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:
specified in Appendix K were promulgated in January 1974 after extensive rulemaking hearings and were based on the under standing of ECCS performance available at that time. In the years following the promulgation of those rules, the NRC, the nuclear industry, and several for eign institutions have conducted an extensive pro gram of research that has greatly improved the un derstanding of ECCS performance during a postulated loss-of-coolant accident. The methods specified in Appendix K were found to be highly con servative; that is, the fuel cladding temperatures ex pected during a loss-of-coolant accident would be much lower than the temperatures calculated using Appendix K methods. In addition to showing that Appendix K is conservative, the ECCS research pro- vided information that allows for quantification of that conservatism. The results of experiments, com puter code development, and code assessment allow more accurate calculations, along with reasonable es timates of uncertainty, of ECCS performance during a postulated loss-of-coolant accident than is possible using the Appendix K procedures. It was also found that some plants were being re stricted in operating flexibility by limits resulting from conservative Appendix K requirements.,Based on the research performed, it was determined that these re strictions could be relaxed through the use of more realistic calculations without adversely affecting safety. The Appendix K requirements tended to di vert both NRC and industry resources from matters that are relevant to reactor safety to analyses with as sumptions known to be nonphysical. In recognition of the known conservatisms in Ap pendix K, the NRC adopted an interim approach in 1983, described in SECY-83-472,5 to accommodate industry requests for improved evaluation 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.