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:
not be extrapo lated beyond the range over which they were devel oped or assessed. If the model is to be extrapolated beyond the conditions for which valid data compari sons have been made, judgments should be made as to the effect of this extrapolation and the effect should be accounted for in the uncertainty evalu ation. The fundamental laws of physics, well established data bases (e.g., steam tables), and sensi tivity studies should be used to assist in estimating the uncertainty that results from extrapolation. A best-estimate code contains all the models nec essary to predict the important phenomena that might occur during a loss-of-coolant accident. Best-estimate code calculations should be compared with applicable experimental data (e.g., separate-effects tests and in tegral simulations of loss-of-coolant accidents) to de termine the overall uncertainty and biases of the cal culation. In addition to providing input to the uncertainty evaluation, integral simulation data com parisons should be used to ensure that important phe nomena that are expected to occur during a loss-of coolant accident are adequately predicted. This is an idealized characterization of a best-estimate code. In practice, best-estimate codes may contain certain models that are simplified or that contain conserva tism to some degree. This conservatism may be intro duced for the following reasons: 1. The model simplification or conservatism has little effect on the result, and therefore the development of a better model is not justified. 2. The uncertainty of a particular model is diffi cult to determine, and only an upper bound can be determined. 3. The particular application does not require a totally best-estimate calculation, so a bias in the calculation is acceptable. The introduction of conservative bias or simplifi cation in otherwise best-estimate codes should not, 1.157-3 however, result in calculations that are unrealistic, that do not include important phenomena, or that contain