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:
R), have been found acceptable by the NRC staff in the past at the 95% probability level. This 95% probability level would also be applied to small-break loss-of-coolant accidents, which have a higher probability than large breaks. The dominant factors influencing risk from small-break loss-of coolant accidents include equipment availability and operator actions. Calculational uncertainties are much less important than factors such as operator recognition of the event, the availability of equip ment, and the correct use of this equipment.. The use 1.157-12 of a best-estimate calculation with reasonable and quantifiable uncertainty is expected to provide a re duction in the overall risk from a small-break loss-of coolant accident by providing more realistic calcula tions with which to evaluate operator guidelines and -' determine the true effect of equipment availability. Regulatory Position 3 provides a description of the features that should be included in the overall code uncertainty evaluation that is called for in para graph 50.46(a) (1). This uncertainty evaluation should make use of probabilistic and statistical meth ods to determine the code uncertainty. For a calcula tion of this complexity, a completely rigorous mathe matical treatment is neither practical nor required. In many cases, approximations and assumptions may be made to make the overall calculational uncertainty evaluation possible. A careful statement of these as sumptions and approximations should be made so that the NRC staff may make a judgment as to the validity of the uncertainty evaluation. The purpose of the uncertainty evaluation is to provide assurance that for postulated loss-of-coolant accidents a given plant will not, with a probability of 95% or more, ex ceed the applicable limits specified in paragraph 50.46(b). 4.2 Code Uncertainty This regulatory guide makes a distinction be tween the terms "code uncertainty" and "overall cal culational uncertainty." The latter term is defined