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:
l-cladding gap is a strong function of hot gap size and of the composi tion and pressure of the gases in the fuel rod. The calculation of hot gap size should take into account U0 2 or mixed-oxide fuel swelling, densification, creep, thermal expansion and fragment relocation, and cladding creep. Fuel swelling is a function of temperature and burnup. Fuel densification is a func tion of burnup, temperature, and initial density. Den sification can result from hydrostatic stresses imposed on fuel during pellet-cladding mechanical interaction and should be considered. Fuel creep is a function of time, temperature, grain size, density, fission rate, oxygen-to-metal ratio, and external stress. Fuel ther mal expansion represents dimensional changes in unirradiated fuel pellets caused by changes in tem perature. An acceptable model for the above fuel pa rameters should be based on in-pile and out-of-pile test data. Cladding creep introduces compressive creep strain in cladding during steady-state operation, reducing the gap between the fuel pellet and clad ding. Cladding creep is a function of fast neutron flux (>1 MeV), cladding temperature, hoop stress, and material. Cladding materials may be cold worked and stress-relieved or fully recrystallized, and there is a significant difference in the magnitude of creepdown between these materials. During pellet cladding mechanical interaction, cladding experi ences deformation from tensile creep, which is sig nificantly different from that caused by compressive creep. An acceptable model for cladding tensile creep should be based on in-reactor tensile creep data. Best-estimate fuel models will be considered ac ceptable provided the models include essential phe nomena identified above and provided their technical basis is demonstrated with appropriate data and analyses. 3.2.1.1 Model Evaluation Procedure for Stored Energy and Heat Transfer in Fuel Rods. A model to be used in ECCS evaluations to calculate internal fuel rod heat