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Federal Register / Vol. 86, No. 20 / Tuesday, February 2, 2021 / Proposed Rules
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of these welded fittings have been met prior to use is essential in ensuring the structural integrity of these Class 1, 2
and 3 systems is maintained. Therefore, the NRC is proposing conditions requiring the licensee to verify the design and testing activities associated with qualification of welded fittings required by Section III, NB/NC/ND
3671.7 that are performed by the fabricator.
Code Case N889 Supplement 7, 2017
Edition Type: New.
Title: Reference Stress Corrosion Crack Growth Rate Curves for Irradiated Austenitic Stainless Steel in Light-Water Reactor Environments, Section XI, Division 1.
Code Case N889 provides a new crack growth rate CGR law for irradiation-assisted stress corrosion cracking. The Code Case is applicable to wrought austenitic stainless steels and associated weld metals, as well as cast austenitic stainless steels. The proposed CGR law requires the user to first calculate irradiated yield stress from the dose to the material. There are two yield stress models: One for Molybdenum bearing stainless steels and one for stainless steels without Molybdenum.
Once irradiated yield stress has been determined, the user calculates the CGR
as a function of applied crack driving force and temperature.
The staff identified three concerns with the technical basis of this Code Case. The first concern relates to the limited CGR data at dose levels greater than 20 displacements per atom dpa.
The proposed CGR law indicates that the irradiated yield stress and, consequently, the CGR increases with fluence up to a dose of 20 dpa, at which point the irradiated yields stress ceases to increase appreciably with further dose accumulation. While the data at dose levels greater than 20 dpa does show a plateau behavior in the CGR, the staffs analyses of that data suggests that areas of high CGR were averaged over the industry calculation of CGR, which increases the uncertainty in the high dose CGRs. Therefore, due to the limited data and the associated high uncertainty at high fluence, the staffs confidence in CGRs at dose levels greater than 20 dpa is low.
The second concern is the effects of uncertainty in the irradiated yield strength value for an individual material-heat. This topic is discussed in Section 4.7 of the technical basis report for Code Case N889. The NRC also conducted separate analyses. While the results of the NRCs findings are generally consistent with the results in
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Section 4.7, the interpretation of their significance is not consistent. For materials with yield strengths greater than 600 MPa i.e., more highlyirradiated materials, the expected CGR
for a material with a yield strength in the 95th percentile is less than two times the CGR predicted by the Code Case, which is not a significant difference. However, for materials with yield strength values less than 250 MPa i.e., unirradiated or minimally irradiated materials, the expected CGR
for a material in the 95th percentile can be more than five times greater than the CGR predicted by the Code Case. Hence, the NRCs concern is that the CGRs for individual low yield strength materials, or materials with low fluence, could be significantly underpredicted by the Code Case.
The final concern is related to the data used in the development of the irradiated yield stress model. The methodology for addressing cold work in this model was developed in MRP
135, Revision 1, while the model itself was developed in MRP211, Revision 0.
The database underlying the model included hundreds of yield strength measurements on initially annealed and cold-worked Types 304, 316, and 347/
348 stainless steel materials. However, most of the data were for annealed Type 304 and cold-worked Type 316 stainless steels. Revision 1 of MRP211 contained additional yield strength data, including significantly more data for cold-worked Types 304 and 347 stainless steel. The authors of the Code Case, as documented in Section 4.5 of the Additional Basis Report dated February 5, 2018, evaluated the Code Case yield stress model with some of this additional data and found agreement between the model and the additional data. However, the Code Case authors excluded new data for cold-worked Type 304 and 347 stainless steel materials. Therefore, the technical basis document for Code Case N889 does not directly address whether cold-worked Type 304 and 347 non-Molybdenum bearing materials are adequately predicted by the irradiated yield strength model in the Code Case.
Therefore, the NRC is proposing three conditions on this Code Case.
The first proposed condition states that this Code Case may not be applied for neutron exposures greater than 20
dpa. This condition addresses the NRC
concern that there is sparse data with high uncertainty beyond 20 dpa. Given that the predicted CGR saturates at higher fluence, this condition prevents potential underprediction of the CGR in this fluence regime.

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The second proposed condition states that at dose levels below 0.75 dpa, the user must use the higher of the Code Case N889 or the Section XI, Nonmandatory Appendix C, C8520
CGR predictions. This condition addresses the NRC concern related to possible underprediction of CGR in Code Case N889 for materials with calculated irradiated yield strength less than 250 MPa.
The final proposed condition states that the irradiated yield stress model for cold-worked Molybdenum bearing materials must be used for cold-worked non-Molybdenum bearing stainless steels including Type 204 and 247
stainless steels. This condition addresses the NRC concern that data for cold-worked non-Molybdenum bearing steels were not appropriately considered during development of Code Case N
889. The NRC performed its own evaluation of cold-worked Type 304 and 347 stainless steels in the MRP211
database and found that the yield strength was better predicted by the Code Cases Molybdenum bearing model than with the Code Cases nonMolybdenum bearing model.
Code Case N890 Supplement 0, 2019
Edition Type: New.
Title: Materials Exempted From G
2110b Requirements, Section XI, Division 1.
Code Case N890 provides an alternative to Section XI, G2110b which removes the requirement of, obtaining fracture toughness data for at least three heats, for using the static fracture toughness curve KIc curve for specific materials with a minimum specified yield strength at room temperature between 50 kilopound per square inch ksi and 90 ksi. Code Case N890 would allow the toughness of four ferritic steels SA508 Grade 2
Class 2, SA508 Grade 3 Class 2, SA
533 Type A Class 2 and SA533 Type B Class 2 with specified minimum yield strength greater than 50 ksi to be characterized by Figure G21101 i.e., the Section XI KIc curve.
The NRC identified one technical concern when reviewing the technical basis of this Code Case. The technical basis provided appropriate data to justify use of the KIc curve for several materials listed in the Code Case.
However, for SA533 Type B, Class 2
materials, the NRC observed that in the technical basis document, there is no fracture toughness data associated with the weld and heat affected zone to support exclusion of the fracture toughness testing requirements for these materials.

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