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
CFR Part: 
CFR Title: 

Content:
ent Room, 2120 L Street NW., Washington, DC. 1.157-2 the criteria will not be exceeded. In this manner, more realistic calculations are available for regulatory decisions, yet an appropriate degree of conservatism would be maintained. Many of the methods and models needed for a S best-estimate calculation are the same as those used previously for evaluation model analyses. Although licensees and applicants are well acquainted with them, explicit guidance on acceptable methods and models (based on NRC experience with its own best estimate advanced codes such as TRAC-PWR, TRAC-BWR, RELAP5, COBRA, and FRAP) would be useful. Further, the NRC has not previously pub lished acceptable methods for uncertainty analyses. Therefore, guidance on methods acceptable to the NRC staff for calculating ECCS performance and for estimating the uncertainty are provided in the follow ing Regulatory Position. C. REGULATORY POSITION 1. BEST-ESTIMATE CALCULATIONS A best-estimate calculation uses modeling that attempts to realistically describe the physical proc esses occurring in a nuclear reactor. There is no unique approach to the extremely complex modeling of these processes. The NRC has developed and as sessed several best-estimate advanced thermal hydraulic transient codes. These include TRAC PWR, TRAC-BWR, RELAP5, COBRA, and the . FRAP series of codes (References 1 through 6). These codes reasonably predict the major phenom ena observed over a broad range of thermal hydraulic and fuel tests. Licensees and applicants may use, but are not limited to, these codes and the specific models within them to perform best-estimate calculations of emergency core cooling system (ECCS) performance. Since the NRC staff has not performed the plant-specific uncertainty analysis re quired by the revised § 50.46 of 10 CFR Part 50, the licensee must demonstrate that the code and models used are acceptable and applicable to the specific fa cility over the intended operating range and must quantify the