Document: NRC Regulatory Guide
Document ID: b4874b3a-bbf8-4eb6-8b86-db32ecf45ac6
Document Type: regulatory_guide
Title: Guidance for Implementation of 10 CFR 50.59, Changes, Tests, and Experiments (Rev. 2)
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML1904/ML19045A435.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.187
CFR Part: 
CFR Title: 

Content:
CFR 50.59(a)(2)(i) of the departure definition (i.e., “the results of the analysis are conservative or essentially the same”) is not applicable. b. Section 4.3.5 of NEI 96-07, Revision 1, states, in part: Certain accidents are not discussed in the UFSAR because their effects are bounded by other related events that are analyzed. For example, a postulated pipe break in a small line may not be specifically evaluated in the UFSAR because it has been determined to be less limiting than a pipe break in a larger line in the same area. Therefore, if a proposed design change would introduce a small high energy line break into this area, postulated breaks in the smaller line need not be considered an accident of a different type. The last sentence of Section 4.3.5 of NEI 96-07, Revision 1, states, “Accidents of a different type are credible accidents that the proposed activity could create that are not bounded by UFSAR- evaluated accidents.” The UFSAR evaluates a broad spectrum of transients and accidents or initiating events. Accidents are categorized by type based on their effects on the plant. For example, one type of accident will cause the reactor coolant system (RCS) to pressurize and possibly jeopardize RCS integrity. Categorizing accidents by type provides a basis for comparison between events, which makes it possible to identify and evaluate the limiting cases (i.e., the cases that can challenge the analysis acceptance criteria) and eliminate non-limiting cases from further consideration. To assist in identifying accidents of a different type, consider that plant UFSAR analyses were based on credible failure modes of existing equipment and determine whether a proposed modification would change the basis for the most limiting scenario. Accidents that are not limiting cases are not discussed in the UFSAR. DG-1356, Page 8 An accident of a different type is any new accident, distinct from any previously evaluated in the UFSAR but of similar frequency and