Document: NRC Regulatory Guide
Document ID: db0cf2ba-a5df-49f8-9003-005c7f8bd08a
Document Type: regulatory_guide
Title: Combining Modal Responses and Spatial Components in Seismic Response Analysis (Rev. 3)
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML1814/ML18142B201.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.92
CFR Part: 
CFR Title: 

Content:
Regulatory Guide Number: 1.92, Revision 3 Title: Combining Modal Responses and Spatial Components in Seismic Response Analysis Office/division/branch: RES/DE/SGSEB Technical Lead: Jinsuo Nie Staff Action Decided: Reviewed with issues identified for future consideration 1. What are the known technical or regulatory issues with the current version of the Regulatory Guide (RG)? RG 1.92 Revision 3, published in October 2012, provides guidance on methods that the NRC staff considers acceptable for combining modal responses and spatial components in seismic response analysis of nuclear power plant structures, systems and components important to safety (SSCs). The changes from Revision 2 were editorial with no substantive change in the Staff Regulatory Positions. RG 1.92 Revision 2 was issued in 2006 and implemented recommendations provided in NUREG/CR-6645, “Reevaluation of Regulatory Guidance on Combination Methods for Seismic Response Spectrum Analysis,” published in December 1999. One issue to be addressed is clarifying the regulatory position related to Section 1.1.1 on closely spaced modes to reflect the staff experience gained in licensing reviews of new reactor designs. Specifically, Revision 3 considers that the modal combination methods described in Revision 1 remain acceptable. These methods rely on a 10% frequency window criterion to define closely spaced modes regardless of damping ratios. However, the Revision 1 criterion differs from the current criterion in Section 1.1.1 of Revision 3, which is damping dependent and the 10% criterion is only applicable when the critical damping ratio is ≤ 2%. This apparent confusion needs to be addressed in a future revision of the RG. It should be noted that the general methods explicitly provisioned in Regulatory Position 1.1 of Revision 3 remain acceptable. Based on recent licensing experience, staff also identified three other technical issues that need further confirmatory research to address issues potentially