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:
values of gap conductance and other thermal parameters as functions of temperature and time. Best-estimate methods to calculate the swelling of the cladding should take into account spatially varying cladding temperatures, heating rates, anisotropic material properties, asymmetric deformation of cladding, and fuel rod thermal and mechanical parameters. Best estimate methods will be considered acceptable pro vided their technical basis is demonstrated with ap propriate data and analyses. 3.3.2 Other Core Thermal Parameters As necessary and appropriate, physical and chemical changes in in-core materials (e.g., eutectic formation, phase change, or other phenomena caused by material interaction) should be accounted for in the reactor core thermal analysis. Best-estimate models will be considered acceptable provided their technical basis is demonstrated with appropriate data and analyses. 1.157-6 3.4 Blowdown Phenomena 3.4.1 Break Characteristics and Flow In analyses of hypothetical loss-of-coolant acci dents, a spectrum of possible break sizes should be -- ' considered, as indicated in Regulatory Position 3.1. The discharge flow rate should be calculated with a critical flow rate model that considers the fluid conditions at the break location, upstream and down stream pressures, and break geometry. The critical flow model should be justified by comparison to ap plicable experimental data over a range of conditions for which the model is applied. The model should be a best-estimate calculation, with uncertainty in the critical flow rate included as part of the uncertainty evaluation. Best-estimate models will be considered acceptable provided their technical basis is demon strated with appropriate data and analyses. 3.4.1.1 Model Evaluation Procedure for Discharge Flow Rate. Critical flow models to be employed in ECCS evaluations should: a. Be checked against an acceptable set of rele vant data, b. Recognize thermal nonequilibrium conditions when the fluid is subcooled,