Document: NRC Regulatory Guide
Document ID: 6f84e3bf-9ff3-49fb-a16a-0de3b89e6bc6
Document Type: regulatory_guide
Title: An Approach for Plant-Specific Risk-Informed Decisionmaking for Inservice Inspection of Piping (Rev. 2)
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML2103/ML21036A105.pdf
Revision Date: 2023-05
Chapter: 
Section ID: RG-1.178
CFR Part: 
CFR Title: 

Content:
inues to be an effective way to account for uncertainties in equipment and human performance and, in particular, to account for the potential for unknown and unforeseen failure mechanisms or phenomena that, because they are unknown or unforeseen, are not reflected in either the PRA or deterministic engineering analyses. The staff requirements memorandum on SECY-98-144, “Staff Requirements—SECY-98-144—White Paper on Risk-Informed and Performance- Based Regulation,” dated March 1, 1999 (Ref. 27), provides additional information on defense in depth as an element of the NRC’s safety philosophy. The engineering evaluation should demonstrate whether the implementation of the proposed ISI program change is consistent with the defense-in-depth philosophy. The intent of this key principle of RG 1.178, Page 13 risk-informed decisionmaking is to ensure that any impact of the proposed licensing-basis change on defense in depth is fully understood and addressed and that consistency with the defense-in-depth philosophy is maintained. The intent is not to prevent changes in the way defense in depth is achieved. The licensees should fully understand how the proposed licensing-basis change impacts plant design and operation from both risk and deterministic engineering perspectives. RG 1.174 should be used to evaluate the impact of a proposed licensing-basis change on defense in depth to determine whether that consistency is achieved. An important element of defense in depth for RI-ISI is maintaining the reliability of independent barriers to fission product release. Class 1 piping (primary coolant system) is the second boundary between the radioactive fuel and the general public. Therefore, even if the RI-ISI program categorized all segments in the hot and cold legs of the primary system piping as having low safety significance and calculated that, with no inspections, the frequency of leaks would not increase beyond existing performance history of the ASME BPV Code, the