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:
aluation models for the purpose of reducing reactor operating restrictions. This interim approach was a step in the direction of basing licensing decisions on realistic calculations of plant behavior. Although the approach permits many "best-estimate" methods and models to be used for licensee submittals, it retains those features of Ap pendix K that are legal requirements. The current revision of § 50.46 permits ECCS evaluation models to be fully "best-estimate" and re moves the arbitrary conservatisms contained in the required features of Appendix K for those licensees wishing to use these improved methods. Safety is best served when decisions concerning the limits within which nuclear reactors are permitted to operate are based upon realistic calculations. This approach is currently being used in the resolution of almost all reactor safety issues (e.g., anticipated transients with out scram, pressurized thermal shock, and operator guidelines) and is now available for one of the last remaining major issues still treated in a prescriptive manner, the loss-of-coolant accident. The NRC staff amended § 50.46 of 10 CFR Part 50 to allow realistic methods to be used for the ECCS performance calculations in place of the evaluation models that use the required Appendix K features. This rule change also requires analysis of the uncer tainty of the best-estimate calculations and requires that this uncertainty be considered when comparing the results of the calculations to the limits of para graph 50.46(b) so that there is a high probability that "6Information Report from William J. Dircks to the Commissioners, dated November 17,1983, "Emergency Core Cooling System Analy sis Methods," SECY-83-472. Available for inspection or copying for a fee in the NRC Public Document 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