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:
area equal to twice the cross-sectional area of the pipe. The range of break sizes considered should be 1.157-4 sufficiently broad that -the system response as a func tion of break size is well enough defined so that inter polations between calculations, without considering unexpected behavior between the break sizes, may be made confidently. Other boundary and initial conditions and equip ment availability should be based on plant technical specification limits. These other conditions include, but may not be limited to, availability and perform ance of equipment, automatic controls, and operator actions. Appendix A to 10 CFR Part 50 requires that a single failure be considered when analyzing safety system performance and that the analysis consider the effect of using only onsite power and only offsite power. 3.2 Sources of Heat During a Loss-of-Coolant Accident Models should account for the sources of heat discussed below and the distribution of heat production. 3.2.1 Initial Stored Energy of the Fuel The steady-state temperature distribution and stored energy in the fuel before the postulated acci dent should be calculated in a best-estimate manner for the assumed initial conditions, fuel conditions, and operating history. To accomplish this, the ther mal conductivity of the fuel pellets and the thermal conductance of the gap between the fuel pellet and '• the cladding should be evaluated. Thermal conduc tivity of fuel is a function of temperature and is de graded by the presence of gases in crack voids be tween fuel fragments. An acceptable model for thermal conductivity should be developed from the in-pile test results for fuel centerline and off-center temperatures, taking into account the conductivity of gases in crack voids. Thermal conductance of the fuel-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