Document: NRC Regulatory Guide
Document ID: a4944d20-ef6e-480b-a45d-14425bedd2e5
Document Type: regulatory_guide
Title: ASME Code Cases Not Approved for Use (Rev. 8)
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML2219/ML22196A065.pdf
Revision Date: 2023-05
Chapter: 
Section ID: RG-1.193
CFR Part: 
CFR Title: 

Content:
rves the NRC intends to use in NUREG/CR-6909, Revision 1) because the Code Case fatigue curve uses a margin of 20 on cycles, whereas the NRC curves use a margin of 12. The Code Case design fatigue air curve is conservative with respect to the NRC fatigue design air curves; however, item (b) below must also be considered when evaluating the adequacy of Fen usage factors calculated using the design curve. b. Carbon and Low-Alloy Steel Fen Expression: Equation (1) of the Code Case uses the carbon steel Fen expression from the initial revision of NUREG/CR-6909 that was adjusted to account for the difference in the margin term used to develop the ASME and NRC design fatigue curves. This equation is different from the Fen expression recently developed by the NRC, and the equations for the transformed environmental parameters are different; therefore, the Fen equation may yield nonconservative values of Fen for the following reasons: (1) The use of average temperature with the Code Case Fen expression may be nonconservative (see item (f)). (2) The Code Case Fen expression was adjusted to account for the difference in the margin used to develop the design curve (i.e., the factor of 20 versus 12 discussed under item (a) 8/12/12 DG-1408, Page 9 CODE CASE NUMBER TABLE 1 UNACCEPTABLE SECTION III CODE CASES SUMMARY DATE OR SUPPLEMENT/ EDITION above). As a result, the constant in the Fen expression is 0.121 compared to 0.632 for carbon steel material in the initial revision to NUREG/CR-6909. Such adjustment is not appropriate and may be nonconservative for Fen application to the portion of the fatigue design air curve that is controlled by the factor of 2 on stress rather than the factor of 20 on cycles. (3) The Code Case Fen expression is for carbon steel material and it is used for application to both carbon and low-alloy steel materials. Use of this expression for low-alloy steel may be nonconservative because the constant is higher for low-alloy steel compared to carbon steel