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:
hod used within the code represents the physical phenomenon it addresses. These individual uncertainties, when taken together, comprise the "code uncertainty." The combined uncertainty associated with indi vidual models (i.e., code uncertainty) within the best estimate codes does not account for all of the uncer tainty associated with the model's use. In addition to the code uncertainty, various other sources of uncer tainty are introduced when attempting to use best estimate codes to predict full-scale plant thermal-hydraulic response. These sources include uncertainty associated with the experimental data used in the code assessment process (including appli cability of the data to full-scale reactors), the input boundary and initial conditions, and the fuel behav ior. Additional sources of uncertainty stem from the use of simplifying assumptions and approximations. A careful statement of these assumptions and approxi mations should be made, and the uncertainty associ ated with them should be taken into account. There fore, the "overall calculational uncertainty" is defined as the uncertainty arrived at when all the contributions from the sources identified above, in cluding the code uncertainty, are taken into account. A 95% probability level is considered acceptable to the NRC staff for comparison of best-estimate pre dictions to the applicable limits of paragraph 50.46(b) of 10 CFR Part 50 to meet the requirement of paragraph 50.46(a)(1)(i) to show that there is a high probability that the criteria will not be exceeded. The basis for selecting the 95% probability level is primarily for consistency with standard engineering practice in regulatory matters involving thermal hy draulics. Many parameters, most notably the depar ture from nucleate boiling ratio (DNBR), 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