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Federal Register / Vol. 86, No. 20 / Tuesday, February 2, 2021 / Proposed Rules
material e.g., stainless steel alloys.
These conditions are currently located in 50.55ab2xiiA and B. The conditions located in 50.55ab2xiiA and B are identical to the conditions that were imposed on Code Case N5164 that were approved by the NRC in Revision 19 of RG 1.147. When the ASME revised N516, the Code Case was not modified in a way that would make it possible for the NRC to remove the conditions.
Therefore, the conditions will be retained in Revision 20 of RG 1.147 by stating the provisions of 50.55ab2xiiA and B must be met when applying this Code Case.
Code Case N7051 Supplement 2, 2017 Edition Type: Revised.
Title: Evaluation Criteria for Temporary Acceptance of Degradation in Moderate Energy Class 2 or 3 Vessels and Tanks, Section XI, Division 1.
The proposed condition on Code Case N7051 is identical to the condition on N705 that was approved by the NRC in Revision 19 of RG 1.147. When the ASME revised N705, the Code Case was not modified in a way that would make it possible for the NRC to remove the condition. Therefore, the condition would be retained in Revision 20 of RG
1.147.

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Code Case N7663 Supplement 2, 2017 Edition Type: Revised.
Title: Nickel Alloy Reactor Coolant Inlay and Onlay for Mitigation of PWR
Full Penetration Circumferential Nickel Alloy Dissimilar Metal Welds in Class 1
Items, Section XI, Division 1.
The proposed conditions on Code Case N7663 are identical to the conditions on N7661 that were approved by the NRC in Revision 19 of RG 1.147. When the ASME revised N
766, the Code Case was not modified in a way that would make it possible for the NRC to remove the conditions.
Therefore, the conditions would be retained in Revision 20 of RG 1.147.
Code Case N8311 Supplement 7, 2017 Edition Type: Revised.
Title: Ultrasonic Examination in Lieu of Radiography for Welds in Ferritic or Austenitic Pipe, Section XI, Division 1.
The proposed condition on Code Case N8311 is identical to the condition on N831 that was approved by the NRC in Revision 19 of RG 1.147. When ASME
revised N831, the Code Case was not modified in a way that would make it possible for the NRC to remove the condition. Therefore, the condition
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would be retained in Revision 20 of RG
1.147.
Code Case N847 Supplement 0, 2017
Edition Type: New.
Title: Partial Excavation and Deposition of Weld Metal for Mitigation of Class 1 Items, Section XI, Division 1.
ASME Code Case N847 provides guidelines for a repair/mitigation process for welds. The process, excavation and weld repair EWR, removes susceptible material from the outside diameter of the pipe, and replaces it with more resistant weld material. This technique allows for the potential of two mitigation methods, the use of more crack resistant material and the potential for compressive stresses on the inside surface of the repaired/
mitigated weld to arrest or prevent cracking. Finally, the excavation can be done 360-degrees around the weld or only for a partial arc of the weld.
The Code Case would allow for application of this process to both BWR
and PWR designs. However, the EWR
process, as defined in this code case, has certain challenges addressing the cracking mechanisms in these operating enviornments and materials. In addition the regulatory requirements or guidelines related to the Code Case vary depending on the design of the reactor.
For PWR designs, the inservice inspection rules are provided by 50.55ag6iiF, which mandates the implementation of a version of ASME
Code Case N770. For BWR designs, the inservice inspection guidelines are provided by Generic Letter 8801, NRC
Position on Intergranular Stress Corrosion Cracking IGSCC in BWR
Austenitic Stainless Steel Piping or BWRVIP75A, BWR Vessel and Internals Project Technical Basis for Revisions to Generic Letter 8801
Inspection Schedules. Therefore, the NRC is proposing six conditions to ensure the inservice inspection frequency guidelines of the code case are inline with the previous requirements and guidance, which are based on the effectiveness of the overall design of the repair/mitigation to address the various cracking mechanisms of these operating reactor designs.
The first proposed condition is a continuation of the condition of 50.55ag6iiF16 which requires that a partial arc EWR, as described in Inspection Item O of ASME Code Case N7705, cannot be used without NRC
review and approval for PWR designs.
The NRC notes that the issues addressed in the final rule incorporating by reference the 2015 and 2017 Editions of
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the ASME BPV Code and the 2015 and 2017 Editions of the ASME OM Code remain applicable, and futher apply to BWR design application of a partial arc EWR. These concerns are for the effectiveness of the repair through a weld residual stress calculation and flaw growth analysis to confirm design of the mitigation for the required inspection interval, non-destructive examination uncertainty analysis of the as-found flaw remaining in the reactor coolant pressure boundary, and the potential for further crack initiation or growth. Therefore, the NRC requires, through the first condition, that approval of the use of this Code Case is only for the application of the 360-degee EWR.
The second proposed condition is related to Figure 1A and Figure 1B of the Code Case. The NRC has experience with relief request submittals, where the details associated with the configuration of the prep area, where the defect is being removed, have shown sharp bottom edges and steep walls. This geometry can result in welding issues, which could result in unfused material, leading to stress risers, which may promote cracking. Therefore, the NRC
requires, through the second condition, that the intersection points at the interface between EWR metal and existing base metal must be rounded to minimize stress concentration.
The third proposed condition is related to Section 2d2 of the Code Case which discusses the flaw evaluations required for the design considerations of the EWR. In recent testing conducted for the NRC
measurable stress corrosion cracking SCC growth was detected past the interface between the SCC-susceptible and less susceptible material. It was demonstrated that the crack can branch and propagate in a direction normal to the original direction along a SCCsusceptible path. In the Alloy 52M
deposited onto Alloy 182 specimens tested, this occurred in the diluted region of the Alloy 52M material as well as the weld metal. Therefore, the NRC
requires, through the third condition, that flaw analysis include the potential for crack growth through the dilution zone. As NRC-approved crack growth rates are not available for all material types e.g., Alloy 690 weld material, the alternative requirements for development of crack growth rates should be consistent with ASME
Section XI Appendix C, Flaw Growth Rate Due to Stress Corrosion Cracking, C3220a.
The fourth proposed condition is related to Section 2 of the Code Case.
The NRC is requiring the use of
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