Document: NRC Regulatory Guide
Document ID: bcbe26a0-004f-4616-b7f4-b32b2bfe4909
Document Type: regulatory_guide
Title: Setpoints for Safety-Related Instrumentation (Rev. 4)
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML2005/ML20055G823.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.105
CFR Part: 
CFR Title: 

Content:
d provide the licensee flexibility to revise setpoints without requiring a license amendment. The setpoint control program should implement a setpoint calculation methodology that the NRC has reviewed and accepted specifically for that purpose. This RG provides guidance that the staff considers appropriate for such a methodology. 2.2.2 Adequacy of the Allowance for Channel Uncertainties between the Limiting Trip Setpoint and the Analytical Limit Limiting trip setpoints should allow for instrument channel performance uncertainties. In prior revisions, ANSI/ISA 67.04.01 described estimating the instrument channel total loop uncertainty by combining random and bias uncertainties. However, these prior revisions did not provide acceptance criteria for establishing the appropriate magnitude of estimates for individual random uncertainties. In the 2018 revision of the standard, Section 4.4(i) states, “The uncertainty tolerance interval for random, independent uncertainty terms shall be estimated using statistical and bounding methods such that the tolerance interval estimate bounds the uncertainty of interest with a 95-percent probability, at a 95-percent confidence level.” In addition, Section 4.4(i) states, “Approaches for determining the tolerance interval may vary, based on the type of distribution and will generally consider the sample size and standard deviation of the sample population to obtain the desired probability and confidence levels.” The 2018 revision also provides guidance for cases in which the sample population is not large enough to support a usable statistical estimate at the 95/95 tolerance interval. In such cases, the licensee should determine a bounding uncertainty term and document the basis for determining this term. The licensee can use this bounding value as a 95/95 tolerance interval term in the uncertainty analysis. DG-1363, Page 7 According to ANSI/ISA 67.04.01-2018, the uncertainty terms contributing to the estimate of total