector, San Onofre Units 2 and 3
Inspection and Mitigation Information Table SONGS Unit 2 Inspection and Mitigation Summary for Alloy 600/82/182 Pressurizer Butt Welds MRP-139 Volumetric Mitigation Nozzle Inspection Requirement Met Completed or to be or to be Met Completed Comments Function Susceptible Outage Start Date Outage Designation Description Designation (MM/YYY)
Designation Alloy 82/182 U2C14 1/3/2006 Full Structural Weld Spray Refueling Overlay Completed in U2C14 U2C14 Entire volume of Alloy 82/182 weld material was Safety PSV 200 Alloy 82/182 See 1/3/2006 Full Structural Weld inspected with PDI qualified technique. Approximately Comments Overlay Completed 75% of the MRP-139 volume inspected.
in U2C14 U2C14 Entire volume of Alloy 82/182 weld material was Safety PSV 201 Alloy 82/182 See 1/3/2006 Full Structural Weld inspected with PDI qualified technique. Approximately Comments Overlay Completed 75% of the MRP-139 volume inspected.
in U2C14 U2C14 Entire volume of Alloy 82/182 weld material was Safety Spare Alloy 82/182 See 1/3/2006 Full Structural Weld inspected with PDI qualified technique. Approximately Comments Overlay Completed 75% of the MRP-139 volume inspected.
in U2C14 MRP-1 39 U2C14 Compliance Prior to Entire volume of Alloy 82/182 weld material was Surge Alloy 82/182 See 1/3/2006 Plant Operation inspected with PDI qualified technique. Approximately Comments Past 12/31/2007 75% of the MRP-139 volume inspected.
Inspection and Mitigation Information Table SONGS Unit 3 Inspection and Mitigation Summary for Alloy 600/82/182 Pressurizer Butt Welds MRP-139 Volumetric Mitigation Nozzle Inspection Requirement Met Completed or to be or to be Met Completed Comments Function /
Susceptible Outage Start Date Outage Material Designation Description Designation (MMIYYYY)
Designation Alloy 82/182 U3C14 10/16/2006 Full Structural Weld Spray Refueling Overlay Completed in U2C14 U3C14 Full Structural Weld Entire volume of Alloy 82/182 weld material was Safety PSV 200 Alloy 82/182 See 10/16/2006 Overlay Completed inspected with PDI qualified technique. Approximately comments in U2C14 75% of the MRP-139 volume inspected U3C14 Full Structural Weld Entire volume of Alloy 82/182 weld material was Safety PSV 201 Alloy 82/182 See 10/16/2006 Overlay Completed inspected with PDI qualified technique. Approximately comments in U2C14 75% of the MRP-139 volume inspected U3C14 Full Structural Weld Entire volume of Alloy 82/182 weld material was Safety Spare Alloy 82/182 See 10/16/2006 Overlay Completed inspected with PDI qualified technique. Approximately comments in U2C14 75% of the MRP-139 volume inspected MRP-139 U3C14 Compliance Prior to Entire volume of Alloy 82/182 weld material was Surge Alloy 82/182 See 10/16/2006 Plant Operation inspected with PD1 qualified technique. Approximately comments Past 12/31/2007 75% of the MRP-139 volume inspected