Document: NRC Regulatory Guide
Document ID: 5f799693-27fd-4e13-a5e1-4c02f393d90a
Document Type: regulatory_guide
Title: Best-Estimate Calculations of Emergency Core Cooling System Performance + HISTORY –HISTORY 04/2013 – Periodic Review of Revision 0 – Reviewed with issues identified for future consideration 03/1987 – Draft RS 701-4, Proposed Revision 0
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML0037/ML003739584.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.157
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CFR Title: 

Content:
of the two standard deviations compared to the 95th percentile is used to account for uncertainty in the probability distribution. Other techniques that account for the uncertainty in a more detailed manner may be used. These techniques may require the use of confidence levels, which are not required by the above approach. The evaluation of the peak cladding temperature at the 95% probability level need only be performed for the worst-case break identified by the break spec trum analysis in order to demonstrate conformance with paragraph 50.46(b). However, in order to use this approach, justification must be provided that demonstrates that the overall calculational uncer tainty for the worst case bounds the uncertainty for other breaks within the spectrum. It may be neces sary to perform separate uncertainty evaluations for large- and small-break loss-of-coolant accidents be cause of the substantial difference in system thermal hydraulic behavior. The revised paragraph 50.46(a) (1)(i) requires that it be shown with a high probability that none of the criteria of paragraph 50.46(b) will be exceeded, and is not limited to the peak cladding temperature criterion. However, since the other criteria are strongly dependent on peak. cladding temperature, explicit consideration of the probability of exceeding the other criteria may not be required if it can be demonstrated that meeting the temperature criterion at the 95% probability level ensures with an equal or greater probability that the other criteria will not be exceeded. 4.5 NRC Approach to LOCA Uncertainty Evaluation Chapter 4 of the "Compendium of ECCS Research for Realistic LOCA Analysis" (Ref. 7) presents a methodology that has been used for evaluating the overall calculational uncertainty in peak cladding temperature predictions for best-estimate thermal-hydraulic transient codes that the NRC has developed. D. IMPLEMENTATION The purpose of this section is to provide informa tion to applicants and licensees