ML21258A408
| ML21258A408 | |
| Person / Time | |
|---|---|
| Site: | Grand Gulf, River Bend  |
| Issue date: | 09/21/2021 |
| From: | Siva Lingam Plant Licensing Branch IV |
| To: | Halter M Entergy Operations |
| Lingam S, 301-415-1564 | |
| References | |
| EPID L-2020-LLR-0079 | |
| Download: ML21258A408 (13) | |
Text
September 21, 2021 Mrs. Mandy Halter Vice President, Regulatory Assurance Entergy Services, LLC M-ECH-29 1340 Echelon Parkway Jackson, MS 39213
SUBJECT:
GRAND GULF NUCLEAR STATION, UNIT 1 AND RIVER BEND STATION, UNIT 1 - REQUEST TO UPDATE AMERICAN SOCIETY OF MECHANICAL ENGINEERS CODE RELIEF REQUEST SAFETY EVALUATIONS WITH NRC-APPROVED REVISION OF BOILING WATER REACTOR VESSEL AND INTERNALS PROJECT GUIDELINES (EPID L-2020-LLR-0079)
Dear Mrs. Halter:
The U.S. Nuclear Regulatory Commission (NRC) has authorized your request to update American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code),
Section XI, relief request safety evaluations with the NRC-approved revision of the Boiling Water Reactor (BWR) Vessel and Internals Project Guidelines (BWRVIP) for Grand Gulf Nuclear Station, Unit 1 (Grand Gulf) and River Bend Station, Unit 1 (River Bend). This action is in response to your request dated June 8, 2020 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML20160A032), as supplemented by letters dated November 9, 2020, April 16, 2021, and August 25, 2021 (ADAMS Accession Nos. ML20314A125, ML21106A216, and ML21237A550, respectively).
Entergy Operations Inc. (the licensee) submitted Relief Requests GG-ISI-020 and RBS-ISI-019 in accordance with paragraph 50.55a(z)(1) of Title 10 of the Code of Federal Regulations (10 CFR) to revise certain authorized alternatives to the requirements of 10 CFR 50.55a, Codes and standards, for Grand Gulf and River Bend.
The licensees June 8, 2020, submittal, contains an updated revision of the original alternative requests, which were submitted by a letter dated March 15, 2017 (ADAMS Accession No. ML17074A625), in Relief Request GG-ISI-020 for Grand Gulf, and by a letter dated February 13, 2018 (ADAMS Accession No. ML18045A151), in Relief Request RBS-ISI-019 for River Bend.
By letter dated January 23, 2018 (ADAMS Accession No. ML18003B126), the NRC staff reviewed and approved the original submittal of Relief Request GG-ISI-020 for Grand Gulf.
Similarly, by letter dated November 16, 2018 (ADAMS Accession No. ML18310A013), the NRC staff reviewed and approved the original submittal of Relief Request RBS-ISI-019 for River Bend.
In the previously approved Relief Requests GG-ISI-020 and RBS-ISI-019, the NRC staff authorized the licensee to use specified BWRVIP guidelines as an alternative to the requirements in 10 CFR 50.55a and the ASME Code for each of the subject facilities.
Specifically, the licensee is currently authorized to use specified BWRVIP guidelines as an
alternative to the requirements for inservice inspection (ISI) of reactor vessel internals components (i.e., interior surfaces, attachments, and support structures).
The licensee requested to allow the use of BWRVIP-18, Revision 2-A, BWR Vessel and Internals Project: BWR Core Spray Internals Inspection and Flaw Evaluation Guidelines (ADAMS Accession No. ML16302A123), BWRVIP-41, Revision 4-A, BWR Vessel and Internals Project: BWR Jet Pump Assembly Inspection and Flaw Evaluation Guidelines (ADAMS Accession No. ML19297G484), and BWRVIP-48, Revision 1, BWR Vessel and Internals Project, BWR Vessel ID Attachment Weld Inspection and Flaw Evaluation Guidelines (ADAMS Accession No. ML043290377; not publicly available, proprietary information; by NRC approval letter dated July 25, 2005 (ADAMS Accession No. ML052130284)), as applicable, in lieu of the currently authorized versions of these guidelines. Pursuant to 10 CFR 50.55a(z)(1), the licensee requested to revise these alternatives on the basis that they would provide an acceptable level of quality and safety.
The NRC staff has reviewed the licensees submittals, and concludes, as set forth in the enclosed safety evaluation, that the licensees proposed alternative has provided an acceptable level of quality and safety. Accordingly, the NRC staff concludes that the licensee has adequately addressed all the regulatory requirements set forth in 10 CFR 50.55a(z)(1).
Therefore, the NRC staff authorizes the use of the licensees proposed alternative, for the remainder of the fourth 10-year ISI interval at Grand Gulf and River Bend.
The NRC approval of the subject inspection frequency for core spray piping bracket welds addressed in BWRVIP-48, Revision 1, does not imply or infer the NRCs approval of BWRVIP-48, Revision 1 for generic use.
All other requirements of ASME Code,Section XI, for which relief was not specifically requested and authorized by the NRC staff remain applicable, including the third-party review by the Authorized Nuclear In-Service Inspector.
Enclosed is the NRC staffs safety evaluation.
If you have any questions, please contact the Project Manager, Siva P. Lingam, at 301-415-1564 or by e-mail to Siva.Lingam@nrc.gov.
Sincerely, Jennifer L. Dixon-Herrity, Chief Plant Licensing Branch IV Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket Nos. 50-416 and 50-458
Enclosure:
Safety Evaluation cc: Listserv Jennifer L.
Dixon-Herrity Digitally signed by Jennifer L. Dixon-Herrity Date: 2021.09.21 17:34:00 -04'00'
Enclosure SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELIEF REQUESTS GG-ISI-020 AND RBS-ISI-019 - REQUEST TO UPDATE AMERICAN SOCIETY OF MECHANICAL ENGINEERS CODE RELIEF REQUEST SAFETY EVALUATIONS WITH NRC APPROVED REVISION OF BOILING WATER REACTOR VESSEL AND INTERNALS PROJECT GUIDELINES ENTERGY OPERATIONS, INC.
GRAND GULF NUCLEAR STATION, UNIT 1 AND RIVER BEND STATION, UNIT 1 DOCKET NOS. 50-416 AND 50-458
1.0 INTRODUCTION
By application dated June 8, 2020 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML20160A032), as supplemented by letters dated November 9, 2020, April 16, 2021, and August 25, 2021 (ADAMS Accession Nos. ML20314A125, ML21106A216, and ML21237A550, respectively), Entergy Operations Inc. (the licensee) submitted Relief Requests GG-ISI-020 and RBS-ISI-019 in accordance with paragraph 50.55a(z)(1), Acceptable level of quality and safety, of Title 10 of the Code of Federal Regulations (10 CFR) to revise certain authorized alternatives to the requirements of 10 CFR 50.55a, Codes and standards, for Grand Gulf Nuclear Station, Unit 1 (Grand Gulf) and River Bend Station, Unit 1 (River Bend).
The June 8, 2020, submittal contains an updated revision of the original alternative requests, which were submitted by a letter dated March 15, 2017 (ADAMS Accession No. ML17074A625),
in Relief Request GG-ISI-020 for Grand Gulf, and by letter dated February 13, 2018 (ADAMS Accession No. ML18045A151) in Relief Request RBS-ISI-019 for River Bend.
By letter dated January 23, 2018 (ADAMS Accession No. ML18003B126), the U.S. Nuclear Regulatory Commission (NRC) staff reviewed and approved the original submittal of Relief Request GG-ISI-020 for Grand Gulf. Similarly, by letter dated November 16, 2018 (ADAMS Accession No. ML18310A013), the NRC staff reviewed and approved the original submittal of Relief Request RBS-ISI-019 for River Bend.
In the previously NRC-approved Relief Requests GG-ISI-020 and RBS-ISI-019, the NRC staff authorized the licensee to use specified Boiling Water Reactor (BWR) Vessel and Internals Project (BWRVIP) guidelines as an alternative to requirements in 10 CFR 50.55a and the American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code) for each of the subject facilities. Specifically, the licensee is currently authorized to use specified BWRVIP guidelines as an alternative to the requirements for inservice inspection (ISI) of reactor vessel internals (RVI) components (i.e., interior surfaces, attachments, and support structures).
The licensee requested to allow the use of BWRVIP-18, Revision 2-A, BWR Vessel and Internals Project: BWR Core Spray Internals Inspection and Flaw Evaluation Guidelines (ADAMS Accession No. ML16302A123); BWRVIP-41, Revision 4-A, BWR Vessel and Internals Project: BWR Jet Pump Assembly Inspection and Flaw Evaluation Guidelines (ADAMS Accession No. ML19297G484); and BWRVIP-48, Revision 1, BWR Vessel and Internals Project BWR Vessel ID [Inside Diameter] Attachment Weld Inspection and Flaw Evaluation Guidelines (ADAMS Accession No. ML043290377; not publicly available, proprietary information; NRC Approval Letter ADAMS Accession No. ML052130284), as applicable, in lieu of the currently authorized versions of these guidelines. Pursuant to 10 CFR 50.55a(z)(1), the licensee requested to revise these alternatives on the basis that they would provide an acceptable level of quality and safety.
2.0 REGULATORY EVALUATION
The regulations in 10 CFR 50.55a(g)(4), Inservice inspection standards requirement for operating plants, state, in part, that ASME Code Class 1, 2, and 3 components (including supports) must meet the requirements, except the design and access provisions and the preservice examination requirements, set forth in Section XI of the applicable editions and addenda of the ASME Code to the extent practical within the limitations of design, geometry, and materials of construction of the components.
The regulations in 10 CFR 50.55a(z), Alternatives to codes and standards requirements, state, in part, that alternatives to the requirements in paragraphs (b) through (h) of 10 CFR 50.55a may be authorized by the NRC if the licensee demonstrates that: (1) the proposed alternative provides an acceptable level of quality and safety, or (2) compliance with the specified requirements would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety.
3.0 NRC TECHNICAL EVALUATION 3.1 Licensees Proposed Alternatives 3.1.1 Grand Gulf Nuclear Station, Unit 1 By letter dated March 15, 2017, the licensee submitted Relief Request GG-ISI-020 requesting alternative inspections of B-N-1 and B-N-2 components for the fourth ISI interval at Grand Gulf.
On January 23, 2018, the NRC staff authorized the use of, among others, BWRVIP-18, Revision 1-A, BWRVIP-41, Revision 3, and BWRVIP-48-A for the fourth ISI interval (to be ending on June 1, 2027), as an alternative to the requirements in 10 CFR 50.55a and the ASME Code,Section XI, at Grand Gulf.
3.1.2 River Bend Station, Unit 1 On February 13, 2018, the licensee submitted Relief Request RBS-ISI-019 requesting alternative inspections of B-N-1 and B-N-2 components for the fourth ISI interval at River Bend. On November 16, 2018, the NRC staff authorized the use of, among others, BWRVIP-18, Revision 2-A, BWRVIP-41, Revision 4A, and BWRVIP-48-A, for the fourth ISI interval (to be ending on November 20, 2027) as an alternative to the requirements in 10 CFR 50.55a and the ASME Code,Section XI, at River Bend.
3.1.3 Licensees Current Proposed Alternatives for Grand Gulf and River Bend In the current licensees submittal, dated June 8, 2020, the licensee requested the NRC staffs approval to implement later versions of the BWRVIP reports that were used in Relief Requests GG-ISI-020 for Grand Gulf and RBS-ISI-019 for River Bend. This latest submittal supplements the original March 15, 2017, and February 13, 2018, submittals for Grand Gulf and River Bend units, respectively. The licensees did not revise the contents of the original submittals except for the following BWRVIP reports, which will replace the previous revisions of the corresponding BWRVIP reports. The BWRVIP Inspection and Evaluation (I&E) guidelines addressed in the following reports will supersede the guidelines addressed in the previous revisions upon NRC staffs approval:
(1)
BWRVIP-18, Revision 2-A, BWR Vessel and Internals Project BWR Core Spray Internals Inspection and Flaw Evaluation Guidelines, (2)
BWRVIP-41, Revision 4-A, BWR Vessel and Internals Project BWR Jet Pump Assembly Inspection and Flaw Evaluation Guidelines, and, (3)
BWRVIP-48, Revision 1, BWR Vessel and Internals Project BWR Vessel ID Attachment Weld Inspection and Flaw Evaluation Guidelines. The NRC staff neither reviewed nor approved this report.
According to the licensees proposal, the above three BWRVIP reports will be used for the fourth ISI interval as an alternative to the requirements in 10 CFR 50.55a and the ASME Code,Section XI, at Grand Gulf.
The licensee proposed to use BWRVIP-18, Revision 2-A, BWRVIP-41, Revision 4-A, and BWRVIP-48, Revision 1, for Grand Gulf. However, the licensee stated that it will only use BWRVIP-48, Revision 1, for River Bend, because BWRVIP-18, Revision 2-A was already approved by the NRC staff on February 22, 2016 (ADAMS Accession No. ML16011A190), and BWRVIP-41, Revision 4-A was already approved by the NRC staff on June 26, 2018 (ADAMS Accession No. ML18130A050). Also, the NRC staff approved the use of these two BWRVIP reports as part of its approval of Relief Request RBS-ISI-019 as discussed in its safety evaluation (SE) dated November 16, 2018.
3.1.4 Qualitative Risk Assessment for Extension of the Core Spray Piping Brackets Attachment Weld Examination Interval in Revision 1 to BWRVIP-48 In its supplemental letter dated November 9, 2020, the licensee submitted a technical evaluation developed by BWRVIP for implementing a revised inspection frequency for the primary and supplemental core spray piping brackets addressed in BWRVIP-48, Revision 1. The BWRVIP stated that the vessel attachment welds for core spray piping brackets are susceptible to stress corrosion cracking (SCC), which is culminated by the combined effects of stresses in the weld, high oxygen concentration in the core spray systems due to lack of hydrogen water protection and susceptible materials (i.e., stainless steel weld materials heat affected zone (HAZ) and Inconel alloy 182 welds) used in core spray piping bracket attachment welds. The degradation in these attachment welds was identified as SCC, and it is confined to the groove weld joint between the weld buildup (for some locations) on the vessel and the core spray attachments, or welds between core spray piping brackets and vessel stainless steel cladding material on the ID of the vessel.
The licensee included the following items (addressed by the BWRVIP) related to aging management of core spray piping brackets. The NRC staff has addressed the details of following items in Section 3.2.4 of this SE. The items are:
(1)
Historical Performance of Core Spray Piping Brackets (2)
SCC Susceptibility Discussion (3)
Weld Metal Susceptibility (4)
HAZ on the (Vessel Side) Susceptibility (5)
State of Stress (Weld and HAZ on the Vessel Side)
(6)
SCC Risk Assessment (7)
Qualitative Risk Assessment Summary and Conclusion 3.2
NRC Staff Evaluation
The NRC staff evaluated the licensees proposed revision to its previously approved alternatives to determine whether the revised alternative would provide an acceptable level of quality and safety as compared to the ASME Code,Section XI, requirements. The scope of the NRC staffs review is limited to only those changes associated with the BWRVIP-18 and BWRVIP-41 revisions.
In addition, in this submittal, the licensee has requested NRC staffs approval for using the revised inspection frequency for the core spray weld attachments addressed in the BWRVIP-48, Revision 1, which is not approved by the NRC staff. The NRC staffs review of the core spray attachments is also addressed in this SE.
3.2.1 BWRVIP-18, Revision 2-A The licensee requested that the previously approved alternatives, discussed in Section 2.0 of this SE, be revised to allow the licensee to use Revision 2-A of the BWRVIP-18 report. On February 22, 2016, the NRC staff generically approved Revision 2 of BWRVIP-18 with certain conditions (see ADAMS Accession No. ML16302A123). Revision 2-A of BWRVIP-18 incorporates the NRC staffs approval and associated conditions for the use of the guidelines.
For each facility, the NRC staff considered whether circumstances existed that were not included in BWRVIP-18, Revision 2-A, including the associated NRC conditions on their use that would preclude the use of these guidelines at the licensees facilities. The NRC staff found no such issues. The NRC staff determined that Revision 2-A of BWRVIP-18 provides an acceptable level of quality and safety as compared to the ASME Code,Section XI, requirements based on the NRC staffs previous approval of Revision 2, and the incorporation of the conditions of the approval into Revision 2-A. Specifically, the licensee proposed to use Revision 2-A, instead of Revision 1, of BWRVIP-18 at the Grand Gulf and River Bend units.
The licensee did not propose any other changes to these previously approved alternatives.
Therefore, the NRC staff finds it acceptable for the licensee to implement the BWRVIP I&E guidelines addressed in BWRVIP-18, Revision 2-A for Grand Gulf and River Bend units.
3.2.2 BWRVIP-41, Revision 4-A On July 2, 2018, the NRC staff generically approved Revision 4 of BWRVIP-41 with certain conditions (see ADAMS Accession No. ML18130A024 and references therein). Revision 4-A of BWRVIP-41 incorporates the NRC staffs approval and associated conditions. For each facility, the NRC staff considered whether circumstances existed that were not included in BWRVIP-41, Revision 4-A, including the associated NRC conditions on their use that would preclude the use of these guidelines at the licensees facilities. The NRC staff found one such case, which addresses the efficacy of hydrogen water chemistry, in combination of noble metal chemical addition, or online noble chemical addition (OLNCA). All these methods provide chemical mitigation in reducing the occurrence of intergranular SCC in BWR units. Both Grand Gulf and River Bend units have implemented the OLNCA method, which facilitates the licensee to take inspection credit for jet pump components. Inspection credits can be taken by the licensees if they conform to the NRC staffs criteria addressed in Section 4.0 of the supplemental final SE of BWRVIP-62, BWR Vessel and Internals Project, Technical Basis for Inspection Relief for BWR Internal Components with Hydrogen Injection (ADAMS Accession No. ML18142A019). Also, Category 3a plants that implement the OLNCA method are required to conform to the NRC staffs criteria addressed in Section 4.0 of the supplemental final SE of BWRVIP-62. Both Grand Gulf and River Bend units fall under Category 3a. The NRC staff concludes that plants which apply the OLNCA method and meet the criteria of Category 3a addressed in BWRVIP-62-A may claim inspection credit based on BWRVIP-62-A.
River Bend: In its supplemental letter dated November 9, 2020, the licensee stated that River Bend complied with the NRC staffs criteria of a Category 3a plant, therefore, it can take credit for adopting the inspection frequencies for the jet pump components addressed in BWRVIP-41, Revision 4-A. Based on its review of the licensees supplemental letter, the NRC staff finds the licensee complied with the NRC staffs criteria in BWRVIP-62-A for a Category 3a plant.
Grand Gulf Unit 1: In its supplemental letter dated November 9, 2020, the licensee stated that except for the minimum required catalyst loading, Grand Gulf complied with the NRC staffs criterion related to verification of the severity of oxidizing nature of the reactor coolant system water in a Category 3a unit. In its supplemental letter dated August 25, 2021, the licensee confirmed that based on its analysis of the Grand Gulf fuel samples, it determined that the average catalyst loading complied with the criterion related to catalyst loading. The NRC staff reviewed this information and finds it acceptable because the licensee complied with the NRC staffs conditions specified in Section 4.0 of the supplemental SE for BWRVIP-62 for a Category 3a unit. Based on its review, the NRC staff determined that the licensee may implement BWRVIP-41 Revision 4-A, at Grand Gulf for the fourth ISI interval.
3.2.3 Re-inspection Issues of BWRVIP-47-A Guidelines for Lower Plenum Components Re-inspection criteria are not addressed for the lower plenum B-N-1 and B-N-2, RVI components in the BWRVIP-47-A, report, BWR Vessel and Internals Project BWR Lower Plenum Inspection and Flaw Evaluation Guidelines (ADAMS Accession No. ML043290063; not publicly available, proprietary information; NRC approval letter dated September 1, 2005 (ADAMS Accession No. ML052490537)). In its supplemental letter dated November 9, 2020, the licensee stated that it had completed baseline inspections of control rod guide tube assembly at Grand Gulf and River Bend, and no indications were found. In addition, the licensee inspected RVI components below the lower core plate, incore housing, dry tubes, and to date, no indications are found. The licensee reiterated that it would perform inspections of the RVI components when they become accessible for inspections during the fourth ISI interval.
The RVI components include all vessel penetrations, incore housing, dry tubes, shroud support legs, and a vessel lower head. Based on the licensees clarification, the NRC staff concludes that the licensee will continue to perform ISI examinations of accessible areas (when available) of B-N-1 and B-N-2, RVI components of the lower plenum as required by the ASME Code,Section XI. The NRC staff finds the licensees response acceptable and considers that this issue is closed.
3.2.4 Qualitative Risk Assessment for Extension of the Core Spray Piping Brackets Attachment Weld Examination Interval Addressed in Revision 1 to BWRVIP-48 The NRC staff reviewed the licensees technical evaluation developed by the BWRVIP for implementing a revised inspection frequency only for the primary and supplemental core spray piping brackets addressed in BWRVIP-48, Revision 1 at Grand Gulf and River Bend. The BWRVIP stated that the vessel attachment welds for core spray piping brackets are susceptible to SCC, which is culminated by the combined effects of stresses in the weld, high oxygen concentration in the core spray systems due to lack of hydrogen water protection, and susceptible materials (i.e., stainless steel materials and Inconel alloy 182 welds) used in core spray piping bracket attachment welds. The aging degradation in these attachment welds due to SCC is confined to the groove weld joint between the weld buildup on the vessel and the core spray piping brackets, or welds between core spray piping brackets and vessel stainless steel cladding material on the ID of the vessel.
Historical Performance of Core Spray Piping Bracket Attachment Welds The BWRVIP stated that, to date, there have been 450 detailed visual examinations performed in the U.S. BWR fleet using Enhanced Visual Testing (EVT-1) for these welds. The NRC staff reviewed the operating experience to date, which indicates that no SCC has been reported in the core spray piping bracket attachment welds to the vessel. The EVT-1 technique has the capability of identifying very small size cracking, and the inspection coverage obtained in these welds ranged from 80-100 percent for the primary core spray piping brackets and for supplemental brackets, the coverage ranged from 60-95 percent. Based on these observations, the NRC staff determined that the EVT-1 inspection technique is effective in identifying small cracks on the surface. In the absence of SCC in the core spray bracket attachment welds, the NRC staff considers that if there were to be any SCC cracks in these welds, they would have been identified in the BWR fleet.
SCC Susceptibility Discussion The BWRVIP stated that main essential variables for causing SCC are: (1) environment; (2) material susceptibility; and (3) tensile stresses. The NRC staff reviewed the combined effects of these variables on the SCC, and its evaluation is addressed below:
(1)
Environment: The core spray piping bracket attachment welds are exposed to a highly oxidizing environment with no effective protection from hydrogen. Therefore, a sample population in the core spray piping system is frequently inspected to monitor the potential for SCC during each outage in the BWR fleet.
(2)
Material Susceptibility: Stainless steel base metals, when they are sensitized due to heat generated in the welding process, are more susceptible to SCC. This is called the sensitization process, and it occurs in the vicinity of HAZ in the base metal adjacent to the weld joint, characterized by a Chromium depleted zone, which enhances the susceptibility to SCC. With respect to weld metal, stainless steel weld metal is less susceptible to SCC than Alloy 182 weld metal due to the presence of ferrite in the stainless steel welds. Alloy 182 weld metal does not have ferrite metallurgical phase; therefore, it is more susceptible to SCC.
(3)
Tensile Stresses: The BWRVIP stated that the only applied load to the core spray brackets is the dead weight of the core spray pipe. According to the BWRVIP, the loading due to the dead weight is minimal, therefore, the predominant loading for causing SCC is the residual stresses of the weld that joins the core spray attachment brackets to the vessel wall. The NRC staff considers that residual stresses associated with the weld joint is probably a predominant factor that could case SCC. This is due to accumulation of residual stresses due to weld repairs that could potentially occur during the fabrication. The residual stress from the weld repair could accentuate the cracking during service.
Weld Metal and HAZ (Vessel Side) Assessment The licensee stated that core spray piping bracket attachment welds at Grand Gulf and River Bend were fabricated using Type 308/308L austenitic stainless steel weld materials. As stated above, stainless steel welds are less susceptible to SCC due to presence of ferrite metallurgical phase in the weld metal. The ASME Code,Section III, requires 5 percent of ferrite in the stainless steel weld metal. Consistent with this expectation, the operating experience to date indicated that cracks are not commonly observed in stainless steel welds unless they were subjected to heavy cold work during fabrication. Based on the BWRVIPs recommendation, the licensees perform routine inspections in stainless steel weld joints and focus on the inspections of HAZ areas where Chromium depletion occurs. For Grand Gulf and River Bend, the core spray piping bracket attachment welds were fabricated with stainless steel weld metal, which has more Chromium than the HAZ area in the stainless steel base metal. Therefore, the weld metal is less prone to SCC as opposed to the Chromium depleted zone in HAZ areas. Since the piping bracket attachment welds are attached to the stainless steel weld buildup of the vessel or stainless steel vessel cladding, the licensee will inspect the HAZ areas at the piping bracket attachment welds in accordance with BWRVIP-48, Revision 1. The NRC staff finds this type of inspection is acceptable because the licensees are inspecting the most susceptible areas of the weld joints (i.e., HAZ areas) of the vessel base metal.
State of Stress (Weld and HAZ on Vessel Side)
The BWRVIP stated that the applied loads on the core spray piping brackets during normal operations are low, therefore, it can be concluded that the predominant driving force for SCC is the residual weld stresses associated with the double welded groove joint of the core spray bracket welds. These bracket welds are addressed in Figure 2-8 in BWRVIP-48-A. The BWRVIP stated that shrinkage stresses associated with the solidification cooling of the weld metal could increase the residual stresses in the weld joint. This increase in the weld residual stress is directly proportional to weld joint restraint offered by the thicker member in the weld joint assembly. Based on the weld design of the double welded groove joint, as shown in Figure 2-8 in the BWRVIP-48-A report, the residual stress in this weld joint might not have reached the threshold limit (approximately yield stress) to cause SCC.
The NRC staff noted that generally, the residual stresses can be minimized by using a proper welding sequence in the top and bottom portions of the double groove to balance the stresses and minimize distortion. The NRC staff further noted that in addition, the piping bracket was not completely restrained, and it was completely free to move during the welding process. Therefore, in the absence of a weld joint restraint, it can be concluded that over the years, the SCC did not manifest in the core spray piping bracket welds attached to the vessel as a result of moderate weld residual stress. Furthermore, the NRC staff concludes that low to moderate weld heat input and limited weld repair could also have contributed to the absence of SCC in the piping bracket vessel attachment welds. The NRC staff determined that weld residual stresses in the attachment weld joints did not exceed the threshold limit to cause SCC and this conclusion can be based on the following reasons: (1) Moderate weld heat input during the fabrication of the core spray bracket attachment welds, (2) limited weld restraint during welding, (3) proper weld sequencing, (4) minimum amount of grinding during welding and/or, (5) few weld repairs.
SCC Risk Assessment The NRC staff determined that even though the core spray piping systems are exposed to highly oxygenated reactor coolant, no SCC, to date, was discovered in the core spray piping bracket welds to the vessel. During the first 10 years of operation, several cracks were identified in the core spray piping systems; however, after repair and/or replacement with SCC resistant materials in the core spray systems, a lower number of cracks were identified. This declining trend can be due to self-limiting nature of cracks in stainless steel materials. With respect to core spray piping brackets, since no cracks were observed, it can be concluded that these welds are less likely to crack during the fourth ISI interval at Grand Gulf and River Bend.
However, as a means of verification, the core spray piping brackets will be routinely inspected in accordance with the inspection criterion addressed in the BWRVIP-48, Revision 1. The NRC staff determined that due to the time dependent nature of SCC growth rates, if any new cracks were to occur in these welds, they would be identified already, and corrective action would be taken by the licensee in a timely manner. Therefore, the NRC staff determines that the risk factor in not identifying the SCC in a timely manner is low.
Qualitative Risk Assessment and Conclusions Even though there are no cracks identified thus far in the core spray piping bracket welds, the NRC staff noted that the BWRVIP implemented a conservative approach to monitor SCC in the core spray piping bracket welds. To date, a total of 450 inspections of core spray piping bracket welds have been performed in the BWR fleet, and no cracks have been found. In the absence of any SCC in these welds, the NRC staff concludes that the licensees proposed inspection frequency addressed in BWRVIP-48, Revision 1, for core spray piping bracket welds is acceptable based on the following observations:
(1)
If any detection of SCC in core spray piping welds were to occur in any of the units in the BWR fleet, the BWRVIP would alert the industry and the NRC staff to further evaluate the inspection frequencies in these welds. Field inspection data will be available to the fleet regularly because of different outages schedules of each BWR unit. If any SCC were to occur in the future, the operating experience will provide opportunities for modifying the re-examination schedules of the core spray piping bracket welds; (2)
Previous inspection results addressed in BWRVIP-18, Revision, 2-A, indicated that over the time interval, average crack growth rates due to SCC tend to decrease, which suggests that the cracking may be self-limiting. Since SCC is a time dependent phenomenon, based on item (1) above, any emerging cracks could be identified in a timely fashion in the BWR units: and, (3)
All the requirements addressed in BWRVIP-48-A, including re-inspection, and scope expansion criteria are applicable to the core spray piping bracket welds. These inspection criteria provide reasonable assurance that the effective aging management program is implemented at Grand Gulf and River Bend during the fourth ISI interval.
Based on these observations, the NRC staff concludes that the proposed inspection frequency addressed in the November 9, 2020, supplement provides reasonable assurance that the aging degradation due to SCC in the core spray piping bracket welds is adequately managed by the licensee during the fourth ISI interval. The NRC staff approval of the subject inspection frequency for core spray piping bracket welds addressed in BWRVIP-48, Revision 1, is on a plant-specific basis only for the duration of the fourth ISI interval at Grand Gulf and River Bend.
The NRC staff-approved inspection frequencies for core spray bracket vessel attachment welds addressed in BWRVIP-48, Revision 1, will supersede the previous inspection frequencies approved by the NRC staff for these welds at Grand Gulf and River Bend.
3.3
SUMMARY
The NRC staff finds that the implementation of the BWRVIP I&E guidelines specified in the licensee's proposed alternative in its letters dated June 8, 2020, and November 9, 2020, will ensure the structural integrity of the RVI components with an acceptable level of quality and safety. Therefore, the NRC staff authorizes only the approved versions of the BWRVIP I&E guidelines addressed in this alternative. In the event the licensee decides to take exceptions to, or deviations from the authorized alternative, the licensee must revise and resubmit its request for authorization to use the proposed alternative under 10 CFR 50.55a(z)(1).
4.0 CONCLUSION
With respect to Relief Request GG-ISI-020 for Grand Gulf, the NRC staff approves the use of BWRVIP-18, Revision 2-A for the fourth ISI interval. With respect to BWRVIP-41, Revision 4-A, the licensee complied with the NRC staffs conditions, which confirm the efficacy of the OLNCA method. Therefore, the licensee is authorized to implement BWRVIP-41, Revision 4-A, for the fourth ISI interval at Grand Gulf. With respect to BWRVIP-48, Revision 1, the NRC staff finds that all the requirements addressed in the BWRVIP-48-A guidelines for RVI components other than core spray piping bracket welds remain valid for the fourth ISI interval at Grand Gulf. With respect to the core spray piping bracket welds, the NRC staff finds that subject inspection frequency for core spray piping bracket welds addressed in BWRVIP-48, Revision 1, is valid for the fourth ISI interval at Grand Gulf.
With respect to Relief Request RBS-ISI-019 for River Bend, the NRC staff finds that BWRVIP-18, Revision 2-A, and BWRVIP-41, Revision 4-A remain valid for the fourth ISI interval at River Bend. The NRC staff finds that all the requirements addressed in the BWRVIP-48-A guidelines for RVI components other than core spray piping bracket welds remain valid for the fourth ISI interval at River Bend. The NRC staff further finds that core spray piping bracket welds addressed in BWRVIP-48, Revision 1, is valid for the fourth ISI interval at River Bend.
The NRC staff determines that the licensees proposed alternative as stated in the submittals provide an acceptable level of quality and safety. Accordingly, the NRC staff concludes that the licensee has adequately addressed all the regulatory requirements set forth in 10 CFR 50.55a(z)(1). Therefore, the NRC staff authorizes the use of the licensees proposed alternative for the remainder of the fourth 10-year ISI interval at Grand Gulf and River Bend.
The NRC approval of the subject inspection frequency for core spray piping bracket welds addressed in BWRVIP-48, Revision 1, does not imply or infer the NRCs approval of BWRVIP-48, Revision 1 for generic use.
All other requirements of ASME Code,Section XI, for which relief was not specifically requested and authorized by the NRC staff remain applicable, including the third-party review by the Authorized Nuclear In-Service Inspector.
Principal Contributors: O. Yee, NRR G. Cheruvenki, NRR Date: September 21, 2021
- via e-mail OFFICE NRR/DORL/LPL4 /PM*
NRR/DORL/LPL4/LA*
NRR/DNRL/NVIB/BC*
NAME SLingam PBlechman ABuford DATE 9/14/2021 9/21/2021 9/10/2021 OFFICE NRR/DORL/LPL4/BC*
NAME JDixon-Herrity DATE 9/21/2021