ML14184A781
| ML14184A781 | |
| Person / Time | |
|---|---|
| Site: | Robinson  |
| Issue date: | 01/23/1991 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML14184A780 | List: |
| References | |
| NUDOCS 9101250045 | |
| Download: ML14184A781 (6) | |
Text
0 UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D. C. 20555 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION REGARDING REQUEST FOR RELIEF FROM ASME CODE REQUIREMENTS FOR THE SERVICE WATER SYSTEM WELD REPAIR CAROLINA POWER & LIGHT COMPANY H.B. ROBINSON NUCLEAR POWER PLANT DOCKET NO. 50-261
1.0 INTRODUCTION
During the radiographic examination of welds at H.B. Robinson Steam Electric Plant, Unit No. 2 (HBR-2) indications of microbiologically induced corrosion (MIC) were discovered at the service water (SW) supply and return piping for the containment HVH-4 fan cooler and, subsequently, at the containment penetra tions. Carolina Power & Light Company (CP&L) is proposing to perform a temporary non-code weld repair consisting of installing welded pipe sleeves over the MIC areas to restore the piping to its original design capability.
2.0 EVALUATION CP&L Evaluation Service water system piping inside the containment vessel (CV) has been identi fied with MIC. Nondestructive and destructive examinations have determined that the identified flaws are currently within American Society of Mechanical Engineers (ASME) Code allowable limits. However, flaw growth rates, in the presence of MIC, are not predictable; therefore, justification for continued operation of the service water system for an additional operating cycle is not possible. Relief is requested to allow the installation of welded stainless steel sleeves over the susceptible welds in the HVH-4 piping and over welds at containment penetrations that have MIC indications. The installation of sleeves constitutes an engineered repair that would restore the piping to its full structural and pressure-retaining capability, and complies with ASME Code,Section XI, repair requirements with the single exception of not removing the existing flaws. The proposed repair method is consistent with repairs performed previously at HBR-2 for similar MIC indications and reflects both experience with the application of the repair method and its effectiveness in restoring the system to full operability. CP&L notes that should it not be possible to install a sleeve over some existing weld joints in this piping, a ASME Code repair will be made. For example, it is anticipated that small drain lines will be removed, any MIC defects in the piping will be repaired, and new drain lines will be installed.
9101250045 910123 PDR ADOCK 05000261 P
-2 Generic Letter 90-05, "Guidance for Performing Temporary Non-Code Repair of ASME Code Class 1,2, and 3 Piping," provides a framework for requesting relief for the proposed repair. Although the plant is currently not operating and does not strictly adhere to the conditions outlined in the Generic Letter, it is CP&L's belief that the schedular impact and the timing associated with the discovery of the flaws provides some level of mitigation and latitude in considering this request based upon the impracticality of performing a ASME Code repair. This request is further supported by the relatively low safety significance associated with the indications based upon their presence in a ASME Code Class 3, moderate energy system. The non-destructive examination that identified the flaws was performed late in the current refueling outage on the basis of its impact on critical path activities if performed earlier and the belief that it was unlikely that MIC would be found based on previous inspections and plant conditions. The section of HVH-4 piping for which relief is requested is a 316L stainless steel material which was installed in 1985.
This material was expected to be less susceptible to MIC than the 304SS which it replaced and was examined via non-destructive examinatons in 1988 with no MIC indications identified.
The requested relief is based on the guidelines of Generic Letter 90-05.
Specific issues to be addressed in support of the relief have been taken from the Generic Letter and are identified below, along with the action, as appropriate, being taken by CP&L.
- 1. Document the Characterization of the Flaw Radiographic examination of 16 out of a total of 53 joints on the six-inch diameter, schedule 40, 316L stainless steel pipe revealed indications appearing to be MIC in 11 joints.
Eight service water containment penetration sleeve weld joints containing 304L stainless steel were also radiographed with four joints containing what appears to be MIC indications.
Metallographic cross-sections of the worst location identified by RT examination of the 316L piping revealed a cavity measuring approximately 7/8" long starting in the weld, extending axially into the adjacent pipe base metal, and 7/16" circumferentially in the weld metal.
The remaining wall measured 0.120" thick. The path of attack appeared to be the weld metal/base metal joint interface. The RT examinations show indications of an average size in length of 5/16 to 1/2" and the average number of indications ranging from two to four indications per joint. Since no baseline RT was made when the pipe was originally installed, some of the RT indications may be weld joint defect type indicati9ns, this was found to be the case for one location in the joint that was metallographically examined.
RT indications found in the penetration weld joints are similar to the piping indications, but were of a smaller size.
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- 2. Assess the Structural Integrity of the Flawed Piping. Evaluate for the Design Loading Condition Since a flaw growth rate for the MIC in the HVH-4 supply and return lines could not be established to show acceptable stress limits for a full fuel cycle, in accordance with flaw evaluation requirements outlined in Generic Letter 90-05 and ASME Code Case N-480, a decision was made to add welded sleeves over the flawed welds. An operability evaluation was performed for the worst as-found flawed condition utilizing the minimum wall with analyzed loads. This evaluation concluded that the piping systems was in an operable condition at the time of the flaw detection.
For qualification of the piping for an additional fuel cycle, a welded sleeve and welding design was performed by Structural Integrity Associates.
A seismic analysis to qualify the existing line for the additional weight of the welded sleeves was performed by CP&L. The welded sleeves will restore the original structural and pressure-retaining capability of the piping.
The design of the welded sleeve to be installed on the penetrations containing MIC indications will also restore the original structural integrity of the penetrations.
- 3. Assess the Overall Degradation by an Augmented Inspection The remaining 37 weld joints in the piping will not be radiographed since the 16 joints already radiographed provide sufficient data to assess the extent of the MIC attack. Additional radiographs are deemed unnecessary since welded sleeves will be installed over the 316L pipe joints except the one that was replaced, providing a new pressure boundary. Other piping in the containment and auxiliary buildings has been replaced with a more MIC-resistant material, AL6XN, which is being monitored by RT inspection during each refueling outage for evidence of MIC attack. The eight service water penetrations were examined by RT and those where MIC indications appear will be sleeved.
- 4. Assess the Integrity of the Affected Piping at Least Every Three Months by a Suitable NDE Method (UT or RT)
The proposed repair method complies with ASME Code,Section XI, except that it does not.remove the existing flaws. The sleeves will restore the original structural and pressure-retaining capability of the piping. The affected piping is located within the containment vessel that is an adverse environment during plant operation with respeft to both heat stress and radiation levels. In that the repair restores the piping to the equivalent of a "like new" status and considering the environment in which NDE would need to be performed, ongoing periodic NDE is not appropriate and will not be performed for this repair.
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- 5. Perform a qualitative assessment of leakage through the temporary non code repair at least every week during plant walkdown inspections.
As stated above, the repairs are located inside the containment and are designed to restore the piping to its original capability. Normal inspections of the containment are performed on a monthly basis and will, for the duration that the repair is in place, include a visual inspection of the vicinity of the affected piping for leakage. Leakage during plant operation will also be detected by waste system instrumentation.
- 6.
Determine the.Impracticality of Performing a ASME Code Repair As of January 15, 1991, HBR-2 is in the late stages of Refueling Outage
- 13. The current schedule includes entering hot shutdown on January 23, 1991, at which time the containment fan coolers are required to be operable. A ASME Code repair of the HVH-4 piping would require, as a minimum, the removal and replacement of the affected welds, with a more complete solution being to replace the 316L piping. Material to perform a complete replacement will not be available until January 24, 1991,,after which the installation work will require a minimum of an additional 14 days. In addition, a ASME Code allowable repair method for the containment penetrations has not yet been determined and, therefore, there is no schedule for materials or repairs. From these schedule considerations, it is clear that performance of the Code repairs would delay the unit's return to service and could not be completed within the limiting condition for operation (LCO) for the containment fan coolers specified in the Technical Specifications; therefore, this situation is considered to meet the intent of impracticality as outlined in Generic Letter 90-05.
- 7.
Perform a Root Cause Determination The worst pipe weld joint, cut out for a metallurgical lab analysis at the CP&L Harris Energy and Environmental Center, revealed that the inside pipe surface had slime deposits high in manganese associated with MIC attack. Laboratory analysis has confirmed that the wall thinning resulted from MIC attack at the weld joints. The path of attack initiation for the 316L stainless steel was along the base metal/weld metal interface and the weld. Whereas, past MIC indications in 304 stainless steel piping were initiated in the weld joint heat-affected zone adjacent to the weld.
HBR-2 first reported MIC attack in 304 stainless steel service water piping in late 1984 after a long plant outage. This is the first confir mation of MIC attack on low carbon grades of 304L and 316L at HBR-2.
- 8. Consider System Interactions (Flooding, Spraying Water on Equipment, and Loss of Flow)
As discussed above, the proposed repair returns the affected piping to its original structural and.pressure-retaining capability; therefore, the potential for system interactions is not increased over that of the original design. Hence, the capability to detect a pipe rupture
-5 in the service water system inside containment is available should it be needed. This capability includes waste system instrumentation, containment sump level instrumentation, and service water system flow instrumentation. The Final Safety Analysis Report, Chapter 15, accident analysis is not impacted by the proposed repair as the piping is restored to its original design capability. Additionally, the containment cooling function can be satisfied by any one of the following combinations: four fan-cooler units; two containment spray pumps; or any two fan-cooler units with one containment spray pump.
- 9. Schedule Implementation of Permanent Code Repair CP&L intends to replace the 316L stainless steel supply and return piping to HVH-4 during Refueling Outage 14 in 1992, which will constitute a permanent Code repair. A plan for permanent repair of the containment penetrations has not been developed. The potential for long lead time procurement in the event that penetration replacement is warranted makes it impractical to commit to a schedule for permanent repair at this time.
A plan and schedule for permanent repair of the containment penetrations will be provided to the NRC by September 30, 1991.
NRC Staff Evaluation
The NRC staff reviewed CP&L's reasoning and basis for requesting relief from the ASME Code requirements for the service water system weld repair. The staff concludes that the proposed temporary non-code weld repair provides an acceptable alternative to the ASME Code weld repair for one operating cycle.
There is reasonable assurance that the integrity of the service water system would not be adversely affected during plant operation as a result of this alternative weld repair.
3.0 CONCLUSION
The NRC staff has reviewed the information submitted by CP&L in support of the relief from the ASME Code requirement associated with the weld repair of the service water system at HBR-2. The staff concludes that the performance of a temporary non-code weld repair provides adequate assurance that the structural integrity of the service water system will be maintained during the next operating cycle. Therefore, pursuant to 10 CFR 50.55a(a)(3)(i), the relief should be granted.
Dated: January 23, 1991 Principal Contributor:
G. Georgiev
DISTRIBUTION Docket FiTe NRC PDR Local PDR S. Varga 14-E-4 G. Lainas 14-H-3 L. Reyes Ru E. Adensam 14-B-20 P. Anderson R. Lo 14-8-20 OGC (For inform. Only) 15-B-18 E. Jordan MNBB-3302 ACRS (10)
P-315 C. Y. Cheng G. Georgiev Robinson File 14--4